Environmental science

Biodiversity and Conservation Biology Upon completing this chapter, you will be able to:

➤ Characterize the scope of biodiversity on Earth ➤ Contrast the background extinction rate with periods of mass extinction ➤ Evaluate the primary causes of biodiversity loss ➤ Specify the benefits of biodiversity ➤ Assess the science and practice of conservation biology ➤ Analyze efforts to conserve threatened and endangered species ➤ Compare and contrast conservation efforts above the species level

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A Siberian tiger in the Sikhote-Alin Mountains

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CENTRAL CASE STUDY

Saving the Siberian Tiger “Future generations would be truly saddened that this century had so little foresight, so little compassion, such lack

of generosity of spirit for the future that it would eliminate one of the most dramatic and beautiful animals this world has ever seen.”

—George Schaller, Wildlife Biologist, on the tiger

“If you kill a tiger, you can buy a motorbike.” —Anonymous poacher, on selling tiger parts

Historically, tigers roamed widely across Asia from Turkey to northeast Russia to In-donesia. However, people have driven the majestic striped cats from nearly all of their range. Today, tigers are exceedingly rare and are sliding toward extinction. Just over 3,000 tigers survive, down from 100,000 a century ago.

Sikhote-Alin Mountains

RUSSIA

CHINA

MONGOLIA

INDIA

Tigers of the subspecies known as the Siberian tiger are the largest cats in the world. These regal animals today find their last refuge in the forests of the remote Sikhote-Alin Mountains of the Russian Far East. For thou- sands of years the Siberian tiger coexisted with the re- gion’s native people and held a prominent place in their lore. These people viewed it as a guardian of the moun- tains and forests, and they rarely killed a tiger unless it had preyed on a person.

The Russians who moved into the region in the early 20th century had no such cultural traditions. They hunted tigers relentlessly for sport and hides, and the tiger population dipped to perhaps just 20–30 animals. In response, the Russian government banned the hunting of tigers, and the population be- gan to recover. However, poachers started killing ti- gers illegally to sell their body parts to China and oth- er Asian countries, where they are used in traditional medicine and as alleged aphrodisiacs. Meanwhile, logging, road building, and agriculture degraded and fragmented tiger habitat, providing easy access for still more poachers.

International conservation groups got involved just in time, working with Russian biologists to save the

dwindling tiger population. One such group was the Hor- nocker Wildlife Institute, now part of the Wildlife Conserva- tion Society (WCS). In 1992 the group helped launch the Sibe- rian Tiger Project, devoted to studying and conserving the tiger and its habitat. The team put together a plan to protect the tiger, began educating people on the animal’s value, and worked closely with peo- ple who live near the big cats.

Today, WCS biologists track tigers with radio- collars, monitor their movements and health, deter- mine causes of death when they die, and study as- pects of the tiger’s ecosystem. They also work with the region’s people and help fund local wildlife of- ficials to deter and capture poachers.

Thanks to such efforts, the Siberian tiger popu- lation stabilized, even while the world’s other tiger populations were declining. The last range-wide sur- vey, in 2005, found between 428 and 502 Siberian ti- gers in the wild, while 1,500 more survived in zoos and captive breeding programs. However, govern- ment funding and law enforcement to deter poaching were reduced, and data since 2005 suggest that tiger numbers are falling yet again.

Many dedicated scientists, conservationists, and policymakers continue trying to save these

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endangered animals. In November 2010, leaders of the 13 nations where tigers still survive met at a historic summit in St. Petersburg, Russia, marking the first time that multiple heads of state had ever convened to fo- cus on saving a single species of wild animal. Russian prime minister Vladimir Putin, Chinese premier Wen Jiabao, World Bank president Robert Zoellick, and actor Leonardo DiCaprio were among the luminaries participating in the conference. At this International Ti- ger Forum, the leaders signed a declaration that set in motion a strategic multinational plan called the Global Tiger Recovery Program.

This program aims to double the tiger population by 2022 (the next “Year of the Tiger” by the Chinese zodiac) by protecting habitat, cracking down on poach- ing, and addressing illegal trade in pelts and body parts. National governments, conservation organiza- tions, and the World Bank promised millions of dollars, although more is needed—an estimated $350 million over the first 5 years of the program. Representatives of the 13 nations planned to work out details of financ- ing during 2011.

Some proponents of tiger conservation criticized the program, worrying that funding would not be ad- equate, that specific measures to reduce demand for tiger body parts were not spelled out, and that pro- posed actions were not focused enough. Nonetheless, by demonstrating support for tiger conservation at the highest political level, the summit gave tiger conser- vation efforts a clear boost. The struggle to save the tiger from imminent extinction is one of numerous ef- forts around the world today to stem the loss of our planet’s priceless biological diversity. ■

OUR PLANET OF LIFE Our rising human population and resource consumption are putting ever-greater pressure on the flora and fauna of our planet, from tigers to tiger beetles. We are diminishing Earth’s diversity of life, the very quality that makes our planet so special.

Biodiversity encompasses multiple levels Biological diversity, or biodiversity (p. 49), describes the va- riety of life across all levels of biological organization, includ- ing the diversity of species, their genes, their populations, and their communities. Biodiversity is a concept as multifaceted as life itself, and biologists employ different working defini- tions according to their own aims and philosophies. Yet sci- entists agree that the concept applies across the major levels in the organization of life (FIGURE 8.1). The level that is easiest to visualize and most commonly used is species diversity.

Species diversity A species (p. 46) is a distinct type of organism, a set of individuals that uniquely share certain characteristics and can breed with one another and produce fertile offspring. Biologists use differing criteria to distin- guish one species from another. Some emphasize character- istics shared because of common ancestry, whereas others

Ecosystem diversity

Species diversity

Genetic diversity

FIGURE 8.1  The concept of biodiversity encompasses several levels in the hierarchy of life. Species diversity (middle frame of the figure) refers to the number or variety of species. Genetic diversity (bottom frame) refers to variation in DNA composition among individuals within a species. Ecosystem diversity (top frame) and related concepts refer to variety at levels above the species level, such as ecosystems, communities, habitats, or landscapes.

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by the same processes that drive speciation (pp. 49–50) but re- sult when divergence stops short of forming separate species. Scientists denote subspecies with a third part of the scientific name. The Siberian tiger, Panthera tigris altaica, is one of five (or perhaps only four) subspecies of tiger still surviving (FIG- URE 8.3). Tiger subspecies differ in color, coat thickness, stripe patterns, and size. For example, Panthera tigris altaica is taller at the shoulder than the Bengal tiger (Panthera tigris tigris) of India and Nepal, and it has a thicker coat and larger paws.

Genetic diversity Scientists designate subspecies when they recognize substantial, genetically based differences among individuals from different populations of a species. However, all species consist of individuals that vary geneti- cally from one another to some degree, and this genetic di- versity is an important component of biodiversity. Genetic diversity encompasses the differences in DNA composition (p. 28) among individuals.

Genetic diversity provides the raw material for adaptation to local conditions. A diversity of genes for coat thickness in tigers allowed natural selection (pp. 46–48) to favor genes for thin fur in Bengal tigers living in warm regions, and genes for thick fur in Siberian tigers living in cold regions. In the long term, populations with more genetic diversity may be more likely to persist, because their variation better enables them to cope with environmental change.

Populations with little genetic diversity are vulnerable to environmental change because they may happen to lack ge-

emphasize the ability to interbreed. In practice, however, sci- entists generally agree on species identities.

We can express species diversity in terms of the number or variety of species in a particular region. One component of species diversity is species richness, the number of species. Another is evenness or relative abundance, the extent to which species differ in numbers of individuals.

Speciation (pp. 49–50) generates new species, whereas extinction (p. 51) diminishes species richness. Immigration, emigration, and local extinction may change species richness locally, but only speciation and extinction change it globally.

Taxonomists classify species by their similarity into a hi- erarchy of categories meant to reflect evolutionary relation- ships. Related species are grouped together into genera (sin- gular: genus); related genera are grouped into families; and so on (FIGURE 8.2). Every species is given a two-part Latin or Latinized scientific name denoting its genus and species. The tiger, Panthera tigris, is similar to the world’s other species of large cats, such as the jaguar (Panthera onca), the leopard (Panthera pardus), and the African lion (Panthera leo). These four species are closely related in evolutionary terms, and this is indicated by the genus name they share, Panthera. They are more distantly related to cats in other genera such as the chee- tah (Acinonyx jubatus) and the bobcat (Felis rufus), although all cats are classified together in the family Felidae.

Biodiversity exists below the species level in the form of subspecies, populations of a species that occur in separate geo- graphic areas and differ in some characteristics. Subspecies arise

Domain: Eukarya

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: Felidae

Genus: Panthera

Species: Panthera tigris

FIGURE 8.2  Taxonomists classify organisms using a hierarchical system meant to reflect evolutionary relation- ships. Species that are similar in their appearance, behavior, and genetics (because they share recent common ancestry) are placed in the same genus. Organisms of similar genera are placed within the same family. Families are placed within orders, orders within classes, classes within phyla, phyla within kingdoms, and kingdoms within domains. For instance, tigers belong to the class Mammalia, along with elephants, kangaroos, and bats. However, the differences among these species, which have evolved and diverged over millions of years, are great enough that they are placed in different orders, families, and genera.

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good gauge for overall biodiversity. Yet we still are profound- ly ignorant of the number of species that exist in the world. So far, scientists have described about 1.8 million species of plants, animals, and microorganisms. However, estimates for the number that actually exist range from 3 million to 100 million, with the most widely accepted estimates in the neighborhood of 14 million.

Our knowledge of species numbers is incomplete for several reasons. First, many species are tiny and easily over- looked. These include bacteria, nematodes (roundworms), fungi, protists, and soil-dwelling arthropods. Second, many organisms are so difficult to identify that ones thought to be identical sometimes turn out, once biologists look more closely, to be multiple species. Third, some areas of Earth remain little explored. We have barely sampled the ocean depths, hydrothermal vents (p. 260), or the canopies and soils of tropical forests. As one example, a 2005 expedition to the remote Foja Mountains of New Guinea discovered over 40 new species of vertebrates, plants, and butterflies in less than a month, while research in marine waters nearby turned up another 50 new species.

Biodiversity is unevenly distributed Some taxonomic groups hold more species than others. In this respect, insects show a staggering predominance over all other forms of life (FIGURE 8.4). Within insects, about 40% are beetles, and beetles alone outnumber all non-insect ani- mals and all plants. No wonder the 20th-century British bi- ologist J.B.S. Haldane famously quipped that God must have had “an inordinate fondness for beetles.”

netic variants that would help them adapt to novel conditions. Populations with low genetic diversity may also be more vul- nerable to disease and may suffer inbreeding depression, which occurs when genetically similar parents mate and produce weak or defective offspring. Scientists have sounded warn- ings over low genetic diversity in species that have dropped to low population sizes, including cheetahs, bison, and elephant seals, but the full consequences of reduced diversity in these species remain to be seen. Diminished genetic diversity in our crop plants is a prime concern to humanity (p. 136).

Ecosystem diversity Biodiversity encompasses levels above the species level, as well. Ecosystem diversity refers to the number and variety of ecosystems, but biologists may also refer to the diversity of biotic communities or habitats within some specified area. If the area is large, scientists may also consider the geographic arrangement of habitats, communities, or eco- systems at the landscape level, including the sizes and shapes of patches and the connections among them. Under any of these concepts, a seashore of rocky and sandy beaches, for- ested cliffs, offshore coral reefs, and ocean waters would hold far more biodiversity than the same acreage of a monocultural cornfield. A mountain slope whose vegetation changes with elevation from desert to hardwood forest to conifer forest to alpine meadow would hold more biodiversity than an equal- sized area consisting of only desert, forest, or meadow.

Many species await discovery Scientists often express biodiversity in terms of species rich- ness because that component is most easily measured and is a

Siberian (Amur) tiger

South China tiger

Indochina tiger

Bengal tiger

Sumatran tiger

Caspian tiger (extinct)

Historical range

Current range

Bali tiger (extinct)

Javan tiger (extinct)

FIGURE 8.3  Three of the eight sub- species of tiger—the Bali, Javan, and Caspian tigers—were driven extinct during the 20th century. Today only the Siberian, Bengal, Indochina, and Sumatran tigers persist, while the South China tiger has not been seen in 25 years and may be extinct. Defor- estation, hunting, and other pressures from people have caused tigers of all subspecies to disappear from 93% of the geographic range they historically occupied. Researchers estimate that the majority of surviving individuals are crowded into less than half of 1% of the species’ original range. This map contrasts the ranges of the eight subspecies in the years 1800 (orange) and 2000 (red). Data from the Tiger Information Center.

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tion zones where habitats intermix; p. 33) often support high biodiversity. Because human disturbance can sometimes in- crease habitat diversity, species diversity may rise in disturbed areas. However, this is true only at local scales. At larger scales, human disturbance decreases diversity because specialists dis- appear when habitats are homogenized and because species that rely on large expanses of habitat disappear when habitats are fragmented.

EXTINCTION AND BIODIVERSITY LOSS Biodiversity at all levels is being lost to human impact, most irretrievably in the extinction of species. Extinction (p. 51) occurs when the last member of a species dies and the species ceases to exist. The disappearance of a particular population from a given area, but not the entire species globally, is re- ferred to as extirpation. The tiger has been extirpated from most of its historic range (see Figure 8.3), but it is not yet ex- tinct. Extirpation is an erosive process that can, over time, lead to extinction.

Human impact is responsible for most cases of extirpa- tion and extinction today, but these processes also occur natu- rally, albeit at a much slower rate. If species did not naturally go extinct, we would be up to our ears in dinosaurs, trilobites, and millions of other creatures that vanished from Earth long before we appeared. Paleontologists estimate that roughly 99% of all species that ever lived are now extinct, and that the

Living things are distributed unevenly across our planet, as well. For instance, species richness generally increases as one nears the equator. This pattern of variation with latitude is called the latitudinal gradient, and hypotheses abound to explain it. A leading idea is that greater amounts of solar en- ergy, heat, and humidity at tropical latitudes lead to more plant growth, making areas nearer the equator more produc- tive and able to support more animals. The relatively sta- ble climates of equatorial regions, in turn, discourage single species from dominating ecosystems and, instead, allow nu- merous species to coexist. Whereas variable environmental conditions favor generalists (species that can tolerate a wide range of circumstances), stable conditions favor specialists (species that do particular things especially well). Another proposed explanation for the latitudinal gradient is that gla- ciation events repeatedly forced organisms toward tropical latitudes, leaving the polar and temperate regions relatively species-poor.

The latitudinal gradient influences the species diversity of Earth’s biomes (pp. 78–84). Tropical dry forests and rainfor- ests support far more species than tundra and boreal forests, for instance. At smaller scales, diversity varies with habitat type. Structurally diverse habitats tend to allow for more eco- logical niches (p. 53) and support greater species richness and evenness. For instance, forests generally support more diver- sity than grasslands.

For any given area, species diversity tends to increase with diversity of habitats, because each habitat supports a somewhat different set of organisms. Thus, ecotones (transi-

Mammals

Arachnids

Fungi

Reptiles

Bacteria

Birds

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PlantsPlants

ArchaeaAmphibians

Annelids

Roundworms

Annelids

Roundworms Echinoderms Fishes

Flatworms

Protists

Jellyfish

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Molluscs

Crustaceans

FIGURE 8.4  This illustration shows organisms scaled in size to the number of species known from each major taxo- nomic group. This gives a visual sense of the disparity in species richness among groups. However, because most species are not yet discovered or described, some groups (such as bacteria, archaea, insects, nematodes, protists, fungi, and others) may contain far more species than we now know of. Data from Groom- bridge, B., and M.D. Jenkins, 2002. Global

biodiversity: Earth’s living resources in the 21st

century. UNEP-World Conservation Monitoring

Centre. Cambridge, U.K.: Hoechst Foundation.

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of all species. Although similar in scale to previous mass ex- tinctions, today’s ongoing mass extinction is different in two primary respects. First, we are causing it. Second, we will suf- fer as a result.

We are setting the sixth mass extinction in motion Over just the past few centuries, we have recorded hun- dreds of instances of species extinction caused by people. Among North American birds in the past two centuries alone, we have driven into extinction the Carolina para- keet, great auk, Labrador duck, passenger pigeon (pp. 53– 54), probably the Bachman’s warbler and Eskimo curlew, and possibly the ivory-billed woodpecker (FIGURE 8.6). Several more species, including the whooping crane, Kirt- land’s warbler, and California condor (p. 177), teeter on the brink of extinction.

However, species extinctions caused by people precede written history. Archaeological evidence shows that in case after case, a wave of extinctions followed close on the heels of human arrival on islands and continents (FIGURE 8.7). Af- ter Polynesians reached Hawaii, half its birds went extinct. Birds, mammals, and reptiles vanished following human arrival on many other oceanic islands, including large land masses such as New Zealand and Madagascar. Dozens of species of large vertebrates died off in Australia after people arrived roughly 50,000 years ago, and North America lost 33 genera of large mammals once people arrived more than 10,000 years ago.

Today, species loss is accelerating as our population growth and resource consumption put increasing strain on

remaining 1% comprises the wealth of species on our planet today.

Most extinctions preceding the appearance of human beings occurred one by one for independent reasons, at a pace referred to as the background extinction rate (p. 51). By studying traces of organisms preserved in the fossil record (p. 50), scientists infer that for mammals and marine ani- mals, each year, on average, 1 species out of every 1–10 mil- lion vanished.

Earth has experienced five mass extinction episodes Extinction rates rose far above this background rate at sev- eral points in Earth’s history. In the past 440 million years, our planet experienced five major episodes of mass extinc- tion (p. 51; FIGURE 8.5). Each event eliminated more than one-fifth of life’s families and at least half its species. The most severe episode occurred at the end of the Permian pe- riod (see APPENDIX: Geologic Time Scale for Earth’s geologic periods). At this time, 248 million years ago, close to 90% of all species went extinct. The best-known episode occurred 65 million years ago at the end of the Cretaceous period, when an apparent asteroid impact brought an end to the di- nosaurs and many other groups.

If current trends continue, the modern era, known as the Quaternary period, may see the extinction of more than half

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FIGURE 8.5  The fossil record shows evidence for five episodes of mass extinction during the past half-billion years of Earth his- tory. At the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods, 50–95% of the world’s species appear to have gone extinct. (This graph shows families, not species, which is why the drops appear less severe.) Each time, biodiversity later rebounded to equal or higher levels, but each rebound required millions of years. Data from Raup, D.M., and J.J. Sepkoski, 1982. Mass extinctions in the marine fossil record. Science 215:1501–1503. Reprinted with

permission from AAAS.

FIGURE 8.6  The ivory-billed woodpecker was one of North America’s most majestic birds and lived in old-growth forests throughout the southeastern United States. Forest clearing and timber harvesting eliminated the mature trees it needed for food, shelter, and nesting, and this symbol of the South appeared to go extinct. In recent years, fleeting, controversial observations in Arkansas, Louisiana, and Florida have raised hopes that the species persists, but proof has been elusive.

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habitats and wildlife. In 2005, scientists with the Millennium Ecosystem Assessment (p. 16 ) calculated that the current global extinction rate is 100 to 1,000 times greater than the background rate. They projected that the rate would increase tenfold or more in future decades.

To monitor endangered species, the International Union for Conservation of Nature (IUCN) maintains the Red List , an updated list of species facing high risks of extinction. The 2010 Red List reported that 21% (1,131) of mammal species, 13% (1,240) of bird species, and 30% (1,898) of amphibian species are threatened with extinction. Among other major groups (for which assessments are not complete), 17% to 73% of species are judged to be at high risk of extinction. In the United States alone over the past 500 years, 237 animals and 30 plants are known to have gone extinct. For all these figures, the actual numbers are without doubt greater than the known numbers.

Among the 1,131 mammals facing possible extinction is the tiger, which despite—or perhaps because of—its tremendous size and reputation as a fierce predator, is one of the most endangered large animals on the planet. In 1950, eight tiger subspecies existed (see Figure 8.3 ). Today, three are extinct. The Bali tiger went extinct in the 1940s, the Caspian tiger in the 1970s, and the Javan tiger in the 1980s. The South China tiger has not been seen in 25 years and little of its habitat remains, so scientists fear it too will soon be extinct, if it is not already.

North America ~10,000–11,500 yr ago 72% of large mammal genera

Eurasia >30,000 yr ago 36% of large mammal genera

Pacific Islands ~1,000–3,000 yr ago 50+% of endemic landbird species

New Zealand ~1,000 yr ago moas, other birds

South America ~10,000–15,000 yr ago 83% of large mammal genera

?

Africa ~160,000 yr ago 18% of large mammal genera

?

Madagascar ~1,500 yr ago lemurs, elephant birds, others

Australia ~44,000–72,000 yr ago 88% of large mammal genera

FIGURE 8.7  This map shows for each region the time of human arrival and the extent of the recent extinction wave. Illustrated are representative extinct megafauna from each region. The human hunter icons are sized accord- ing to the degree of evidence that human hunting was a cause of extinctions; larger icons indicate more certainty that humans (as opposed to climate change or other factors) were the cause. Data for South America and Africa are so far too sparse to be conclusive, and future archaeological and paleontological research could well alter these interpretations. Adapted from Barnosky, A.D., et al., 2004. Assessing the causes of late Pleistocene extinctions on the continents. Science 306: 70–75; and Wilson, E.O., 1992. The diversity of life . Cambridge, MA: Belknap Press.

FAQ

Q: If a mass extinction is happening, why don’t I see species going extinct all around me? A: There are two reasons that most of us don’t personally sense the scale of biodiversity loss. First, if you live in a town or city, the plants and animals you see from day to day are generalist species that thrive in disturbed areas. In contrast, the species in trouble are those that rely on less-disturbed habitats, and you may need to go further afield to find them.

S econd, a human lifetime is very short! The loss of populations and species over the course of our lifetime may seem a slow process to us, but on Earth’s timescale it is sudden—almost instantaneous. Because each of us is born into a world that has already lost many species, we don’t recognize what’s already been lost. Likewise, our grandchildren won’t appreciate what we’ve lost in our lifetimes. Each human generation experiences just a portion of the overall phenomenon, so we have difficulty sensing the big picture. Nonetheless, researchers who study biology and naturalists who spend their time outdoors are seeing a great deal of biodiversity loss—and that’s precisely why they feel so passionate about it.

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prairies native to North America’s Great Plains are today al- most entirely converted to agriculture. Less than 1% of prai- rie habitat remains. As a result, grassland bird populations have declined by an estimated 82–99%.

Habitat destruction has occurred widely in nearly every biome (FIGURE 8.9). Over half of the world’s temperate for- ests, grasslands, and shrublands had been converted by 1950 (mostly for agriculture). Across Asia, scientists estimate that 40% of the tiger’s remaining habitat has disappeared just in the last decade. Today habitat is being lost most rapidly in tropical rainforests, tropical dry forests, and savannas.

Because organisms are adapted to the habitats in which they live, any major change in their habitat is likely to render it less suitable for them. Many human activities alter, degrade, or destroy habitat. Farming replaces diverse natural commu- nities with simplified ones composed of only a few plant spe- cies. Grazing modifies the structure and species composition of grasslands. Either type of agriculture can lead to desertifi- cation. Clearing forests removes the food, shelter, and other resources that forest-dwelling organisms need to survive. Hydroelectric dams turn rivers into reservoirs upstream and affect water conditions and floodplain communities down- stream. Urban sprawl supplants natural ecosystems, driving many species from their homes.

Biodiversity loss involves more than extinction Extinction is only part of the story of biodiversity loss. The larger part involves declining population sizes. As a species’ numbers decline, its geographic range often shrinks as it is extirpated from parts of its range. Thus, many species today are less numerous and occupy less area than they once did. Tigers numbered well over 100,000 worldwide in the 19th century but number only 3,000 to 3,500 today. Such declines mean that genetic diversity and ecosystem diversity, as well as species diversity, are being lost.

To measure and quantify this degradation, scientists at the World Wildlife Fund and the United Nations Environ- ment Programme (UNEP) developed a metric called the Liv- ing Planet Index. This index summarizes trends in the popula- tions of 2,544 vertebrate species that are sufficiently monitored to provide reliable data. Between 1970 and 2007, the Living Planet Index fell by roughly 30% (FIGURE 8.8), driven prima- rily by biodiversity losses in tropical regions.

Several major causes of biodiversity loss stand out Scientists have identified four primary causes of popula- tion decline and species extinction: habitat loss, pollution, overharvesting, and invasive species. Global climate change (Chapter 14) now is becoming the fifth. Each of these causes is intensified by human population growth and by our in- creasing per capita consumption of resources.

Habitat loss Habitat loss is the single greatest cause of biodiversity loss today. It is the primary cause of popula- tion declines in 83% of threatened mammals and 85% of threatened birds, according to UNEP data. For example, the

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FIGURE 8.8  The Living Planet Index serves as an indicator of the state of global biodiversity. Index values summarize trends for 7,953 populations of 2,544 vertebrate species. Between 1970 and 2007, the Living Planet Index fell by roughly 30%. The index for terrestrial species fell by 25%; for freshwater species, 35%; and for marine species, 24%. Most losses are in tropical regions, where the index has declined by 60%. In contrast, temperate areas are recov- ering, showing an improvement of 29%. Data from World Wide Fund for Nature, 2010. The Living Planet Report, 2010. Gland, Switzerland.

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Sa va

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FIGURE 8.9  Habitat loss has affected all of Earth’s biomes, as a result of human impacts from housing development (inset photo), agriculture, mining, and other activities. Bars show for each biome the percentage of original area converted for human use through 1990. Temperate grassland and chaparral have lost over 70% of their area, whereas tundra and boreal forest have lost very little. In recent decades, tropical dry forest and savanna have lost the greatest fraction. These data are for outright conversion of habitat and do not include areas indirectly affected by human activity in other ways. Adapted from Millennium Ecosystem Assessment, 2005. Ecosystems and human

well-being: Biodiversity synthesis. World Resources Institute, Washington, D.C.

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Of course, human habitat alteration benefits some spe- cies. Animals such as house sparrows, pigeons, gray squir- rels, rats, and cockroaches thrive in cities and suburbs. How- ever, the species that benefit from our presence are relatively few; for every species that wins, more lose. Furthermore, the species that do well in our midst tend to be weedy generalists that are in little danger of disappearing any time soon.

Habitat loss occurs most commonly through gradual, piecemeal degradation such as habitat fragmentation (FIGURE 8.10). When farming, logging, road building, or development intrude into a forest, they break up a continuous expanse of for- est habitat into an array of fragments, or patches. As habitat fragmentation proceeds across a landscape, animals and plants adapted to the forest habitat disappear from one fragment after

Original habitat1

Gaps form as habitat becomes fragmented

2

Gaps become larger; fragments become smaller and more isolated

3

Species disappear due to habitat fragmentation

4

FIGURE 8.10  Fragmentation of habitat ➊ begins when gaps are created ➋ within a natural habitat. As development proceeds, these gaps expand ➌, join together, and eventually come to domi- nate the landscape ➍, stranding islands of habitat in their midst. As habitat becomes fragmented, fewer populations can persist, and numbers of species in the fragments decline.

FIGURE 8.11  Body parts from tigers are sold as traditional medicines and aphrodisiacs in some Asian cultures. Poachers are il- legally killing tigers to satisfy the surging market demand for these items. Here a street vendor in northern China displays tiger body parts for sale.

another. In response to habitat fragmentation, conservation bi- ologists design landscape-level strategies to prioritize areas to be preserved (pp. 201–203).

Pollution Pollution harms organisms in many ways. Air pollution (Chapter 13) degrades forest ecosystems. Water pol- lution (Chapter 12) impairs fish and amphibians. Agricul- tural runoff (including fertilizers, pesticides, and sediments; Chapters 2 and 7) harms many terrestrial and aquatic species. Heavy metals, polychlorinated biphenyls (PCBs), endocrine- disrupting compounds, and other toxic chemicals poison peo- ple and wildlife (Chapter 10). Plastic garbage in the ocean can strangle, drown, or choke marine creatures (pp. 270–271). The effects of oil and chemical spills on wildlife (pp. 267, 338–340) are dramatic and well known. However, although pollution is a substantial threat, it tends to be less significant than pub- lic perception holds it to be, and it is far less influential than habitat loss.

Overharvesting For most species, hunting or harvest- ing by people will not in itself pose a threat of extinction, but for species like the Siberian tiger, it can. Large in size, few in number, long-lived, and raising few young in its lifetime— a classic K-selected species (p. 59)—the Siberian tiger is just the type of animal to be vulnerable to hunting. The advent of Russian hunting nearly drove the animal extinct, whereas decreased hunting after World War II allowed the population to increase. By the 1980s, the Siberian tiger population was likely up to 250 individuals. The political freedom that came with the Soviet Union’s breakup in 1989, however, brought with it a freedom to harvest Siberia’s natural resources, in- cluding the tiger, without regulations or rules, and poachers illegally killed at least 180 Siberian tigers between 1991 and 1996. This coincided with an economic expansion in many Asian countries, where tiger penises are believed to boost human sexual performance and where tiger bones, claws, whiskers, and other body parts are used to try to treat a va- riety of health problems (FIGURE 8.11). Although no proof of

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Invasive species Our introduction of non-native species to new environments, where some may become invasive (pp. 75–77), also displaces native species (FIGURE 8.12). Some in- troductions are accidental. Examples include aquatic organ- isms transported in the ballast water of ships (such as zebra mussels; Chapter 4), animals that escape from the pet trade, and weeds whose seeds cling to our socks as we travel from place to place. Other introductions are intentional. People have long brought food crops and animals with them as they colonized new places, and today we continue international trade in exotic pets and ornamental plants.

Most organisms introduced to new areas perish, but the few types that survive may do very well, especially if they are freed from the predators and parasites that attacked them back home or from the competitors that had limited their access to resources. Once released from the limiting factors (p. 58) of predation, parasitism, and competition, an introduced species

their effectiveness has been demonstrated, sale of body parts from one tiger fetches at least $15,000 on the black market—a powerful economic temptation for poachers in poor regions.

Hunting has reduced the populations of many K-selected animals. The Atlantic gray whale was driven ex- tinct, and several other whales remain threatened or endan- gered. Gorillas and other primates that are killed for their meat may face extinction soon. Thousands of sharks are killed each year simply so their fins can be used in soup. Today the oceans contain only 10% of the large animals they once did (p. 274).

To combat overharvesting, governments have passed laws, signed treaties, and strengthened anti-poaching efforts. Scientists have begun using genetic analyses to expose illegal hunting and wildlife trade. For instance, DNA testing can re- veal the geographic origins of elephant ivory and determine whether whale meat sold in markets came from animals caught illegally (see THE SCIENCE BEHIND THE STORY, pp. 178–179).

Species Invasive in… Effects

Kudzu (Pueraria montana) Southeastern

United States

(Native to Japan)

Asian long-horned beetles (Anoplophora glabripennis) United States

(Native to Asia)

Rosy wolfsnail (Euglandina rosea)

Hawaii

(Native to Southeastern United States and Latin America)

Invasive Species

European starling (Sturnus vulgaris)

North America

(Native to Europe)

Gypsy moth (Lymantria dispar) Northeastern

United States

(Native to Eurasia)

Cheatgrass (Bromus tectorum) Western United

States

(Native to Eurasia)

Brown tree snake (Boiga irregularis)

Guam

(Native to Southeast Asia)

Kudzu is a vine that can grow 30 m (100 ft) in a single season. The U.S. Soil Conservation Service introduced kudzu in the 1930s to help control erosion. Adaptable and extraordinarily fast-growing, kudzu has taken over thousands of hectares of forests, fields, and roadsides.

Having arrived in imported lumber in the 1990s, these beetles burrow into trees and interfere with the trees’ ability to absorb and process water and nutrients. They may wipe out the majority of hardwood trees in an area. Several U.S. cities, including Chicago and Seattle, have cleared thousands of trees after detecting these invaders.

In the 1950s, well-meaning scientists introduced the rosy wolfsnail to Hawaii to prey upon and reduce the population of another invasive species, the giant African land snail. Within a few decades, however, the carnivorous rosy wolfsnail had instead driven more than half of Hawaii’s native species of banded tree snails to extinction.

In the 1860s, a scientist introduced the gypsy moth to Massachusetts in the belief that it might help produce a commercial-quality silk. The moth failed to start a silk industry, and instead spread through the northeastern United States, where its outbreaks defoliate trees over large regions every few years.

The bird was first introduced to New York City in the late 19th century by Shakespeare devotees intent on bringing every bird mentioned in Shakespeare’s plays to America. It only took 75 years for starlings to spread to all corners of North America, becoming one of the continent’s most abundant birds. Starlings are thought to outcompete native birds for nest holes.

In just 30 years after its introduction to Washington state in the 1890s, cheatgrass has spread across much of the western United States. It crowds out other plants, uses up the soil’s nitrogen, and burns readily. Fire kills many of the native plants, but not cheatgrass, which grows back even stronger amid the lack of competition.

Nearly all native forest bird species on the South Pacific island of Guam have disappeared. The culprit is the brown tree snake, brought to the island inadvertantly as stowaways in cargo bays of military planes in World War II. Guam’s birds had not evolved with tree snakes, and so had no defenses against the snake’s nighttime predation. The snakes have spread to other islands where they are repeating their ecological devastation. The arrival of this snake is the greatest fear of conservation biologists in Hawaii.

FIGURE 8.12  Invasive species are species that thrive in areas where they are introduced, often harming native species. This chart shows a few of the many thousands of invasive species.

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may proliferate and displace native species. Invasive species cause billions of dollars in economic damage each year.

Climate change The preceding four types of human impacts affect biodiversity in discrete places and times. In contrast, our manipulation of Earth’s climate (Chapter 14) is having global impacts. As our emissions of greenhouse gas- es from fossil fuel combustion cause temperatures to warm worldwide, we modify weather patterns and increase the frequency of extreme weather events.

FIGURE 8.13  As Arctic warming melts the sea ice from which they hunt seals, polar bears must swim farther for food. A lawsuit brought by environmental groups forced the U.S. Fish and Wild- life Service in 2008 to list the polar bear as threatened under the Endangered Species Act as a result of climate change.

Extreme weather events such as droughts increase stress on populations, and warming temperatures force organisms to shift their geographic ranges toward the poles and higher in altitude. Some species will not be able to adapt. Mountain- top organisms (such as the cloud-forest fauna at Monteverde in Chapter 3) cannot move further upslope to escape warm- ing temperatures, so they may perish. Trees may not be able to move poleward fast enough. As ranges shift, animals and plants may find themselves among new communities of prey, predators, and parasites to which they are not adapted. In the Arctic, where warming has been greatest, the polar bear (FIGURE 8.13) has been listed as a threatened species under the U.S. Endangered Species Act (p. 176) because thawing ice hinders its ability to hunt seals (p. 314). All in all, scien- tists predict that a 1.5–2.5 ºC (2.7–4.5 ºF) global temperature rise could put 20–30% of the world’s plants and animals at increased risk of extinction.

All five of these causes of biodiversity loss are intensi- fied by human population growth and rising per capita re- source consumption. As researchers gain a solid scientific understanding of the causes of biodiversity loss, we are also coming to appreciate its consequences (FIGURE 8.14). More- over, these causes may interact, resulting in impacts greater than the sum of their parts. The current collapse of amphib- ian populations throughout the world provides an example. Today entire populations of frogs, toads, and salamanders are vanishing without a trace. Nearly 2,500 of the 6,300 known species are in decline, and roughly 170 species stud-

As you progress through this chapter, try to identify as many solutions to biodiversity loss as you can. What could you personally do to help address this issue? Consider how each action or solution might affect items in the concept map above.

Economic loss

Loss of ecosystem services

Degradation of ecosystem function

Health impacts

Social disruption Loss of aesthetic and spiritual ties

with nature

Loss of food sources

Loss of tourism and recreation

Loss of sources of medicines

Overharvesting

Pollution

Habitat alteration

Invasive species

Globalization

Human population growth

Global climate change

More greenhouse gas

emissions

Growth in per capita consumption

Solutions

Biodiversity loss

Causes Consequences

FIGURE 8.14  The loss of biodiversity stems from a variety of causes (ovals on left) and results in a number of consequences (boxes on right) for ecological systems and human well-being. Arrows in this concept map lead from causes to consequences. Note that items grouped within outlined boxes do not necessari- ly share any special relationship; the outlined boxes are intended merely to streamline the figure.

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ied just years or decades ago are thought to be gone. As these creatures disappear before our eyes, scientists are racing to discover the reasons, and they have found evidence impli- cating habitat destruction, chemical pollution, disease, inva- sive species, and climate change. In some cases, researchers surmise that factors in combination are multiplying one an- other’s effects.

BENEFITS OF BIODIVERSITY Biodiversity loss matters from an ethical perspective, be- cause many people feel that organisms have an intrinsic right to exist. However, losing biodiversity is also a problem for human society because of the many tangible, pragmat- ic ways that biodiversity benefits people and supports our society.

Biodiversity provides ecosystem services Contrary to popular opinion, some things in life can indeed be free—as long as we protect the ecological systems that pro- vide them. Intact forests provide clean air and water, and they buffer hydrologic systems against flooding and drought. Na- tive crop varieties provide insurance against disease. Wildlife can attract tourism and boost economies. Intact ecosystems provide these and other valuable processes, known as ecosys- tem services (pp. 2, 36, 90, 95), for all of us, free of charge. According to UNEP, biodiversity:

▶ Provides food, fuel, fiber, and shelter. ▶ Purifies air and water. ▶ Detoxifies and decomposes wastes. ▶ Stabilizes Earth’s climate. ▶ Moderates floods, droughts, and temperatures. ▶ Cycles nutrients and renews soil fertility. ▶ Pollinates plants, including many crops. ▶ Controls pests and diseases. ▶ Maintains genetic resources for crop varieties, livestock

breeds, and medicines. ▶ Provides cultural and aesthetic benefits. ▶ Gives us the means to adapt to change.

In these ways, organisms and ecosystems support vital processes that people cannot replicate or would need to pay for if nature did not provide them. The annual economic val- ue of just 17 of these ecosystem services has been estimated at more than $46 trillion per year (p. 95).

Biodiversity helps maintain ecosystem function Ecological research shows that biodiversity tends to en- hance the stability of communities and ecosystems. Re- search also finds that biodiversity tends to increase the resilience (p. 74) of ecological systems—their ability to

weather disturbance, bounce back from stress, or adapt to change. Thus, when we lose biodiversity this can diminish a natural system’s ability to function and to provide serv- ices to our society.

Will the loss of a few species really make much differ- ence in an ecosystem’s ability to function? Consider a meta- phor first offered by Paul and Anne Ehrlich (pp. 115, 117): The loss of one rivet from an airplane’s wing—or two, or three—may not cause the plane to crash. But as more rivets are removed the structure will be compromised, and eventu- ally the loss of just one more rivet will cause it to fail.

Research suggests that removing a top predator such as a tiger can indeed have a strong impact, because top preda- tors are often keystone species (pp. 71–72). A single tiger may prey on many herbivores, each of which may consume many plants—so the removal of a species like the tiger can have con- sequences that multiply as they cascade down the food chain.

“Ecosystem engineers” (pp. 72–73) such as ants and earthworms can be every bit as influential as keystone species, so the loss of an ecosystem engineer from a system can like- wise set major changes in motion. Ecosystems are complex, and it can be difficult to predict which species are most im- portant. Thus, many people prefer to apply the precautionary principle (pp. 152, 222) in the spirit of Aldo Leopold (p. 14), who advised, “To keep every cog and wheel is the first precau- tion of intelligent tinkering.”

Biodiversity enhances food security Biodiversity provides the food we eat. Throughout history, people have used 7,000 plant species and several thousand animal species for food. Today nutritional experts worry that industrial agriculture has narrowed our diet. Globally, we now get 90% of our food from just 15 crop species and 8 livestock species, and this lack of diversity leaves us vulnerable to fail- ures of particular crops. In a world where 1 billion people go hungry and more are malnourished, we can improve food security (the guarantee of an adequate, safe, nutritious, and reliable food supply; p. 134) by finding sustainable ways to harvest or farm novel or underutilized wild species and rare crop varieties (FIGURE 8.15). For example, the babassu palm of the Amazon produces more vegetable oil than any other plant. The serendipity berry generates a sweetener 3,000 times sweeter than table sugar. Several salt-tolerant grasses and trees are so hardy that farmers can irrigate them with salt water to produce animal feed, a vegetable oil substitute, and other products.

Moreover, crop relatives and wild ancestors of crops hold reservoirs of genetic diversity (p. 136) that can help save our monocultural crops from catastrophe when we transfer help- ful genes by crossbreeding or genetic engineering. We have already received tens of billions of dollars’ worth of disease resistance from the wild relatives of potatoes, wheat, barley, and other crops.

Organisms provide drugs and medicines People have made medicines from plants for centuries, and many of today’s pharmaceuticals are derived from

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chemical compounds from wild plants (FIGURE 8.16). The rosy periwinkle produces compounds that treat Hodgkin’s disease and a deadly form of leukemia. Had this plant from Madagascar become extinct, these two fatal diseases would have claimed far more victims. In Australia, a rare species of cork, Duboisia leichhardtii, provides hyoscine, a compound that physicians use to treat cancer, stomach disorders, and motion sickness. The Pacific yew of North America’s Pacific Northwest produces a compound that forms the basis for the anti-cancer drug taxol. Each year, pharmaceutical products owing their origin to wild species generate up to $150 billion in sales and save thousands of human lives.

The world’s biodiversity holds an even greater treasure chest of medicines still to be discovered. Yet with every spe- cies that goes extinct, we lose one more opportunity to find cures for cancer, AIDS, or other maladies (FIGURE 8.17).

Species Native to… Potential usesand benefits

Food Security and Biodiversity: Potential new food sources

Amaranths (three species of Amaranthus)

Tropical and Andean America

Grain and leafy vegetable; livestock feed; rapid growth, drought resistant

“Tree of life” to Amerindians; vitamin-rich fruit; pith as source for bread; palm heart from shoots

Amazon lowlands

Buriti palm (Mauritia flexuosa)

Cold-resistant root vegetable resembling radish, with distinctive flavor; near extinction

Andes Mountains

Maca (Lepidium meyenii)

A deep-forest pig; thrives on vegetation high in cellulose and hence less dependent on grain

Indonesia: Moluccas and Sulawesi

Babirusa (Babyrousa babyrussa)

World’s largest rodent; meat esteemed; easily ranched in open habitats near water

South America

Capybara (Hydrochoeris hydrochoeris)

Threatened species related to llama; source of meat, fur, and hides; can be profitably ranched

Central Andes

Vicuna (Lama vicugna)

South and Central America

Tropical birds; adaptable to human habitations; fast-growing

Chachalacas (Ortalis, many species)

FIGURE 8.15  By protecting biodiversity, we enhance food security. The wild species shown here are a fraction of the many plants and animals that could supplement our food supply. Adapted from Wilson, E.O., 1992. The diversity of life. Cambridge, MA:

Belknap Press.

Plant Drug Medical application

Pineapple (Ananas comosus)

Medicines and Biodiversity: Natural sources of pharmaceuticals

Autumn crocus (Colchicum autumnale)

Yellow cinchona (Cinchona ledgeriana)

Common thyme (Thymus vulgaris)

Pacific yew (Taxus brevifolia)

Colchicine

Quinine

Thymol

Taxol

Anticancer agent

Antimalarial

Cures fungal infection

Anticancer (especially ovarian cancer)

Velvet bean (Mucuna deeringiana)

L-Dopa Parkinson’s disease suppressant

Common foxglove (Digitalis purpurea)

Digitoxin

Bromelain Controls tissue inflammation

Cardiac stimulant

FIGURE 8.16  By protecting biodiversity, we enhance our ability to treat illness. Shown are just a few of the plants found to provide chemical compounds of medical benefit. Adapted from Wilson, E.O., 1992. The diversity of life. Cambridge, MA: Belknap Press.

FIGURE 8.17  We lost opportunities for medical advances when two species of gastric brooding frogs went extinct shortly after their discovery in Australia’s rainforests. Females of these bizarre frogs raised their young inside their stomachs, where the young apparently exuded substances to neutralize their mother’s stomach acids. Any such substance could be of immense value for treating human stomach ulcers, which affect 25 million U.S. citizens. When both frog species went extinct in the 1980s, they took their secrets with them forever.

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direct contact with wild organisms, they suffer what he calls “nature-deficit disorder.” Although it is not a medical condi- tion, Louv argues that this alienation from biodiversity and nature damages childhood development and may lie behind many of the emotional and physical problems young people in developed nations face today.

Do we have ethical obligations toward other species? Aside from all of biodiversity’s pragmatic benefits, many peo- ple feel that living organisms have an inherent right to exist. In this view, biodiversity conservation is justified on ethical grounds alone. As Maurice Hornocker wrote when he and his associates first established the Siberian Tiger Project, “Sav- ing the most magnificent of all the cat species and one of the most endangered should be a global responsibility. . . . If they aren’t worthy of saving, then what are we all about? What is worth saving?”

Today many people are engaged in efforts to save vanish- ing species. The search for solutions to today’s biodiversity crisis is dynamic and exciting, and scientists are developing innovative approaches to maintaining the diversity of life on Earth.

CONSERVATION BIOLOGY: THE SEARCH FOR SOLUTIONS Today, more and more scientists and citizens perceive a need to stop the loss of biodiversity. In his 1994 autobiography, Naturalist, E.O. Wilson wrote:

When the [20th] century began, people still thought of the planet as infinite in its bounty. [Yet] in one lifetime, exploding human populations have reduced wildernesses to threatened nature reserves. Ecosystems and species are vanishing at the fastest rate in 65 million years. Troubled by what we have wrought, we have begun to turn in our role from local conqueror to global steward.

Conservation biology arose in response to biodiversity loss The urge to act as responsible stewards of natural systems, and to use science as a tool in that endeavor, helped spark the rise of conservation biology. Conservation biology is a scientific discipline devoted to understanding the factors, forces, and processes that influence the loss, protection, and restoration of biological diversity.

Conservation biologists aim to develop solutions to such problems as habitat degradation and species loss (see ENVISIONIT, p. 175). Conservation biology is thus an applied and goal-oriented science, with implicit values and ethical standards. Conservation biologists integrate an understanding of evolution and extinction with ecology and the dynamic nature of environmental systems. They use field data, lab data, theory, and experiments to study our impacts on other organisms. They also design, test, and implement ways to alleviate human impact.

Biodiversity boosts economies through tourism and recreation Many people like to travel to observe wildlife and explore natural areas, and in so doing they create economic oppor- tunities for residents living near protected natural areas. Visitors spend money at local businesses, hire local people as guides, and support parks that employ local residents. Such ecotourism (p. 60) can bring jobs and income to areas that otherwise might be poverty-stricken.

Ecotourism has become a vital source of income for Cos- ta Rica, with its rainforests; Australia, with its Great Barrier Reef; Belize, with its reefs, caves, and rainforests; and Kenya and Tanzania, with their savanna wildlife. The United States, too, benefits from ecotourism; its national parks draw mil- lions of visitors from around the world. Although too much development for ecotourism can damage the natural assets that draw people, ecotourism can serve as a powerful financial incentive for nations, states, and local communities to pre- serve natural areas and reduce impacts on the landscape and on native species.

People value and seek out connections with nature Not all of biodiversity’s benefits to people can be expressed in the hard numbers of economics or the practicalities of food and medicine. Some scientists and philosophers argue that people find a deeper value in biodiversity. Harvard Univer- sity biologist and author Edward O. Wilson has popularized the notion of biophilia, asserting that human beings have an instinctive love for nature and feel an emotional bond with other living things (FIGURE 8.18). Wilson and others cite as evidence of biophilia our affinity for parks and wildlife, our love for pets, the high value of real estate with a view of natu- ral landscapes, and our interest in hiking, bird-watching, fishing, hunting, backpacking, and similar outdoor pursuits.

In a 2005 book, writer Richard Louv adds that as today’s children are increasingly deprived of outdoor experiences and

FIGURE 8.18  An Indonesian girl peers into a flower of Rafflesia arnoldii, the largest flower in the world. The concept of biophilia holds that human beings have an instinctive love and fascination for nature and a deep-seated desire to affiliate with other living things.

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Sampling plants in Florida

Tracking rhinos in Africa

Radio tracking birds in Spain

Blood sample from Seychelles Magpie Robin

Sampling insects in Madagascar

You Can Make a Difference

Buy shade-grown coffee, organic produce, and other wildlife-friendly products.

Conservation biologists are racing to save species from decline and extinction.

They work in the �eld, in the lab, at zoos, and with local people in areas that need protecting …

… striving to recover populations of plants and animals threatened by habitat loss and other causes.

Preserve or restore wildlife habitat in your yard, in your community, or on your campus.

Volunteer time or money to organizations working to save species and habitat.

Sea turtle with transmitter, Hong Kong

En v

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At the genetic level, conservation geneticists ask how small a population can become and how much genetic variation it can lose before running into problems such as inbreeding depression (p. 164). By determining a popula- tion’s minimum viable population size, conservation geneti- cists can help wildlife managers decide how vital it may be to increase the population. Studies of genes, populations, and species inform conservation efforts with habitats, com- munities, ecosystems, and landscapes. Because small and isolated subpopulations are most vulnerable to extirpation, conservation biologists pay special attention to them. By ex- amining how organisms disperse from one habitat patch to another, and how their genes flow among subpopulations, conservation biologists try to learn how likely a population is to persist or succumb in the face of habitat change or oth- er threats.

Endangered species are a focus of conservation efforts The primary legislation for protecting biodiversity in the United States is the Endangered Species Act. Passed in 1973, the Endangered Species Act (ESA) forbids the gov- ernment and private citizens from taking actions that de- stroy endangered species or their habitats. The ESA also forbids trade in products made from endangered spe- cies. The aim is to prevent extinctions, stabilize declin- ing populations, and enable populations to recover. As of 2011, there were 1,061 species in the United States listed as “endangered” and 313 more listed as “threatened,” the status considered one notch less severe than endangered. For about half of these species, government agencies are running recovery plans to protect them and stabilize or increase their populations.

The ESA has had a number of notable successes. Follow- ing the 1973 ban on the pesticide DDT and years of effort by wildlife managers, the bald eagle, peregrine falcon, brown pelican, and other birds have recovered and are no longer listed as endangered (FIGURE 8.19). Intensive management programs with other species, such as the red-cockaded wood- pecker, have held populations steady in the face of continued pressure on habitat. In fact, roughly 40% of declining popula- tions have been stabilized.

This success comes despite the fact that the U.S. Fish and Wildlife Service and the National Marine Fisheries Service, the agencies responsible for upholding the ESA, are perennially underfunded for the job. Reauthorization of the ESA faced stiff opposition from the Republican Congresses in power from 1994 to 2006. Efforts to weaken the ESA by stripping it of its ability to safeguard habitat were narrowly averted in 2006 after 5,700 scientists sent Congress a letter of protest.

Polls repeatedly show that most Americans support protecting endangered species. Yet some opponents feel that the ESA places more value on the life of an endangered organism than it does on the livelihood of a person. This was a common perception in the Pacific Northwest in the 1990s, when protection for the northern spotted owl slowed logging in old-growth rainforest and loggers began to fear for their jobs. In addition, many landowners worry that fed-

eral officials will restrict the use of private land on which threatened or endangered species are found. This has led to a practice described as “shoot, shovel, and shut up” among landowners who want to conceal the presence of such spe- cies on their land.

In fact, however, the ESA has stopped few development projects—and a number of provisions of the ESA and its amendments promote cooperation with landowners. Habitat conservation plans and safe harbor agreements are arrange- ments that allow landowners to harm species in some ways if they improve habitat for them in others.

Today a number of nations have laws protecting species, although they are not always effective. When Canada enacted its Species at Risk Act in 2002, the Canadian government was careful to stress cooperation with landowners and provincial governments, rather than presenting the law as a mandate from the national government. Environmental advocates and many scientists protested that the law was too weak and failed to protect habitat adequately.

In Russia, the government issued Decree 795 in 1995, cre- ating a Siberian tiger conservation program and declaring the tiger a natural and national treasure. In 2007 it established a national park in the Sikhote-Alin Mountains to help protect the tiger, and the next year Prime Minister Vladimir Putin personally visited scientists in the field as they tagged a tiger. However, state funding for tiger conservation remained so meager that the Wildlife Conservation Society felt it necessary to help pay for Russians to enforce their own anti-poaching laws. Whether government support increases in the wake of Russia’s hosting of the 2010 International Tiger Forum in St. Petersburg remains to be seen.

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FIGURE 8.19  The recovery of the bald eagle is a success story of the U.S. Endangered Species Act. The national symbol of the United States was close to extinction in the Lower 48 states in the 1960s. Following its protection under the ESA and a ban on the pesticide DDT in 1973, the eagle population began a long re- bound. With its Lower-48 population around 10,000 pairs in 2007, the bald eagle was declared recovered and was removed from the Endangered Species List. Data from U.S. Fish and Wildlife Service, based on annual volunteer eagle surveys. The paucity of data after 2000 is because

surveys began to be discontinued once it became clear that the eagle was

recovering.

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condor, North America’s largest bird (FIGURE 8.20). The suc- cessful program to reintroduce gray wolves to Yellowstone National Park has proven popular with the American public but has met stiff resistance from ranchers, who fear the wolves will attack their livestock. In Arizona and New Mexico, a wolf reintroduction program is making headway, but a number of wolves have been shot.

China is considering a reintroduction program for the Siberian tiger. The Chinese government says it is preparing 600 captive Siberian tigers for release into the far northeast- ern portion of the country. Critics note that forests there are so fragmented that efforts should focus on improving habitat first.

One new idea for saving species from extinction is to cre- ate individuals by cloning them. In this technique, DNA (p. 28) from an endangered species is inserted into a cultured egg without a nucleus, and the egg is implanted into a female of a closely related species that acts as a surrogate mother. Several mammals have been cloned in this way, with mixed results. Some scientists even talk of recreating extinct species from DNA recovered from preserved body parts. Indeed, in 2009 a subspecies of Pyrenean ibex (a type of mountain goat) was cloned from cells taken from the last surviving individual, which had died in 2000. The cloned baby ibex died shortly after birth. Even if cloning can succeed from a technical stand- point, however, such efforts are not an adequate response to biodiversity loss. Without ample habitat and protection in the wild, having cloned animals in a zoo does little good.

Forensics is being used to protect species Forensic science, or forensics, involves the scientific analysis of evidence to make an identification or answer a question relating to a crime or an accident. Conservation biologists are now em- ploying forensics to protect species at risk from illegal harvest- ing. By analyzing DNA from organisms or their tissues sold at

Conservation efforts include international treaties The United Nations has facilitated several international trea- ties to protect biodiversity. The 1973 Convention on Inter- national Trade in Endangered Species of Wild Fauna and Flora (CITES) protects endangered species by banning the international transport of their body parts. When nations enforce it, CITES can protect tigers and other rare species whose body parts are traded internationally.

In 1992, leaders of many nations agreed to the Convention on Biological Diversity. This treaty embodies three goals: to conserve biodiversity, to use biodiversity in a sustainable man- ner, and to ensure the fair distribution of biodiversity’s ben- efits. Among its accomplishments, the treaty has prompted nations to augment protected reserves, has enhanced global markets for shade-grown coffee and other crops grown with- out removing forests, has ensured that African nations share in the economic benefits of ecotourism from wildlife preserves, and has replaced pesticide-intensive farming practices with sustainable ones in some rice-producing Asian nations. Yet the treaty’s overall goal—“to achieve, by 2010, a significant reduction of the current rate of biodiversity loss at the global, regional, and national level”—was not met. Fully 193 nations have become parties to the Convention on Biological Diver- sity. The only ones choosing not to do so are tiny Andorra, the Vatican, and the United States.

Captive breeding, reintroduction, and cloning are being pursued In the effort to save threatened and endangered species, zoos and botanical gardens have become centers for captive breeding, in which individuals are bred and raised in con- trolled conditions with the intent of reintroducing them into the wild. One example is the program to save the California

FIGURE 8.20  To save the California condor from extinction, biologists are raising chicks in captivity. Feeding them with hand puppets designed to look and feel like the heads of adult condors, the biologists shield each chick from all contact with humans, so that when the bird is grown it does not feel an attachment to people. Condors had declined because people killed them, they collided with electrical wires, and they suc- cumbed to lead poisoning after scavenging carcasses of animals killed with lead shot. By 1982, only 22 condors remained, and biolo- gists decided to take all the birds into captiv- ity, in hopes of boosting their numbers and then releasing them. The ongoing program is succeeding. As of 2011, there were 189 birds in captivity and 181 birds living in the wild, having been released at sites in California, Arizona, and Baja California. Several pairs have begun nesting, and so far 24 chicks have been raised in the wild.

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THE SCIENCE BEHIND THE STORY

Using Forensics to Uncover Illegal Whaling

The meat from whales has long been a delicacy in Japan and some other nations. Whaling ships deci- mated populations of most species of whales in the 20th century through overhunting, and the International Whaling Commission (IWC) outlawed commercial whaling worldwide beginning in 1986. Yet whale meat continues to be sold at market to wealthy consumers today (see photo). This meat comes legally from several sources:

1. From scientific hunts. Japan and several other nations negotiated with the IWC to continue to hunt limited numbers of whales for research purposes, and this meat may be sold afterwards.

2. From whales killed accidentally when caught in fishing nets meant for other animals (bycatch, p. 274).

3. Possibly from stockpiles frozen before the IWC’s moratorium.

However, conservation biologists long suspected that much of the whale meat on the market was actually caught illegally for the purpose of selling for food and that fleets from Japan and other nations were killing more whales than international law allowed. Once DNA sequencing technology was

developed, scientists could use this tool to find out.

The detectives in this story are conservation geneticists C. Scott Baker, Stephen Palumbi, Frank Cipriano, and their colleagues. For close to two decades they have been traveling to Asia on what have

amounted to top-secret grocery shopping trips.

It began in 1993 when Baker and Palumbi bought samples of whale meat—all labeled simply as kujira, the generic Japanese term for whale meat—from a number of markets in Japan and sequenced DNA from these samples. Law forbids the export of whale meat, so the researchers had to run their analyses in their hotel rooms with portable genetic kits. Once they were back home in the United States, they compared their data with se- quences from known whale species.

By analyzing which samples matched which, they concluded that they had sampled meat from nine minke whales, four fin whales, one humpback whale, and two dolphins. Moreover, because subspecies of whales from different oceans differ genetically, the researchers were able to analyze the genetic variation in their samples and learn that one fin whale came from the Atlantic whereas the other three were from the Pacific, and that eight of the nine minke whales came from the Southern Hemisphere.

Because several of these species and/or subspecies were off-limits to hunting, the data suggested that some of the meat had been hunted, proc- essed, or traded illegally. Baker and Palumbi concluded in a 1994 paper in the journal Science that “the existence of legal whaling serves as a cover for the sale of illegal whale products.” They urged that the international

Dr. C. Scott Baker of Oregon State University running genetic analyses in a Japanese hotel room

Whale meat is sold at this market in South Korea.

market, researchers can often determine the species or subspe- cies and, sometimes, its geographic origin. This can help detect illegal activity, enhancing the enforcement of laws protecting wildlife (see THE SCIENCE BEHIND THE STORY, above).

One example involves African elephants killed for ivory from their tusks. Trade in ivory has been banned globally in an

effort to stop the slaughter of elephants. After airport customs agents seized 6.5 tons of tusks in Singapore in 2002, research- ers led by Samuel Wasser of the University of Washington ana- lyzed DNA from the tusks to determine the geographic origin of the elephants that were killed. The researchers sought to find out whether the tusks belonged to savanna elephants killed in

As any television buff knows, forensic science is a crucial tool in solving mysteries and fighting crime. In recent years conservation biologists have been using forensics to unearth secrets and catch bad guys in the multibillion-dollar illegal global wildlife trade. One such detective story comes from the Pacific Ocean and Japan.

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community monitor catches more closely.

Two years later, Baker, Pa- lumbi, and Cipriano presented results from markets in South Korea and Japan. Again their genetic sleuthing revealed a diversity of whale species, and they stated that their data were “difficult to reconcile” with records of legal catches (scientific whaling by Japan and fishing bycatch by South Korea) reported by these na- tions to the IWC. Among the whales they detected were two specimens of what seemed to be a subspecies or species of whale new to science.

In 2000, the team ana- lyzed 655 samples labeled as whale meat from Japanese and South Korean markets

and found evidence of 12 species or subspecies of whales, along with orcas, porpoises, and dolphins—and even sheep and horses! Seven of the whale species were internationally protected, and together these constituted 10% of the whale meat for sale in Japanese markets.

Genetic analyses of minke whale samples from Japan’s markets also indi- cated that a surprisingly large percent- age came from animals from the Sea of Japan, where Korea and Japan harvest- ed them as fishing bycatch. One-third of the meat on the market was coming from the Sea of Japan, meaning that four times as many whales were being killed there as Japan was reporting (see top figure). The research team calculat- ed that Japan and Korea together were taking so many minke whales from the Sea of Japan that they would eventually wipe out the population (see bottom figure).

In 2007, Baker led a team that combined genetic forensics with eco-

logical methods to estimate numbers of individual whales whose meat was passing through Korean markets. They inferred that meat from 827 minke whales had passed through South Korea’s market in five years. The nation had reported catching only 458 minke whales as fishing bycatch, leading the researchers to conclude that the remainder had been taken illegally.

The governments of Japan and South Korea have tried to refute these findings. Yet the technology and ap- proaches that turn scientists into forensic detectives are now influencing the debate and the negotiation over whaling policy at the interna- tional level.

Genetic types from Sea of Japan

Genetic types from Pacific Ocean

Genetic types from markets in Japan

SOUTH KOREA JAPAN

The distribution of genetic types from minke whale meat in Japanese markets (center pie chart) shows evidence both of whales from the Sea of Japan (left pie chart) and of whales from the open Pacific Ocean (right pie chart). Note how proportions of the types from the market are intermedi- ate between those of each geographic area of ocean. Adapted with permis- sion from Lukoschek, V., et al., 2009. High proportion of protected minke whales sold on Japanese markets due to illegal, unreported, or unregulated exploitation. Animal Conservation 12:385–395.

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Minke whales in the Sea of Japan declined sharply (orange color in graph) until the 1986 moratorium on their capture. Population models forecast that bycatch of 150, 100, or even 50 minke whales per year would be enough to prevent the population’s recovery. Data suggest that actual bycatch from the Sea of Japan has been close to 150 per year. Adapted from Baker, C.S., et al., 2000. Predicted decline of pro- tected whales based on molecular genetic monitoring of Japanese and Korean markets. Proc. Roy. Soc. Lond. B 267:1191–1199.

Zambia (the origin of the shipment), or forest elephants from other locations. The DNA matched known samples from Zam- bian elephants, revealing that many more elephants were be- ing killed there than Zambia’s government had realized. In re- sponse, the government replaced its wildlife director and began imposing harsher sentences on poachers and ivory smugglers.

Some species act as “umbrellas,” protecting habitat and communities Scientists know that protecting species does little good if the larger ecological systems they rely on are not also sus- tained. Yet no law or treaty exists to protect communities

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The nonprofit group Conservation International maps 34 biodiversity hotspots (FIGURE 8.22). The ecosystems of these areas together once covered 15.7% of the planet’s land sur- face but today, because of habitat loss, cover only 2.3%. This small amount of land is the exclusive home for half the world’s plant species and 42% of all terrestrial vertebrate species. The hotspot concept gives incentive to focus on these areas, where the greatest number of unique species can be protected per unit effort.

We can restore degraded ecosystems Protecting natural areas before they become degraded is the best way to safeguard biodiversity and ecological systems. However, we can often restore degraded natural systems to some semblance of their former condition through ecologi- cal restoration (p. 77). Ecological restoration projects aim not just to bring back populations of animals and plants, but also to reestablish the processes that make ecosystems func- tion. By restoring complex natural systems such as Illinois prairies or the Florida Everglades (pp. 77–78), restoration

or ecosystems. Therefore, conservation biologists often use particular species as tools to conserve habitats, com- munities, and ecosystems. Such species are called umbrella species because they serve as a kind of umbrella to protect many other species. Umbrella species often are large ani- mals that roam great distances, such as the Siberian tiger. Because such animals require large areas, meeting their habitat needs helps meet those of thousands of less charis- matic species that might never elicit as much public interest.

Environmental advocacy organizations have found that using large, charismatic vertebrates as spearheads for biodiversity conservation is an effective strategy. This ap- proach of promoting particular flagship species is evident in the longtime symbol of the World Wide Fund for Nature (World Wildlife Fund in North America), the panda. The panda is a large endangered mammal requiring sizeable stands of undisturbed bamboo forest. Its lovable appear- ance has made it a favorite with the public—and an effective vehicle for soliciting support for conservation efforts that protect far more than just the panda.

Protected areas conserve biodiversity at the ecosystem level Although most legislation, funding, and resources for bio- diversity conservation go toward single-species approaches, our practice of preserving areas of undeveloped land in parks and protected areas helps to conserve habitats, communities, ecosystems, and landscapes (pp. 199–203). So far we have set aside 12% of the world’s land area in national parks, state parks, provincial parks, wilderness areas, biosphere reserves, and other protected areas. Many such lands are managed for recreation, water quality protection, or other purposes, not for biodiversity, and many suffer from illegal logging, poach- ing, or resource extraction. Yet these areas offer animals and plants a degree of protection from human persecution, and some are large enough to preserve whole natural systems that otherwise would be fragmented, degraded, or destroyed.

Protected areas alone may not be enough. India has estab- lished reserves to protect its remaining tigers, yet tigers have disappeared from at least two of these reserves. Today a major challenge is to provide linkages among protected areas across the landscape, so that isolated populations of wide-ranging species like tigers can intermix—and so that organisms can move in response to climate change as it alters habitats within protected areas.

Biodiversity hotspots pinpoint regions of high diversity One international approach oriented around geographic regions, rather than single species, is that of biodiversity hotspots. A hotspot is an area that supports an especially great number of species that are endemic (p. 51) to the re- gion: that is, found nowhere else in the world (FIGURE 8.21). To qualify as a hotspot, a location must harbor at least 1,500 endemic plant species (0.5% of the world’s total). In addition, a hotspot must have already lost 70% of its habi- tat as a result of human impact, and be in danger of losing more.

FIGURE 8.21  The ring-tailed lemur is a primate that is endemic to the island of Madagascar. Over 2,000 individuals survive in zoos worldwide thanks to captive breeding, but the lemur’s natural habi- tat is fast disappearing. Madagascar has lost over 90% of its forests because of human population growth, poverty, and resource extraction. One recent president encouraged conservation and ecotourism, but when his government fell, illegal logging resumed, destroying large areas of protected forest. Many lemurs were killed and sold for their meat, and timber was exported to meet demand from wealthy nations.

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Community-based conservation is growing Helping people, wildlife, and ecosystems at the same time, as the Iraqi marsh restoration project intends, is the focus of many current efforts in conservation biology. In past decades, conser- vationists from developed nations, in their zeal to preserve eco- systems in other nations, often neglected the needs of people in the areas they wanted to protect. Developing nations came to view this as a kind of neocolonialism. Today this has largely changed, and many conservation biologists actively engage lo- cal people in efforts to protect land and wildlife (FIGURE 8.23).

This cooperative approach, called community-based conservation, is being pursued to help protect tigers in many places. In India, multiple projects offer education, health care, and development assistance to communities living amid the

ecologists aim to recreate systems that filter pollutants, cleanse water and air, build soil, and recharge groundwater, providing habitat for wildlife and services for people.

Today’s highest-profile restoration project is an effort to restore the vast marshes of southern Iraq. In the 1990s, Iraqi ruler Saddam Hussein drained the marshes, aiming to devastate the region’s people, whom he viewed as dis- loyal. Today ecologists from many nations have joined the people of the marshes in a multimillion-dollar restoration effort. The project was able to restore natural water flow to 75% of the region, so that vegetation grew back and wildlife and people began returning. The effort was on the verge of success when drought descended and Turkey and Syria began diverting water upriver for their own purposes. As of 2011, ecologists and human rights supporters alike were searching for ways to complete this ambitious restoration project.

FIGURE 8.22  Some areas of the world possess exceptionally high numbers of species found nowhere else. Many conservation biologists support prioritizing habitat preserva- tion in these areas, dubbed biodi- versity hotspots. Shown in red are the 34 biodiversity hotspots mapped by Conservation International. Only about 15% of the area in red is actu- ally habitat for these species; most is developed. Data from Conservation International.

Single-Species Conservation? What would you say are some advantages of focus- ing on conserving single species, versus trying to conserve habitats, communities, ecosystems, or landscapes? What are some disadvantages?

Which do you think is the better approach, or should we pursue both?

FIGURE 8.23  In community-based conservation, conserva- tion biologists partner with local people, empowering them to conserve wildlife and habitat in their own region. Here, Costa Rican schoolgirls plant trees in a park in their nation’s capital, San Jose.

shrinking habitat of the Bengal tiger. In Cambodia, people who used to hunt Indochinese tigers are being retrained and paid salaries as forest guards to protect the animals from poachers, or as wildlife technicians to help with science and monitoring. In Russia, the Wildlife Conservation Society is working with local hunters to reduce poaching of Siberian tigers and to in- crease populations of deer and other animals that are prey for both tigers and hunters. The WCS is also establishing a mar- ket in the West for sustainably harvested products from the region that are certified “tiger-friendly.” Proceeds from sales of the products aim to supplement the incomes of up to 1,000 local people by 12–25%.

Making conservation beneficial for local people requires hard work, investment, and trust on all sides. While setting aside land may deprive local people of short-term access to exploitable resources, it also helps ensure that these resources will not be used up or sold to foreign corporations, but can instead be sustainably managed. Moreover, parks and reserves draw ecotourism that supports local economies. Communi- ty-based conservation has not always been successful, but in a world of rising human population, we will require locally based management for biodiversity that sustainably meets people’s needs.

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biodiversity loss. This loss matters, because human society cannot function without biodiversity’s many pragmatic ben- efits. Conservation biologists are rising to the challenge of conducting science aimed at saving endangered species, pro- tecting their habitats, recovering populations, and preserving and restoring natural ecosystems. The innovative strategies these scientists are pursuing hold promise to slow the loss of biodiversity on Earth.

➤ CONCLUSION Data from scientists worldwide confirm what any naturalist who has watched the habitat change in his or her hometown already knows: From amphibians to tigers, biological diver- sity is being lost rapidly and visibly within our lifetimes. This erosion of biodiversity threatens to result in a mass extinc- tion event equivalent to those of the geologic past. Habitat alteration, invasive species, pollution, overharvesting of bi- otic resources, and climate change are the primary causes of

7. Name two successful accomplishments of the U.S. En- dangered Species Act. Now name two reasons some peo- ple have criticized it.

8. Describe how captive breeding can help with endangered species recovery, and give an example. Now explain why cloning will never be, in itself, an effective response to species loss.

9. What is the difference between an umbrella species and a keystone species? Could one species be both an umbrella species and a keystone species?

10. What is a biodiversity hotspot? Describe community- based conservation.

T E S T I N G Y O U R C O M P R E H E N S I O N

1. What is biodiversity? Describe three levels of biodiversity. 2. What are the five primary causes of biodiversity loss?

Give one specific example of each. 3. List three invasive species, and describe their impacts. 4. Define the term ecosystem services. Give three examples

of ecosystem services that people would have a hard time replacing if their natural sources were eliminated.

5. What is the relationship between biodiversity and food security? Between biodiversity and pharmaceuticals? Give three examples of potential benefits of biodiversity conservation for food security and medicine.

6. Describe three reasons why people suggest biodiversity conservation is important.

you want to introduce legislation to protect your nation’s vanishing biodiversity. Consider the U.S. Endangered Species Act and Canada’s Species At Risk Act, as well as international efforts such as CITES and the Convention on Biological Diversity. What strategies would you write into your legislation? How would your law be similar to and different from each of these efforts?

5. THINK IT THROUGH As a citizen and resident of your community, and a parent of two young children, you attend a town meeting called to discuss the pro- posed development of a shopping mall and condo- minium complex. The development would eliminate a 100-acre stand of forest, the last sizeable forest stand in your town. The developers say the forest loss will not matter because plenty of 1-acre stands still exist scattered throughout the area. Consider the develop- ment’s possible impacts on the community’s biodiver- sity, children, and quality of life. What will you choose to tell your fellow citizens and the town’s decision- makers at this meeting, and why?

S E E K I N G S O L U T I O N S

1. Many arguments have been advanced for the importance of preserving biodiversity. Which argument do you think is most compelling, and why? Which argument do you think is least compelling, and why?

2. Some people declare that we shouldn’t worry about endangered species because extinction has always oc- curred. How would you respond to this view?

3. Advocates of biodiversity preservation from developed na- tions have long pushed to set aside land in biodiversity-rich regions of developing nations. Leaders of developing na- tions have responded by accusing these advocates of neo- colonialism. “Your nations attained prosperity and power by overexploiting their environments decades or centuries ago,” these leaders ask, “so why should we now sacrifice our development by setting aside our land and resources?” What would you say to these leaders? What would you say to the environmental advocates? Do you see ways that both preservation and development goals might be reached?

4. THINK IT THROUGH You are an influential legisla- tor in a nation that has no endangered species act, and

C A L C U L A T I N G E C O L O G I C A L F O O T P R I N T S Of the five major causes of biodiversity loss discussed in this chapter, habitat alteration arguably has the greatest impact. In their 1996 book introducing the ecological footprint concept, authors Mathis Wackernagel and

William Rees present a consumption/land use matrix for an average North American. Each cell in the matrix lists the number of hectares of land of that type required to provide for the different categories of a person’s

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consumption (food, housing, transportation, consumer goods, and services). Of the 4.27 hectares required to support this average person, 0.59 hectares are forest,

with most (0.40 hectares) being used to meet the housing demand. Using this information, calculate the missing values in the table.

1. Approximately two-thirds of the forests’ productivity is consumed for housing. To what use(s) would you specu- late that most of the other third is put?

2. If the harvesting of forest products exceeds the sustain- able harvest rate, what will be the likely consequence for the forest? For communities surrounding the forest?

Hectares of forest used for housing

Total forest hectares used

You 0.40 0.59 Your class Your state United States Data from Wackernagel, M., and W. Rees, 1996. Our ecological footprint: Reducing human impact on the earth. British Columbia, Canada: New Society Publishers.

3. What impacts would you expect on biodiversity in each of the following cases, and why? a. The cutting of small plots of forest within a large forest b. The clear-cutting (p. 194) of an entire forest c. The clear-cutting of an entire forest followed by plant-

ing of a monocultural plantation of young trees

Go to www.masteringenvironmentalscience.com for homework assignments, practice quizzes, Pearson eText, and more.

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Environmental Health and Toxicology Upon completing this chapter, you will be able to:

� Identify major environmental health hazards and explain the goals of environmental health ➤ Describe the types of toxic substances in the environment, the factors that affect their toxicity,

and the defenses that organisms possess against them ➤ Explain the movements of toxic substances and how they affect organisms and ecosystems ➤ Discuss the study of chemical hazards, including wildlife toxicology, epidemiology, animal testing,

and dose-response analysis ➤ Compare and contrast risk assessment and risk management

➤ Compare philosophical approaches to risk and how they relate to regulatory policy

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Bisphenol A: Worldwide

CENTRAL CASE STUDY

Poison in the Bottle: Is Bisphenol A Safe? “Babies in the U.S. are born pre-polluted with BPA. What more evidence do we need to act?”

—Dr. Janet Gray, Director of the Environmental Risks and Breast Cancer Project, Vassar College

“There is no basis for human health concerns from exposure to BPA.” —The American Chemistry Council

H ow is it that a chemical found to alter reproductive development in animals gets

used in baby bottles? How can it be that a substance linked to breast cancer, pros-

tate cancer, and heart disease is routinely used in food and drink containers? The

chemical bisphenol A (BPA for short) has been associated with everything from neurological

effects to miscarriages. Yet it’s in hundreds of products we use every day, and there’s a 90%

chance that it is coursing through your body right now.

To understand how chemicals that may pose health risks come to be widespread in our society, we need to explore how scientists and policymakers study toxic substances and other environmental health risks—and the vexing challenges these pursuits entail.

Bisphenol A is a syn- thetic organic compound (C15H16O2) used in the resins that line metal food cans and drink cans and water supply pipes, and in dental sealants for our teeth. It’s also found in the hard, clear polycarbonate plastic in some water bot- tles, food containers, eating utensils, eyeglass lenses, CDs and DVDs, electronics, baby bottles, and chil- dren’s toys.

Unfortunately, bisphenol A leaches out of these products into our food, air, and bodies. The Centers for Disease Control and Prevention (CDC) reports that 93% of Americans carry detectable concentrations in their urine. Because most of the chemical passes through the body within hours of exposure, its wide- spread presence in urine suggests that most Ameri- cans receive continuous exposure to BPA. Babies and children accumulate the most BPA, because they eat more for their body weight and metabolize the chem- ical less effectively.

What, if anything, is BPA do- ing to us? Over 200 studies with rats, mice, and other animals have shown many apparent ef- fects of BPA, including a wide range of reproductive abnor- malities, and a few recent stud- ies suggest human health im- pacts (see THE SCIENCE BEHIND THE STORY, pp. 214–215). Many of these effects are seen at ex- tremely low concentrations. Sci- entists say this is because BPA mimics the female sex hormone estrogen and can induce some

of estrogen’s effects in animals. Hormones such as estrogen function at very low concentrations in the body, so a synthetic chemical in the body at simi- larly low concentrations can fool the body into re- sponding as it would to estrogen.

In reaction to the burgeoning research, a grow- ing number of researchers, doctors, and consumer advocates are calling on governments to regulate bi- sphenol A and for manufacturers to stop using it. The chemical industry insists that BPA is safe, pointing to industry-sponsored research that finds no health impacts. Expert panels convened to assess the fast- growing body of scientific studies on BPA have strug- gled with the fact that traditional research methods are not geared to test hormone-mimicking substances that exert effects at low doses. These panels have of- ten arrived at divergent conclusions. For instance, the

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U.S. Food and Drug Administration (FDA) insisted in 2008 that it saw no reason to regulate BPA, but its own science advisory committee disagreed, and in 2009 the FDA decided to start a testing program.

In 2008, the Canadian government was the first to declare bisphenol A toxic, banning its sale and impor- tation. As of 2011, the use of BPA in certain products for children was banned in China, Malaysia, and in nine U.S. states. Despite the lack of federal regulation of BPA in the United States, grassroots lobbying efforts have led many companies to voluntarily remove BPA from their products, especially those made for children and infants. The six major U.S. manufacturers of plastic baby bottles promised in 2008 to stop using BPA, and the manufacturer Sunoco stopped selling BPA to compa- nies that use it in children’s products. Nalgene phased out its BPA-containing polycarbonate water bottles. The retailers Walmart and Toys “R” Us decided to stop carrying children’s products with BPA. As a result, con- cerned parents can now more easily find BPA-free prod- ucts for their children, but the rest of us remain exposed through thousands of products (FIGURE 10.1).

Bisphenol A is by no means one of our greatest environmental health threats. However, it provides a timely example of how we as a society assess health risks and decide how to manage them. As scientists and government regulators assess BPA’s potential risks, their efforts give us a window on how hormone- disrupting chemicals are challenging the way we appraise and control the environmental health risks we face. �

ENVIRONMENTAL HEALTH Examining the impacts of human-made chemicals such as bi- sphenol A is just one aspect of the broad field of environmen- tal health. The study and practice of environmental health assesses environmental factors that influence our health and quality of life. These factors include wholly natural aspects of

FIGURE 10.1 ▲ Researchers for Consumer Reports magazine tested these (and more) common packaged foods in 2009; they found that nearly all of them contained bisphenol A that had leached from the linings of their containers.

the environment over which we have little or no control, as well as anthropogenic (human-caused) factors. Practitioners of environmental health seek to prevent adverse effects on human health and on the ecological systems that are essential to our well-being.

We face four types of environmental hazards We can categorize environmental health hazards into four main types: physical, chemical, biological, and cultural. Al- though some amount of risk is unavoidable, much of envi- ronmental health focuses on taking steps to minimize the risks of encountering hazards and to mitigate the impacts of the hazards we do encounter.

Physical hazards Physical hazards arise from processes that occur naturally in our environment and pose risks to human life or health. Some are ongoing natural phenome- na, such as excessive exposure to ultraviolet (UV) radiation from sunlight, which damages DNA and has been tied to skin cancer, cataracts, and immune suppression (FIGURE 10.2A). We can reduce these risks by shielding our skin from intense sunlight with clothing and sunscreen.

(a) Physical hazard (b) Chemical hazard

(c) Biological hazard (d) Cultural hazard

FIGURE 10.2 ▲ Environmental health hazards come in four types. The sun’s ultraviolet radiation is an example of a physical hazard (a). Excessive exposure increases the risk of skin cancer. Chemical haz- ards (b) include both synthetic and natural chemicals. Much of our exposure comes from pesticides and household chemical products. Biological hazards (c) include diseases and the organisms that trans- mit them. Some mosquitoes, for example, are vectors for pathogenic microbes, including those that cause malaria. Cultural or lifestyle hazards (d) include the behavioral decisions we make, as well as the socioeconomic constraints forced on us. Smoking is a lifestyle choice that raises one’s risk of lung cancer and other diseases.

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Other physical hazards include discrete events such as earthquakes, volcanic eruptions, fires, floods, landslides, hurricanes, and droughts. We cannot prevent many of these hazards, but we can minimize risk by preparing our- selves with emergency plans and avoiding common prac- tices that make us vulnerable to certain physical hazards. For example, clearing vegetation from hillsides increases the chance of landslides, and channelizing rivers pro- motes flooding in some areas while preventing it in others (p. 261).

Chemical hazards Chemical hazards include many of the synthetic chemicals that our society manufactures, such as pharmaceuticals, disinfectants, and pesticides (FIGURE 10.2B). Some natural substances that we process for our use (such as hydrocarbons, lead, and asbestos) are also harmful to human health. Following our overview of environmental health, much of this chapter will focus on chemical health hazards and the ways we study and regulate them.

Biological hazards Biological hazards result from eco- logical interactions among organisms (FIGURE 10.2C). When we become sick from a virus, bacterial infection, or other pathogen, we are suffering parasitism (pp. 66–67). This is what we call infectious disease. Infectious diseases such as malaria, cholera, tuberculosis, and influenza (flu) are major environmental health hazards, especially in developing na- tions with widespread poverty and few resources for health care. As with physical and chemical hazards, it is impossi- ble for us to avoid risk from biological agents completely, but through monitoring, sanitation, and medical treatment we can reduce the likelihood and impacts of infection.

Infectious diseases

14.9 million 23.4%

Cardiovascular diseases 29.0%12.6%

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AIDSDiarrheal diseases

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tetanus, etc.)

Cancers

Maternal and perinatal conditions

Injuries

Respiratory and digestive diseases

Other

(a) Leading causes of death across the world (b) Leading causes of death by infectious disease

FIGURE 10.3 ▲ Infectious disease is the second-leading cause of death worldwide (a), accounting for nearly one-quarter of all deaths. Six types of diseases (b)—respiratory infections, diarrhea, AIDS, tuberculosis (TB), malaria, and childhood diseases—account for 80% of all deaths from infectious disease. Data from World Health Organization, 2009. World health statistics 2009. WHO, Geneva, Switzerland.

Cultural hazards Hazards that result from our place of residence, our socioeconomic status, our occupation, or our behavioral choices can be thought of as cultural hazards or lifestyle hazards. We can minimize or prevent some of these cultural or lifestyle hazards, but others may be beyond our control. For instance, individuals can choose whether or not to smoke cigarettes (FIGURE 10.2D), but exposure to second- hand smoke in the home or workplace may be beyond one’s control. Much the same might be said for other cultural hazards such as drug use, diet and nutrition, crime, and mode of transportation. Environmental justice advocates (pp. 14–15) argue that “forced” risks from cultural hazards, such as living near a hazardous waste site, are often higher for people with fewer economic resources or less political clout.

The biological hazard of disease is a focus of environmental health Despite all our technological advances, we still find ourselves battling disease, which causes the vast majority of human deaths worldwide (FIGURE 10.3A). Over half the world’s deaths result from noninfectious diseases, such as cancer and heart disease. These diseases are not spread from one person to another, but rather are influenced by genetics, environ- mental factors, and lifestyle choices. For instance, whether a person develops heart disease depends not only on his or her genes, but also on lifestyle choices such as diet and exercise.

Infectious diseases account for almost one of every four deaths that occur each year—nearly 14 million people worldwide (FIGURE 10.3B). Some pathogenic viruses, bacteria, and protists attack us directly; others cause infection through a vector, an organism that transfers the pathogen to the host.

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Infectious disease is a greater problem in developing coun- tries, where it accounts for close to half of all deaths. Infec- tious disease causes many fewer deaths in developed nations because their wealth allows their citizens better nutrition, sanitation, hygiene, and access to medical care.

Decades of public health efforts have lessened the impacts of infectious disease and even have eradicated some diseas- es—yet other diseases are posing new challenges. Some, such as acquired immunodeficiency syndrome (AIDS), continue to spread globally despite concerted efforts to stop them. Others, such as tuberculosis and strains of malaria, are evolving resist- ance to our antibiotics, in the same way that pests evolve resist- ance to our pesticides (p. 147). Additionally, human-induced global warming of the climate (Chapter 14) is enabling tropical diseases (such as malaria, dengue, and cholera) to gain foot- holds in temperate regions.

In our world of global mobility and dense human popula- tions, novel infectious diseases (or new strains of old diseases) that emerge in one location are more likely to spread quickly to other locations. Recent examples include severe acute res- piratory syndrome (SARS) in 2003, the H5N1 avian flu start- ing in 2004, and the H1N1 swine flu that spread across the globe in 2009–2010. Diseases like influenza, whose pathogens evolve rapidly, give rise to a variety of strains, making it more likely that one may turn exceedingly dangerous and cause a global pandemic (a widespread outbreak of a disease).

Thousands of dedicated people—from doctors and nurs- es to policymakers to philanthropists—are dedicating their lives to reducing the incidence of disease and improving hu- man health. They use a diversity of approaches to better the living conditions of those most affected by infectious disease by improving access to clean drinking water, sanitation, medi- cal care, and nutritious foods.

Toxicology is the study of chemical hazards Although most indicators of human health are improving as the world’s wealth increases, our modern society is exposing us to more and more synthetic chemicals. Some of these sub- stances pose threats to human health, but figuring out which of them do—and how, and to what degree—is a complicated scientific endeavor. Toxicology is the science that examines the effects of poisonous chemicals on humans and other organisms. Toxicologists assess and compare substances to determine their toxicity, the degree of harm a chemical substance can inflict. A toxic substance, or poison, is called a toxicant, but any chemical substance may exert negative impacts if we ingest or expose ourselves to enough of it. Con- versely, a toxicant in a small enough quantity may pose no health risk at all. These facts are often summarized in the catchphrase, “The dose makes the poison.” In other words, a substance’s toxicity depends not only on its chemical identity, but also on its quantity.

In recent decades, our ability to produce new chemicals has expanded, concentrations of chemical contaminants in the environment have increased, and public concern for health and the environment has grown. These trends have driven the rise of environmental toxicology, which deals specifically

TABLE 10.1 Selected Environmental Hazards

Outdoor Air ▶ Chemicals from automotive exhaust ▶ Chemicals from industrial pollution ▶ Photochemical smog (pp. 288–289) ▶ Pesticide dri ▶ Dust and particulate matter

Water ▶ Pesticide and herbicide runo ▶ Nitrates and fertilizer runo ▶ Mercury, arsenic, and other heavy metals in groundwater

and surface water

Food ▶ Natural toxins ▶ Pesticide and herbicide residues

Indoors ▶ Smoking and secondhand smoke ▶ Radon ▶ Lead in paint and pipes ▶ Asbestos ▶ Toxicants (e.g., PBDEs, phthalates, bisphenol A) in plastics

and consumer products ▶ Dust and particulate matter

with toxic substances that come from or are discharged into the environment. Toxicologists generally focus on human health, using other organisms as models and test subjects. En- vironmental toxicologists study animals and plants to deter- mine the ecological impacts of toxic substances, and to see if other organisms can serve as indicators of health threats that could soon affect people.

People face environmental health hazards indoors Modern Americans spend roughly 90% of their lives indoors. Unfortunately, our homes and workplaces, just like the out- doors, can be rife with physical, biological, chemical, and cultural hazards (TABLE 10.1; also see Figure 13.21, p. 295).

Cigarette smoke and radon are leading indoor hazards (pp. 294–296) and are the top two causes of lung cancer in developed nations. Homes and offices can have problems with toxic compounds produced by mold, which can flourish in wall spaces when moisture levels are high. Asbestos, used in the past as insulation in walls and other products, can be dan- gerous when it is inhaled. Lead poisoning from water pipes or old paint can cause damage to the brain, liver, kidney, and stomach; learning problems and behavioral abnormalities; anemia; hearing loss; and even death. Lead poisoning among U.S. children has greatly declined in recent years as a result of education campaigns and the phaseout of lead-based paints and leaded gasoline (p. 5), which was prompted by govern- ment regulation to protect public health.

There are also indoor chemical hazards that we have yet to discover. One recently recognized hazard is polybrominated diphenyl ethers (PBDEs). These compounds are used as fire retardants in computers, televisions, plastics, and furniture,

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and they may evaporate at very slow rates throughout the life- time of the product. Like bisphenol A, PBDEs appear to act as hormone disruptors. The European Union decided in 2003 to ban PBDEs, but in the United States there has so far been little movement to address the issue.

Risks must be balanced against rewards As we review the impacts of toxic substances throughout this chapter, it is important to keep in mind that artificially produced chemicals have played a crucial role in giving us the standard of living we enjoy today. These chemicals have helped create the industrial agriculture that produces our food, the medical advances that protect our health and pro- long our lives, and many of the modern materials and conven- iences we use every day. It is appropriate to remember these benefits as we examine some of the unfortunate side effects of these advances and as we search for better alternatives.

TOXIC SUBSTANCES AND THEIR EFFECTS ON ORGANISMS Our environment contains countless natural substances that may pose health risks. These include oil oozing naturally from the ground; radon gas seeping up from bedrock; and toxins, toxic chemicals manufactured in the tissues of living organisms—for example, chemicals that plants use to ward off herbivores or that insects use to defend themselves from predators. In addition, we are exposed to many synthetic ( human-made) chemicals.

Synthetic chemicals are all around us—and in us Synthetic chemicals surround us in our daily lives, and each year in the United States we manufacture or import 113 kg (250 lb) of chemical substances for every man, woman, and child. Many of these substances, particularly the pesticides we use to control insects and weeds, find their way into soil, air, and water—and into humans and other organisms (FIGURE 10.4).

As a result of all this exposure, every one of us carries traces of hundreds of industrial chemicals in our bodies. The U.S. government’s latest National Health and Nutrition Examination Survey gathered data on 148 foreign compounds in Americans’ bodies. Among these were several toxic per- sistent organic pollutants restricted by international treaty (p. 223). Depending on the pollutant, these were detected in 41–100% of the people tested. Smaller-scale surveys have found similar results. Our exposure to synthetic chemicals be- gins in the womb as substances our mothers ingested while pregnant were transferred to us. A 2009 study by the nonprof- it Environmental Working Group found 232 chemicals in the umbilical cords of 10 newborn babies it tested. Nine of the 10 umbilical cords contained BPA, leading researchers to note that we are born “pre-polluted.”

All this should not necessarily be cause for alarm. Not all synthetic chemicals pose health risks, and relatively few are

known with certainty to be toxic. However, of the roughly 100,000 synthetic chemicals on the market today, very few have been thoroughly tested. For the vast majority, we simply do not know what effects, if any, they may have on us.

Silent Spring changed public attitudes toward synthetic chemicals It was not until the 1960s that people began to learn about the risks of exposure to pesticides. The key event was the publica- tion of Rachel Carson’s 1962 book Silent Spring (pp. 98–100), which brought the insecticide dichlorodiphenyl-trichlo- roethane (DDT) to the public’s attention. The book was written at a time when large amounts of pesticides virtually untested for health effects were indiscriminately sprayed, on the assumption that the chemicals would do no harm to people (FIGURE 10.5).

Carson synthesized scientific studies, medical case histo- ries, and other data to contend that DDT in particular, and artificial pesticides in general, were hazardous to people, wild- life, and ecosystems. The book became a best-seller and helped generate significant social change in views and actions toward the environment. The use of DDT was banned in the United States in 1973 and is now illegal in a number of nations. U.S. chemical companies still manufacture and export DDT, how- ever, because developing countries with tropical climates use it to control disease vectors, such as mosquitoes that transmit malaria. In these countries, malaria represents a greater health threat than do the toxic effects of the pesticide.

A Circle of Poison? Although the United States has banned the use of DDT, U.S. compa- nies still manufacture and export the compound to developing nations. Thus, it is possible that

pesticide-laden food can be imported back into the United States in what has been called a “circle of poison.” How do you feel about this? Is it unethical for one country to sell to others a substance that it has deemed toxic? Or would it be unethical for the United States not to sell DDT to African nations if they desire it for controlling malaria?

Not all toxic substances are synthetic, and not all synthetic chemicals are toxic Although many toxicologists focus on synthetic chemicals, toxic substances also exist naturally in the environment around us and in the foods we eat. Thus, it would be a mis- take to assume that all artificial substances are unhealthy and that all natural substances are healthy. In fact, the plants and animals we eat contain many chemicals that can cause us harm. Recall that plants produce toxins to ward off animals that eat them. In domesticating crop plants, we have selected (p. 48) for strains with reduced toxin content, but we have not eliminated these dangers. Furthermore, when we consume animal meat, we ingest toxins the ani- mals obtained from plants or animals they ate. Scientists

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FIGURE 10.5  Children on a beach in Long Island, New York, are fogged with DDT from a pesticide spray machine being tested in 1945. Before the 1960s, the environmental and health effects of potent pesticides such as DDT were not widely known. Public parks and neighborhoods were regularly sprayed for insect control without safeguards against excessive human exposure.

Human fetuses and babies

Humans

Non-human biota

Industry and manufacturing

Genes, womb, breast milk

Consum er products

W orkplace exposure Non-target effects

F oo

d Plant grow th

Air for bre athing

A ir for breathing

Hunting and harvesting

Work Medical facilities and public spaces

Agriculture: crops, rangeland, feedlots Soil

Water: surface and groundwater AirHome

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FIGURE 10.4 ▲ Synthetic chemicals take many routes in traveling through the environment. People take in only a tiny proportion of these compounds, and many compounds are harmless. However, people receive small amounts of toxicants from many sources, and developing fetuses and babies are particularly sensitive.

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ing chemical pathways that produce energy in mitochondria and depriving cells of energy.

Most recently, scientists have recognized endocrine disruptors, toxic substances that interfere with the endocrine system. The endocrine system consists of chemical messen- gers (hormones) that travel through the bloodstream at ex- tremely low concentrations and have many vital functions. They stimulate growth, development, and sexual maturity, and they regulate brain function, appetite, sex drive, and many other aspects of our physiology and behavior. Some hormone-disrupting toxicants affect an animal’s endocrine system by blocking the action of hormones or accelerating their breakdown. Others are so similar to certain hormones in their molecular structure and chemistry that they “mimic” the hormone by interacting with receptor molecules just as the actual hormone would (FIGURE 10.6). Bisphenol A is one of many chemicals that appear to mimic the female sex hor- mone estrogen and bind to estrogen receptors. Phthalates are another class of hormone-disrupting chemicals that are used widely in children’s toys, perfumes and cosmetics, and other items. Health research on phthalates has linked them to birth defects, breast cancer, reduced sperm counts, and other re-

are actively debating just how much risk natural toxicants pose, and it is clear that more research is required on these questions.

Toxic substances come in different types Toxic substances can be classified based on their particular effects on health. The best-known toxicants are carcinogens, which are substances or types of radiation that cause cancer. In cancer, malignant cells grow uncontrollably, creating tumors, damaging the body, and often leading to death. Cancer fre- quently has a genetic component, but a wide variety of envi- ronmental factors are thought to raise the risk of cancer. In our society today, the greatest number of cancer cases is thought to result from carcinogens contained in cigarette smoke. Carcin- ogens can be difficult to identify because there may be a long lag time between exposure to the agent and the detectable on- set of cancer—up to 15–30 years in the case of cigarette smoke.

Mutagens are substances that cause genetic mutations in the DNA of organisms (p. 28). Although most mutations have little or no effect, some can lead to severe problems, in- cluding cancer and other disorders. If mutations occur in an individual’s sperm or egg cells, then the individual’s offspring suffer the effects.

Chemicals that cause harm to the unborn are called teratogens. Teratogens that affect development of human embryos in the womb can cause birth defects. One example involves the drug thalidomide, developed in the 1950s as a sleeping pill and to prevent nausea during pregnancy. Tragi- cally, the drug turned out to be a powerful teratogen. Its use caused birth defects in thousands of babies, and its use by pregnant women was banned in the 1960s.

Other toxicants, known as neurotoxins, assault the nerv- ous system. Neurotoxins include venoms produced by ani- mals, heavy metals such as lead and mercury, and some pes- ticides. A famous case of neurotoxin poisoning occurred in Japan, where a chemical factory dumped mercury waste into Minamata Bay between the 1930s and 1960s. Thousands of people there ate fish contaminated with the mercury and soon began suffering from slurred speech, loss of muscle control, sudden fits of laughter, and in some cases death.

The human immune system protects our bodies from disease. Some toxic substances weaken the immune system, reducing the body’s ability to defend itself against bacteria, viruses, allergy-causing agents, and other attackers. Others, called allergens, overactivate the immune system, causing an immune response when one is not necessary. One hypothesis for the increase in asthma in recent years is that allergenic synthetic chemicals are more prevalent in our environment. Allergens are not universally considered toxicants, however, because they affect some people but not others and because one’s response does not necessarily correlate with the degree of exposure.

Pathway inhibitors are toxicants that interrupt vital biochemical processes in organisms by blocking one or more steps in important biochemical pathways. Rat poisons, for example, cause internal hemorrhaging in rodents by interfer- ing with the biochemical pathways that create blood clotting proteins. Some herbicides, such as atrazine, kill plants by blocking steps in photosynthesis. Cyanide kills by interrupt-

Hormone

Hormone mimic

Receptor

Cell membrane Inside cell

Response (identical to that

caused by hormone)

Inside cell

Response

Receptor

Cell membrane

Hormone

(b) Hormone mimicry

(a) Normal hormone binding

FIGURE 10.6 ▲ Many endocrine-disrupting substances mimic the structure of hormone molecules. Like a key similar enough to fit into another key’s lock, the hormone mimic binds to a cellular receptor for the hormone, causing the cell to react as though it had encountered the hormone.

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The type of exposure can affect the response The risk posed by a hazard often varies according to whether a person experiences high exposure for short periods of time, known as acute exposure , or low exposure over long periods of time, known as chronic exposure . Incidences of acute ex- posure are easier to recognize because they often stem from discrete events, such as accidental ingestion, an oil spill, a chemical spill, or a nuclear accident. Toxicity tests in labo- ratories generally reflect effects of acute toxicity. However, chronic exposure is more common—and more difficult to detect and diagnose. Chronic exposure often affects organs gradually, as when smoking causes lung cancer, or when alco- hol abuse leads to liver or kidney damage. Because of the long time periods involved, relationships between cause and effect may not be readily apparent.

TOXIC SUBSTANCES AND THEIR EFFECTS ON ECOSYSTEMS When toxicants concentrate in environments and harm the health of many individuals, populations (p. 46 ) of the affect- ed species become smaller. This decline can then affect other species. For instance, species that are prey of the affected or- ganism could experience population growth due to lower lev- els of predation. Predators of the poisoned species, however, would decline as their food source became less abundant.

organisms (for example, fetuses, infants, and young children) tend to be much more sensitive to toxicants than are adults. Regulatory agencies such as the U.S. Environmental Protec- tion Agency (EPA) typically set human chemical exposure standards for adults and extrapolate downward for infants and children. However, many scientists contend that these linear extrapolations often do not offer adequate protection to fetuses, infants, and children.

productive effects. Like bisphenol A, phthalates show how a substance can have multiple effects, by being a carcinogen, mutagen, and endocrine disruptor.

Organisms have natural defenses against toxic substances Although synthetic toxicants are new, organisms have been exposed to natural toxicants for millions of years. Mercury, cadmium, arsenic, and other harmful substances are found naturally in the environment. Some organisms produce bio- logical toxins to avoid predators or capture prey. Examples include venom in poisonous snakes, toxins in sea urchins, and the natural insecticide pyrethrin found in chrysanthe- mums. These exposures have provided selection pressure (pp. 46–48 ) for protection from toxins, and over time, organisms able to tolerate these harmful substances have gained an evo- lutionary advantage.

Barriers such as skin, scales, feathers, and fur are the first line of defense against toxic substances because they help the body to resist uptake from the surrounding environment. However, toxicants can circumvent these barriers and enter the body from vital activities such as eating, drinking, and breath- ing. Once in the organism, they are distributed widely by the circulatory and lymph systems in animals, and by the vascular system in plants.

Organisms possess biochemical pathways that use en- zymes to detoxify harmful chemicals. Some pathways break down, or metabolize, toxic substances to render them inert. Other pathways make toxic substances water soluble so they are easier to excrete through the urinary system. In humans, many of these pathways are found in the liver, so this organ is disproportionately affected by intake of harmful substances such as excessive alcohol.

Some toxic substances cannot be effectively detoxified or made water soluble by detoxification enzymes. These chemi- cals are sequestered in fatty tissues and cell membranes to keep them away from vital organs. Heavy metals, dioxins, and some insecticides (including DDT) are stored in body tissue in this manner.

These defenses can protect organisms against low levels of some toxicants but can be overwhelmed if exposure exceeds critical levels. For other toxicants, harm occurs with any ex- posure if organisms have no defense against the substance. Defense mechanisms for natural toxins have evolved over millions of years. Organisms have not had long-term expo- sure to the synthetic chemicals that are so prevalent in today’s environment, so the impacts of these toxic substances can be severe and unpredictable.

Individuals vary in their responses to hazards Some of the defenses described above have a genetic basis. As a result, individuals may respond quite differently to identical exposures to hazards because they happen to have different combinations of genes. Poorer health also makes an individ- ual more sensitive to biological and chemical hazards. Sensi- tivity also can vary with sex, age, and weight. Because of their smaller size and rapidly developing organ systems, younger

FAQ

Q: Does exposure to a toxic substance cause genetic resistance to the substance? A: When a population of organisms is exposed to a toxicant, such as a pesticide, a few individuals often survive while the vast majority of the population is killed. These individuals survive because they possess genes (which others in the population do not) that code for enzymes that counteract the toxic properties of the toxicant. Because the effects of these genes are only expressed when the pesticide is applied, many people think the toxicant “creates” detoxification genes by mutating the DNA of a small number of individuals. This is not the case. The genes for detoxifying enzymes were present in the DNA of resistant individuals from birth, but their effects were only seen when pesticide exposure caused selective pressure (pp. 46–48 ) for resistance to the toxic substance.

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Essential Environment: The Science Behind the Stories, Fourth Edition, by Jay Withgott and Matthew Laposata. Published by Benjamin Cummings. Copyright © 2012 by Pearson Education, Inc.

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THE SCIENCE BEHIND THE STORY

Testing the Safety of Bisphenol A

At a laboratory at Case Western Reserve University in Ohio in 1998, geneticist Patricia Hunt was mak- ing a routine check of her female lab mice. As she extracted and examined developing eggs from the ovaries, she began to wonder what had gone wrong. About 40% of the eggs showed problems with their chromosomes, and 12% had irregular amounts of genetic material, a dangerous condition called aneuploidy, which can lead to miscar- riages or birth defects in mice and people alike.

A bit of sleuthing revealed that a lab assistant had mistakenly washed the lab’s plastic mouse cages and water bot- tles with an especially harsh soap. The soap damaged the cages so badly that parts of them seemed to have melted.

The cages were made from polycarbonate plastic, which con- tains bisphenol A (BPA). Hunt knew at the time that BPA mimics estrogen and that some studies had linked the chemical to reproductive abnormali- ties in mice, such as low sperm counts and early sexual development. Other research indicated that BPA leaches out of plastic into water and food when

the plastic is treated with heat, acidity, or harsh soap.

Hunt wondered whether the chem- ical might be adversely affecting the mice in her lab. Deciding to re-create the accidental cage-washing incident in a controlled experiment, Hunt in- structed researchers in her lab to wash polycarbonate cages and water bottles using varying levels of the harsh soap. They then compared mice kept in dam- aged cages with plastic water bottles to mice kept in undamaged cages with glass water bottles.

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