Science

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Learning Objectives

After studying this chapter, you should be able to:

• Identify the different terrestrial and aquatic biome types that cover the planet and explain why they might differ in terms of biodiversity and species richness.

• Describe how energy produced through photosynthesis forms the basis for most life on the planet and how this energy flows through different trophic levels in an ecosystem.

• Explain how nutrients, such as nitrogen and phosphorous, cycle within ecosystems and how human activities are altering the flow and location of these nutrients, often with unintended consequences.

• Understand the difference in life history strategy between different organisms, including those between r-selected and K-selected species.

• Explain the concepts of niche, limiting factor, keystone species, and trophic cascades and how these relate to the functioning of ecosystems and the species within them.

• Discuss how interactions between different species in an ecosystem (such as predators and their prey) result in evolutionary changes in these organisms and how ecosystem change and succession over time alters the balance of species present in a given location.

• Describe how toxic substances like mercury can find their way into natural environments far from any source and impact wildlife populations in that area.

Ecosystems 1

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Pre-TesT

Pre-Test

1. Which biome would be expected to have the warmest and wettest conditions? a. Coniferous forest b. Desert c. Tropical forest d. Temperate grassland 2. The major sources of human emissions of the pollutant mercury are a. disposal of thermometers and hospital waste. b. car and truck exhaust. c coal burning and gold mining. d. agriculture and cattle ranching. 3. Which of the following is NOT an example of an important biogeochemical cycle? a. The water cycle b. The phosphorous cycle c. The solar cycle d. The carbon cycle 4. The population biology concept that refers to the maximum number of organisms that a

given environment can support is a. survival rate. b. reproductive rate. c. K-selection. d. carrying capacity. 5. When a top predator is removed from an ecosystem it can have dramatic impacts on the

entire food web. These impacts are referred to as a. biomagnification. b. bioaccumulation. c. trophic cascades. d. photosynthesis. 6. Which of the following is NOT an example of an avoidance/escape feature used to deter

predators from attacking prey? a. A panda feeding only on bamboo b. Fish swimming in a school c. Wildebeests moving in a herd d. A moth with false eye spots on its hind wings 7. Because mercury tends to accumulate in an animal’s tissue, we would expect what kinds

of organisms to carry the highest amounts of this toxin? a. Long-lived predators b. Primary producers c. short-lived predators d. Detritivores

Answers 1. c. tropical forest. The answer can be found in section 1.1. 2. c. coal burning and gold mining. The answer can be found in section 1.2. 3. c. the solar cycle. The answer can be found in section 1.3. 4. d. carrying capacity. The answer can be found in section 1.4. 5. c. trophic cascades. The answer can be found in section 1.5. 6. a. a panda bear feeding only on bamboo. The answer can be found in section 1.6. 7. a. long-lived predators. The answer can be found in section 1.7.

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INTrODUCTION

Introduction Anyone who has spent time outdoors in a favorite patch of forest or other natural setting might gain an appreciation for the complexity of life present in these ecosystems. Though silent and invisible to us, trees and other plants are busy converting sunlight into stored energy through photosynthesis. Birds, insects, and other creatures are on the move searching for food or themselves ending up as food for other organisms. some of these ecosystems seem little changed over time while others might undergo rapid and dramatic transformation over the course of only a few years. For example, mature forests might change little from year to year, while shallow lakes gradually fill with sediment and slowly morph into swamps. such changes to ecosystems move ecologists to investigate the mechanisms that maintain stability in some systems like the forest, yet promote change in others like the lake. Indeed, ecology is the study of all natural systems, including the forest and the lake.

Ecologists study the relationships between living organisms and the physical environment. For example, in an ecosystem, plants compete with one another for sunlight, and some ani- mals eat plants, while others eat plant eaters. Ecologists studying such an ecosystem might ask questions, such as: What mineral qualities of the soil nourish this particular community of plants? And, how does competition and predation among all the billions of soil microor- ganisms affect the nutrient qualities of the soil? such queries help guide researchers as they examine the interactions that occur within a particular ecosystem. Furthermore, the knowl- edge gained from such research helps environmental scientists study the impacts of human actions on the environment, such as the damage done to a salt marsh by an oil spill or the impact of air pollution on trees and plants.

This chapter will explore ecology as the study of change and stasis, balance and imbalance, life and death in all natural systems—rainforests, tundra, grasslands, deserts, rivers, and oceans that constitute our world. It begins with a review of the concept of biomes, major ecological communities like forests, deserts, tundra, and oceans. While biomes differ dramatically in terms of climate and the variety of life present, they all are generally powered by solar energy. The second section examines how energy enters and flows through different trophic levels in an ecosystem. The third section considers how nutrients, such as nitrogen and phosphorous, are cycled within ecosystems. This is followed by an overview of population biology, the study of how different organisms grow and reproduce in different ways. The fifth section intro- duces the concepts of niche, limiting factor, keystone species, and trophic cascades, and how they impact the functioning of various ecosystems. section 1.6 reviews evolution and natural selection and how these processes alter the species composition of ecosystems over time. All of these topics will provide you with a basic foundation in ecology and environmental science needed to understand the subjects presented in later chapters. To illustrate how the topics covered in this chapter connect, the final section presents a case history of how mercury con- tamination is affecting wildlife and ecosystems the world over.

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seCTION 1.1 earTh’s BIOmes

1.1 Earth’s Biomes The diversity of life on Earth is vast. Yet ecologists have found that areas on different continents that share similar climate conditions tend to have similar ecosystem structures and functions. As a result, ecologists use the concept of a biome to classify large areas of the planet into a small number of similar units. Biomes include both terrestrial (land-based) and aquatic (in water) communities. Biomes display huge differences in the number or diversity of species present and how these species interact with one another. The following section, which has been excerpted from habitable Planet: a systems approach to environmental science by Annenberg Learner, discusses the different types of biomes and how they are classified. It will help you gain an appre- ciation for the incredible variety of ecosystems and natural conditions on the planet, and how conditions shape the diversity of life found in each area.

The reading points out that scientists have determined that a handful of factors—namely tem- perature, availability of moisture, abundance of light, and availability of nutrients—are the key influences on the number and variety of organisms in a given ecosystem. Generally speaking, tropical regions with their warm temperatures, abundance of moisture, and relatively constant levels of daylight have the highest number and diversity of organisms. Indeed, tropical, moist for- est ecosystems make up the terrestrial biome with the highest productivity and diversity of life. In contrast, polar regions with their frigid temperatures, low moisture conditions, and months of the year with little or no natural light tend to have the lowest levels of productivity and diversity. Scientists study all types of biomes in order to learn about the life cycle and optimal conditions within different types of climates.

By Annenberg Learner Geography has a profound impact on ecosystems because global circulation patterns and cli- mate zones set basic physical conditions for the organisms that inhabit a given area. The most important factors are temperature ranges, moisture availability, light, and nutrient availabil- ity, which together determine what types of life are most likely to flourish in specific regions and what environmental challenges they will face.

earth is divided into distinct climate zones that are created by global circulation patterns. The tropics are the warmest, wettest regions of the globe, while subtropical high-pressure zones create dry zones at about 308 latitude north and south. Temperatures and precipitation are lowest at the poles. These conditions create biomes—broad geographic zones whose plants and animals are adapted to different climate patterns. since temperature and precipitation vary by latitude, earth’s major terrestrial biomes are broad zones that stretch around the globe. Each biome contains many ecosystems (smaller communities) made up of organisms adapted for life in their specific settings.

Land biomes are typically named for their characteristic types of vegetation, which in turn influence what kinds of animals will live there. soil characteristics also vary from one biome to another, depending on local climate and geology. [. . .]

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seCTION 1.1 earTh’s BIOmes

Figure 1.1: Global biomes

earth’s major biomes result primarily from differences in climate. each biome contains many ecosystems made up of species adapted for life in their specific biome.

Adapted from U.S. Department of Agriculture Natural Resources Conservation Service. Retrieved from http://www.nrcs.usda.gov /wps/portal/nrcs/detail/soils/use/worldsoils/?cid=nrcs142p2_054013

Aquatic biomes (marine and freshwater) cover three-quarters of the Earth’s surface and include rivers, lakes, coral reefs, estuaries, and open ocean. Oceans account for almost all of this area. Large bodies of water (oceans and lakes) are stratified into layers: surface waters are warmest and contain most of the available light, but depend on mixing to bring up nutri- ents from deeper levels. The distribution of temperature, light, and nutrients set broad condi- tions for life in aquatic biomes in much the same way that climate and soils do for land biomes.

marine and freshwater biomes change daily or seasonally. For example, in the intertidal zone where the oceans and land meet, areas are submerged and exposed as the tide moves in and out. During the winter months lakes and ponds can freeze over, and wetlands that are covered with water in late winter and spring can dry out during the summer months.

There are important differences between marine and freshwater biomes. The oceans occupy large continuous areas, while freshwater habitats vary in size from small ponds to lakes cov- ering thousands of square kilometers. As a result, organisms that live in isolated and tem- porary freshwater environments must be adapted to a wide range of conditions and able to disperse between habitats when their conditions change or disappear.

Equator

Tropic of Capricorn

Tropic of Cancer

30° S

30° N

Tropical forest Temperate deciduous forest

Savanna Temperate grassland

Desert Coniferous forest

Chaparral Tundra (arctic and alpine)

Oceans Polar and high- mountain ice

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http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/use/worldsoils/?cid=nrcs142p2_054013
http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/use/worldsoils/?cid=nrcs142p2_054013
seCTION 1.1 earTh’s BIOmes

Biomes and Biodiversity since biomes represent consistent sets of condi- tions for life, they will support similar kinds of organisms wherever they exist, although the spe- cies in the communities in different places may not be taxonomically [the science of classifying animals] related. For example, large areas of Africa, Austra- lia, south america, and India are covered by savan- nas (grasslands with scattered trees). The various grasses, shrubs, and trees that grow on savannas all are generally adapted to hot climates with distinct rainy and dry seasons and periodic fires, although they may also have characteristics that make them well-suited to specific conditions in the areas where they appear.

species are not uniformly spread among earth’s biomes. Tropical areas generally have more plant and animal biodiversity [the diversity of animal and plant life in a region] than high latitudes, measured in species richness (the total number of species present). This pattern, known as the latitudinal bio- diversity gradient, exists in marine, freshwater, and terrestrial ecosystems in both hemispheres. [. . .]

Why is biodiversity distributed in this way? Ecolo- gists have proposed a number of explanations:

• higher productivity in the tropics allows for more species;

• The tropics were not severely affected by glaciation and thus have had more time for species to develop and adapt;

• Environments are more stable and predictable in the tropics, with fairly constant temperatures and rainfall levels year-round;

• more predators and pathogens limit competition in the tropics, which allows more species to coexist; and

• Disturbances occur in the tropics at frequencies that promote high successional diversity.

Consider This Recall that as part of the scientific method scientists regularly formulate and test hypotheses about how the world works. Now, note the language used in the pre- vious paragraph about how “evidence is strongest . . . ” for one proposition over the others. What does this tell you about the scientific method and the kind of lan- guage and terminology used by scientists to describe the natural world?

. luoman/iStock/Thinkstock

Tropical rainforests produce their own moisture. Scientists believe that as these ecosystems are cleared through deforestation—as shown here in the Amazon—there is a threshold beyond which they will no longer produce enough moisture to sustain themselves. The result could be conversion of rainforests to drier savannas.

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seCTION 1.2 eNergy FlOWs ThrOUgh eCOsysTems

Of these hypotheses, evidence is strongest for the proposition that a stable, predict- able environment over time tends to pro- duce larger numbers of species. For exam- ple, both tropical ecosystems on land and deep sea marine ecosystems—which are subject to much less physical fluctuation than other marine ecosystems, such as estuaries—have high species diversity. Predators that seek out specific target species may also play a role in maintain- ing species richness in the tropics.

Excerpted from Weeks, S., Moorcoft, P.R. (2007). Unit 4: Ecosystems. The habitable Planet: a systems approach to environmental science. Retrieved from http://www.learner.org/courses/envsci/unit/text.php?unit=4&secNum =0. Used with permission of Annenberg Learner.

1.2 Energy Flows Through Ecosystems Despite the incredible range of conditions that characterize the ecosystems found in differ- ent biomes, they all have something in common. With few exceptions, Earth’s ecosystems are powered by solar energy. Primary producers such as plants and algae use sunlight in a process known as photosynthesis to convert carbon dioxide and water into glucose (sugars). Glucose represents a form of stored energy that is used by plants for their own growth and maintenance. Other organisms can then consume this plant material and use it as a source of energy. Animals, in turn, can eat the organisms that ate the plants in order to acquire energy. Ecologists use the concept of trophic levels to study how energy moves through ecosystems. The following selection adapted from habitable Planet: a systems approach to environmental science, by Annenberg Learner, explains how energy flows through ecosystems and discusses the impact on the environment. It will introduce you to the critical concept of primary productivity, the basis for almost all life on the planet.

Trophic levels can be best visualized as a series of steps, with the base made up of large amounts of primary producers such as plants and algae. These primary producers have the unique abil- ity to transform solar energy from the sun into stored energy in the form of sugars through the process of photosynthesis. Animals that feed on primary producers are known as primary con- sumers. An example of a primary consumer is a rabbit that eats grass and then utilizes much of the energy stored in the grass for its own growth and bodily functions. In order to sustain the rabbit there must be a huge amount of available grass for it to eat. This is why the trophic level comprised of primary producers is the largest. However, the animals that eat rabbits and other primary consumers are fewer in number than rabbits, so their step is smaller than the one below it that represents plants and algae.

Biomes and Biodiversity since biomes represent consistent sets of condi- tions for life, they will support similar kinds of organisms wherever they exist, although the spe- cies in the communities in different places may not be taxonomically [the science of classifying animals] related. For example, large areas of Africa, Austra- lia, south america, and India are covered by savan- nas (grasslands with scattered trees). The various grasses, shrubs, and trees that grow on savannas all are generally adapted to hot climates with distinct rainy and dry seasons and periodic fires, although they may also have characteristics that make them well-suited to specific conditions in the areas where they appear.

species are not uniformly spread among earth’s biomes. Tropical areas generally have more plant and animal biodiversity [the diversity of animal and plant life in a region] than high latitudes, measured in species richness (the total number of species present). This pattern, known as the latitudinal bio- diversity gradient, exists in marine, freshwater, and terrestrial ecosystems in both hemispheres. [. . .]

Why is biodiversity distributed in this way? Ecolo- gists have proposed a number of explanations:

• higher productivity in the tropics allows for more species;

• The tropics were not severely affected by glaciation and thus have had more time for species to develop and adapt;

• Environments are more stable and predictable in the tropics, with fairly constant temperatures and rainfall levels year-round;

• more predators and pathogens limit competition in the tropics, which allows more species to coexist; and

• Disturbances occur in the tropics at frequencies that promote high successional diversity.

Consider This Recall that as part of the scientific method scientists regularly formulate and test hypotheses about how the world works. Now, note the language used in the pre- vious paragraph about how “evidence is strongest . . . ” for one proposition over the others. What does this tell you about the scientific method and the kind of lan- guage and terminology used by scientists to describe the natural world?

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http://www.learner.org/courses/envsci/unit/text.php?unit=4&secNum=0
http://www.learner.org/courses/envsci/unit/text.php?unit=4&secNum=0
seCTION 1.2 eNergy FlOWs ThrOUgh eCOsysTems

Ecologists study dynamics between and among trophic levels as well as the concept of primary productivity to figure out how much energy is available to support the organisms within a par- ticular ecosystem. For example, net primary productivity is the amount of energy available as plant matter for primary consumers, or the amount left over after plants use some of the energy from photosynthesis for themselves. The last section made clear that tropical, moist forests are the most productive of terrestrial biomes. That’s the same as saying that tropical forests have the highest net primary productivity (NPP). Since it is the NPP of an ecosystem that supports all life at higher trophic levels, the high NPP in tropical forests helps explain the abundance and diversity of life in these ecosystems.

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