Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.
12.1 Species Interactions Can Be Classified Based on Their Reciprocal Effects
If we designate the positive effect of one species on another as +, a detrimental effect as −, and no effect as 0, we can use this qualitative description of the different ways in which populations of two species interact to develop a classification of possible interactions between two co-occurring species (Table 12.1). When neither of the two populations affects the other, the relationship is (00), or neutral. If the two populations mutually benefit, the interaction is (++), or positive, and the relationship is called mutualism (Chapter 15). When one species maintains or provides a condition that is necessary for the welfare of another but does not affect its own well-being, the relationship (+0) is called commensalism . For example, the trunk or limb of a tree provides the substrate on which an epiphytic orchid grows (Figure 12.1). The arrangement benefits the orchid, which gets nutrients from the air and moisture from aerial roots, whereas the tree is unaffected.
When the relationship is detrimental to the populations of both species (−−), the interaction is termed competition (Chapter 13). In some situations, the interaction is (−0). One species reduces or adversely affects the population of another, but the affected species has no influence in return. This relationship is amensalism . It is considered by many ecologists as a form of asymmetric competition, such as when taller plant species shade species of smaller stature.
Relationships in which one species benefits at the expense of the other (+−) include predation, parasitism, and parasitoidism (see Chapter 14 for more information on predation and Chapter 15 for more information on parasitism and parasitoidism). Predation is the process of one organism feeding on another, typically killing the prey. Predation always has a negative effect on the individual prey. In parasitism , one organism feeds on the other but rarely kills it outright. The parasite and host live together for some time. The host typically survives, although its fitness is reduced. Parasitoidism , like predation, kills the host eventually. Parasitoids, which include certain wasps and flies, lay eggs in or on the body of the host. When the eggs hatch, the larvae feed on it. By the time the larvae reach the pupal stage, the host has succumbed.
12.2 Species Interactions Influence Population Dynamics
The varieties of species interactions outlined in the previous section typically involve the interaction of individual organisms. A predator captures a prey or a bacterium infects a host organism. Yet through their beneficial or detrimental effects on the individuals involved, these interactions influence the collective properties of birth and death at the population level, and in doing so, influence the dynamics of the respective populations. For example, by capturing and killing individual prey, predators function as agents of mortality. We might therefore expect that as the number of predators (Npredator) in an area increases, the number of prey captured and killed will likewise increase. If we assume the simplest case of a linear relationship, we can represent the influence of changes in the predator population (Npredator) on the death rate of the prey population (dprey) as shown in Figure 12.2a. As the number of predators in the population (Npredator) increases, the probability of an individual in the prey population (Nprey) being captured and killed increases. Subsequently, the death rate of the prey population increases. The net effect is a decline in the growth rate of the prey population. Note the similarity in the functional relationship presented in Figure 12.2a with the example of density-dependent population control presented earlier (Chapter 11, Figure 11.1). Previously, we examined how an increase in population size can function as a negative feedback on population growth by increasing the mortality rate or decreasing the birthrate (density-dependent population regulation; Section 11.2 and Figure 11.4). The relationship shown in Figure 12.2a expands the concept of density-dependent population regulation to include the interaction between species. As the population of predators increases, there is a subsequent decline in the population of prey as a direct result of the prey’s increased rate of mortality.
A similar approach can be taken to evaluate the positive effects of species interactions. In the example of predation, whereas the net effect of predation on the prey is negative, the predator benefits from the capture and consumption of prey. Prey provides basic food resources to the predator and directly influences its ability to survive and reproduce. If we assume that the ability of a predator to capture and kill prey increases as the number of potential prey increase (Nprey), and that the reproductive fitness of a predator is directly related to its consumption of prey, then we would expect the birthrate of the predator population (bpredator) to increase as the size of the prey population increases (Figure 12.2b). The result is a direct link between the availability of prey (size of the prey population, Nprey) and the growth rate of the predator population (dNpredator/dt).
In Chapter 11, we developed a logistic model of population growth. It is a model of intraspecific competition and density-dependent population regulation using the concept of carrying capacity, K. The carrying capacity represents the maximum sustainable population size that can be supported by the available resources. The carrying capacity functions to regulate population growth in that as the population size approaches K, the population growth rate approaches zero (dN/dt = 0).
When individuals of two different species share a common limiting resource that defines the carrying capacity, there is potential for competition between individuals of the two species (interspecific competition). For example, let’s define a population of a grazing antelope inhabiting a grassland as N1, and the carrying capacity of the grassland to support that population as K1 (the subscript 1 refers to species 1). The logistic model of population growth (see Section 11.1) would then be:
dN1/dt = r1N1(1 − N1/K1)dN1/dt = r1N1(1 − N1/K1)
Now let’s assume that a second species of antelope inhabits the same grassland, and to simplify the example, we assume that individuals of the second species—whose population we define as N2—have the same body size and exactly the same rate of food consumption (grazing of grass) as do individuals of the first species. As a result, when we evaluate the role of density-dependent regulation on the population of species 1 (N1), we must now also consider the number of individuals of species 2 (N2) because individuals of both species feed on the grass that defines the carrying capacity of species 1 (K1). The new logistic model for species 1, will be:
dN1/dt = r1N1(1 − (N1 + N2)/K1)dN1/dt = r1N1(1 − (N1 + N2)/K1)
For example, if the carrying capacity of the grassland for species 1 is 1000 individuals (K1 = 1000)—because species 2 draws on the exact same resource in exactly the same manner—the combined carrying capacity of the grassland is also 1000. If there are 250 individuals of species 2 (N2 = 250) living on the grassland, it effectively reduces the carrying capacity for species 1 from 1000 to 750 (Figure 12.3a). The population growth rate of species 1 now depends on the population sizes of both species 1 and 2 relative to the carrying capacity (Figure 12.3b). Although we have defined the two antelope species as being identical in their use of the limiting resource that defines the carrying capacity, this is not always the case. In reality, it is necessary to evaluate the overlap in resource use and quantify the equivalency of one species to another (see Quantifying Ecology 12.1).
In all cases in which individuals of two species interact, the nature of the interaction can be classified qualitatively as neutral, positive, or negative, and the influence of the specific interaction can be evaluated in terms of its impact on the survival or reproduction of individuals within the populations. In the discussion that follows, we develop quantitative models to examine how the diversity of species interactions outlined in Table 12.1 influence the combined population dynamics of the species involved (Chapters 13, 14, and 15). In all cases, these models involve quantifying the per capita effect of interacting individuals on the birthrates and death rates of the respective populations.
Quantifying Ecology 12.1 Incorporating Competitive Interactions in Models of Population Growth
When individuals of two different species (represented as populations N1 and N2) share a common limiting resource that defines the carrying capacity for each population (K1 and K2), there is potential for competition between individuals of the two species (interspecific competition). Thus, the population density of both species must be considered when evaluating the role of density-dependent regulation on each population. In Section 12.2, we gave the example of two species of antelope that share the common limiting food resource of grass. We assumed that individuals of the two species were identical in their food selection and the rate at which they feed, therefore, with respect to the carrying capacity of the grassland, individuals of the two species are equivalent to each other; that is, in resource consumption one individual of species 1 is equivalent to one individual of species 2. As a result, when evaluating the growth rate of species 1 using the logistic model of population growth, it is necessary to include the population sizes of both species relative to the carrying capacity (see Figure 12.4):
dN1/dt = r1N1(1 − (N1 + N2)/K1)dN1/dt = r1N1(1 − (N1 + N2)/K1)
However, two species, even closely related species, are unlikely to be identical in their use of resources. So it is necessary to define a conversion factor that can equate individuals of species 2 to individuals of species 1 as related to the consumption of the shared limited resource. This is accomplished by using a competition coefficient, defined as a, that quantifies individuals of species 2 in terms of individuals of species 1 as related to the consumption of the shared resource. Using the example of two antelope species, let us now assume that both species still feed on the same resource (grass), however, individuals of species 2 have on average only half the body mass of individuals of species 1 and therefore consume grass at only half the rate of species 1. Now an individual of species 2 is only equivalent to one-half an individual of species 1 with respect to the use of resources. In this case, a = 0.5, and we can rewrite the logistic model for species 1 shown previously as:
dN1/dt = r1N1(1 − (N1 + αN2)/K1)dN1/dt = r1N1(1 − (N1 + αN2)/K1)
Because in Section 12.2 we defined the carrying capacity of the grassland for species 1 as K1 = 1000, we can substitute the values of a and K1 in the preceding equation:
dN1/dt = r1N1(1 − (N1 + 0.5N2)/1000)dN1/dt = r1N1(1 − (N1 + 0.5N2)/1000)
Now the growth rate of species 1 (dN1/dt) approaches zero as the combined populations of species 1 and 2, represented as N1 + 0.5N2, approach a value of 1000 (the value of K1).
We have considered how to incorporate the effects of competition from species 2 into the population dynamics of species 1 using the competition coefficient a, but what about the effects of species 1 on species 2? The competition for food resources (grass) will also function to reduce the availability of resources to species 2. We can take the same approach and define a conversion factor that can equate individuals of species 1 to individuals of species 2, defined as b. Because individuals of species 1 consume twice as much resource (grass) as individuals of species 2, it follows that an individual of species 1 is equivalent to 2 individuals of species 2; that is, b = 2.0. It also follows that if individuals of species 2 require only half the food resources as individuals of species 1, then the carrying capacity of the grassland for species 2 should be twice that for species 1; that is, K2 = 2000. The logistic growth equation for species 2 is now:
dN2/dt = r2N2(1 − (N2 + βN1)/K2)dN2/dt = r2N2(1 − (N2 + βN1)/K2)
or, substituting the values for b and K2
dN2/dt = r2N2(1 − (N2 + 2.0N1)/2000)dN2/dt = r2N2(1 − (N2 + 2.0N1)/2000)
We now have a set of equations that can be used to calculate the growth of the two competing species that considers their interaction for the limiting food resource. We explore this approach in more detail in the following chapter (Chapter 13).
In the example of the two hypothetical antelope species presented previously, the estimation of the competition coefficients (a and b) appear simple and straightforward. Both species are identical in their diet and differ only in the rate at which they consume the resource (which is defined as a simple function of their relative body masses). In reality, even closely related species drawing on a common resource (such as grazing herbivores) differ in their selection (preferring one group of grasses of herbaceous plants over another), foraging behavior, timing of foraging, and other factors that influence the nature of their relative competitive effects on each other. As such, quantifying species interactions, such as resource competition, can be a difficult task, as we shall see in the following chapter (Chapter 13, Interspecific Competition).
12.3 Species Interactions Can Function as Agents of Natural Selection
For a number of reasons, the interaction between two species will not influence all individuals within the respective populations equally. First, interactions among species involve a diverse array of physiological processes and behavioral activities that are influenced by phenotypic characteristics (physiological, morphological, and behavioral characteristics of the individuals). Secondly, these phenotypic characteristics vary among individuals within the populations (see Chapter 5). Therefore, the variations among individuals within the populations will result in differences in the nature and degree of interactions that occur. For example, imagine a species of seed-eating bird that feeds on the seeds of a single plant species. Individuals of the plant species exhibit a wide degree of variation in the size of seeds that they produce. Some individuals produce smaller seeds, whereas others produce larger seeds (Figure 12.4a), and seed size is a heritable characteristic (genetically determined). Seed size is important to the birds because the larger the seed, the thicker the seed coat, and the more difficult it is for a bird to crush the seed with its bill. If the seed coat is not broken, the seed passes through the digestive system undigested and provides no food value to the bird. As a result, birds actively select smaller seeds in their diet (Figure 12.4c). In doing so, the birds are decreasing the reproductive success of individual plants that produce small seeds while increasing the relative fitness of those individuals that produce larger seeds. The net effect is a shift in the distribution of phenotypes in the plant population to individuals that produce larger, harder seeds (Figure 12.4d). In this situation, the bird population (and pattern of seed predation) is functioning as an agent of natural selection, increasing the relative fitness of one phenotype over another (see Section 5.6). Over time, the result represents a directional change in the genetic structure of the population (gene frequencies), that is, the process of evolution (Chapter 5).
In this example, the predator functions as an agent of natural selection, decreasing the reproduction for certain phenotypes (small seed-producing individuals) within the plant population and increasing the relative fitness of other phenotypes (large seed-producing individuals). But the shifting distribution of phenotypes within the plant population and the resulting change in the distribution of food resources will in turn have a potential influence on the predator population (Figure 12.4b). The directional selection for increased seed size within the plant population decreases the relative abundance of smaller seeds, effectively decreasing the availability of food resources for birds with smaller bill sizes. If the birds with smaller bills are unable to crack the larger seeds, these individuals will experience a decreased probability of survival and reproduction, which increases the relative fitness of individuals with larger bill size. The shift in the distribution of phenotypes in the plant population, itself a function of selective pressures imposed by the bird population, now functions as an agent of natural selection in the predator (bird) population. The result is a shift in the distribution of phenotypes and associated gene frequencies within the bird population toward larger bill size (Figure 12.4e). This process in which two species undergo reciprocal evolutionary change through natural selection is called coevolution .
Unlike adaptation to the physical environment, adaptation in response to the interaction with another species can produce reciprocal evolutionary responses that either thwart (counter) these adaptive changes, as in the previous example, or in mutually beneficial interactions, magnify (reinforce) their effect. An example of the latter can be found in the relationship between flowering plants and their animal pollinators. Many species of flowering plants require the transfer of pollen from one individual to another for successful fertilization and reproduction (outcrossing; Figure 12.5). In some plant species, this is accomplished through passive transport by the wind, but many plants depend on animals to transport pollen between flowers. By attracting animals, such as insects or birds, to the flower, pollen is spread. When the animal comes into contact with the flower, pollen is deposited on its body, which is then transferred to another individual as the animal travels from flower to flower. This process requires the plant species to possess some mechanism to attract the animal to the flower. A wide variety of characteristics has evolved in flowering plants that function to entice animals through either signal or reward. Signals can involve brightly colored flowers or scents. The most common reward to potential pollinators is nectar, a sugar-rich liquid produced by plants, which serves no purpose for the individual plant other than to attract potential pollinators. Nectar is produced in glands called nectaries, which are most often located at the base of the floral tube (see Figure 12.5).
The relationship between nectar-producing flowers and nectar-feeding birds provides an excellent example of the magnification of reciprocal evolutionary responses—coevolution—resulting from a mutually beneficial interaction. The elongated bill of hummingbirds distinguishes them from other birds and is uniquely adapted to the extraction of nectar (Figure 12.6). Their extremely long tongues are indispensable in gaining nectar from long tubular flowers. Let us assume a species of hummingbird feeds on a variety of flowering plants within a tropical forest but prefers the flowers of one plant species in particular because it produces larger quantities of nectar. Thus, the reward to the hummingbird for visiting this species is greater than that of other plant species in the forest. Now assume that flower size (an inherited characteristic) varies among individuals within the plant population and that an increase in nectar production is associated with elongation of the floral tube (larger flower size). Individual plants with larger flowers and greater nectar production would have an increased visitation rate by hummingbirds. If this increase in visitation rate results in an increase in pollination and reproduction, the net effect is an increase in the relative fitness of individuals that produce larger flowers, shifting the distribution of phenotypes within the plant population. The larger flower size and longer floral tube, however, make it more difficult to gain access to the nectar. Individual hummingbirds with longer bills are more efficient at gaining access, and bill size varies among individuals within the population. With increased access to nectar resources, the relative fitness of longer-billed individuals increases at the expense of individuals with shorter bills. In addition, any gene mutation that results in increasing bill length with be selected for because it will increase the fitness of the individual and its offspring (assuming that they exhibit the phenotype). The genetic changes that are occurring in each population are reinforced and magnified by the mutually beneficial interaction between the two species. The plant characteristic of nectar production is reinforced and magnified by natural selection in the form of improved pollination success by the plant and reproductive success by the hummingbird. In turn, the increased flower size and associated nectar production functions as a further agent of natural selection in the bird population, resulting in an increase in average bill size (length). One consequence of this type of coevolutionary process is specialization, wherein changes in phenotypic characteristics of the species involved function to limit the ability of the species to carry out the same or similar interactions with other species. For example, the increase in bill size in the hummingbird population will function to limit its ability to efficiently forage on plant species that produce smaller flowers, restricting its feeding to the subset of flowering plants within the tropical forest that produces large flowers with long floral tubes (see Figure 12.6). In the extreme case, the interaction can become obligate, where the degree of specialization in phenotypic characteristics results in the two species being dependent on each other for survival and successful reproduction. We will examine the evolution of obligate species interactions in detail later (Chapter 15).
Unlike the case of mutually beneficial interactions in which natural selection functions to magnify the intensity of the interaction, interactions that are mutually negative to the species involved can lead to the divergence in phenotypic characteristics that function to reduce the intensity of interaction. Such is the case when the interaction involves competition for essential resources. Consider the case wherein two species of seed-eating birds co-occur on an island. The two populations differ in average body and bill size, yet the two populations overlap extensively in the range of these phenotypic characteristics (Figure 12.7a) and subsequently in the range of seed sizes on which they forage (Figure 12.7b). The selection of seeds by individual birds is related to body and bill size. Smaller individuals are limited to feeding on the smaller, softer seeds, whereas only larger individuals are capable of cracking the larger, harder seeds. Although larger birds are able to feed on smaller seeds, it is energetically inefficient; therefore, their foraging is restricted to relatively larger seeds (see Section 5.8 for an example).
Seed resources on the island are limited relative to the populations of the two species, hence, competition is often intense for the intermediate-sized seeds for which both species forage. If competition for intermediate-sized seeds functions to reduce the fitness of individual birds that depend on these resources, the result would be reduced survival and reproductive rates for larger individuals of the smaller species and smaller individuals of the larger species (Figure 12.7c). This result represents a divergence in the average body and bill size for the two populations that functions to reduce the potential for competition between the two species (Figure 12.7d).
12.4 The Nature of Species Interactions Can Vary across Geographic Landscapes
We have examined how natural selection can result in genetic differentiation, that is, genetic differences among local populations. Species with wide geographic distributions generally encounter a broader range of physical environmental conditions than species whose distribution is more restricted. The variation in physical environmental conditions often gives rise to a corresponding variation in phenotypic characteristics. As a result, significant genetic differences can occur among local populations inhabiting different regions (see Section 5.8 for examples). In a similar manner, species with wide geographic distributions are more likely to encounter a broader range of biotic interactions. For example, a bird species such as the warbling vireo (Vireo gilvus) that has an extensive geographic range in North America, extending from northern Canada to Texas and from coast to coast, is more likely to encounter a greater diversity of potential competitors, predators, and pathogens than will the cerulean warbler (Dendroica cerula), whose distribution is restricted to a smaller geographic region of the eastern United States (see Figure 17.2 for distribution maps). Changes in the nature of biotic interactions across a species geographic range can result in different selective pressures and adaptations to the local biotic environment. Ultimately, differences in the types of species interactions encountered by different local populations can result in genetic differentiation and the evolution of local ecotypes similar to those that arise from geographic variations in the physical environment (see Section 5.8 for examples of the latter). The work of Edmund Brodie Jr. of Utah State University presents an excellent example.
Brodie and colleagues examined geographic variation among western North American populations of the garter snake (Thamnophis sirtalis) in their resistance to the neurotoxin tetrodotoxin (TTX). The neurotoxin TTX is contained in the skin of newts of the genus Taricha on which the garter snakes feed (Figure 12.8a). These newts are lethal to a wide range of potential predators, yet to garter snakes having the TTX-resistant phenotype, the neurotoxin is not fatal. Both the toxicity of newts (TTX concentration in their skin) and the TTX resistance of garter snakes vary geographically (Figure 12.8b). Previous studies have established that TTX resistance in the garter snake is highly heritable (passed from parents to offspring), so if TTX resistance in snakes has co-evolved in response to toxicity of the newt populations on which they feed, it is possible that levels of TTX resistance exhibited by local populations of garter snakes will vary as a function of the toxicity of newts on which they feed. The strength of selection for resistance would vary as a function of differences in selective pressure (the toxicity of the newts).
To test this hypothesis, the researchers examined TTX resistance in more than 2900 garter snakes from 40 local populations throughout western North America, as well as the toxicity of newts at each of the locations. The researchers found that the level of TTX resistance in local populations varies with the presence of toxic newts. Where newts are absent or nontoxic (as is the case on Vancouver Island, British Columbia), snakes are minimally resistant to TTX. In contrast, levels of TTX resistance increased more than a thousand-fold with increasing toxicity of newts (see Figure 12.8b). Brodie and his colleagues found that for local populations, the level of resistance to TTX varies as a direct function of the levels of TTX in the newt population on which they prey (Figure 12.8c). The resistance and toxicity levels match almost perfectly over a wide geographic range, reflecting the changing nature of natural selection across the landscape.
In some cases, even the qualitative nature of some species interactions can be altered when the background environment is changed. For example, mycorrhizal fungi are associated with a wide variety of plant species (see Chapter 15, Section 15.11). The fungi infect the plant root system and act as an extension of the root system. The fungi aid the plant in the uptake of nutrients and water, and in return, the plant provides the fungi with a source of carbon. In environments in which soil nutrients are low, this relationship is extremely beneficial to the plant because the plant’s nutrient uptake and growth increase. (Figure 12.9a). Under these conditions, the interaction between plant and fungi is mutually beneficial. In environments in which soil nutrients are abundant, however, plants are able to meet nutrient demand through direct uptake of nutrients through their root system. Under these conditions, the fungi are of little if any benefit to the plant; however, the fungi continue to represent an energetic cost to the plant, reducing its overall net carbon balance and growth (Figure 12.9b). Across the landscape, the interaction between plant and fungi changes from mutually beneficial (++) to parasitic (+−) with increasing soil nutrient availability.
Interpreting Ecological Data
1. Q1. Given the preceding figure, is there a net benefit to the plant of having an association with mycorrhizal fungi under conditions of low soil nutrients?
2. Q2. At which point along the gradient of soil nutrient concentration is the net benefit to the plant equal to zero (costs = benefits)?
12.5 Species Interactions Can Be Diffuse
The examples of species interactions that we have discussed thus far focus on the direct interaction between two species. However, most interactions (e.g., predator–prey, competitors, mutually beneficial) are not exclusive nor involve only two species. Rather, they involve a number of species that form diffuse associations. For example, most terrestrial communities are inhabited by an array of insect, small mammal, reptile, and bird species that feed on seeds. As a result, there is a potential for competition to occur among any number of species that draw on this limited food resource. Similarly, there are numerous examples of highly specific mutually beneficial interactions between two species (see Figure 12.6); however, most mutually beneficial interactions are somewhat diffuse. In plant-pollinator interactions, most plants are pollinated by multiple animal species, and each animal species pollinates multiple plant species. For example, honey bees (Apis melifera) are known to visit the flowers of hundreds of plant species, and white mangrove (Laguncularia racemosa) is visited by more than 50 different insect species. Species of plants and pollinators form pollination networks, and the resulting selective forces that reinforce the mutually beneficial interactions are likewise diffuse (Figure 12.10). This process in which a network of species undergoes reciprocal evolutionary change through natural selection is referred to as diffuse coevolution.
In diffuse coevolution, groups of species interact with other groups of species, leading to natural selection and evolutionary changes that cannot be identified as examples of specific, pairwise coevolution between two species. For example, the evolution of resistance to the neurotoxin TTX by garter snakes presented in the previous section (see Figure 12.8) is in response to TTX concentrations in the skin of newts of the genus Taricha on which they prey. This genus consists of three species and four subspecies of western newts, so the evolution of resistance by snake populations is not in response to its interaction with a single species but rather a group of closely related species that all produce the neurotoxin and on which they feed. Likewise, the evolution of toxicity by members of the genus Taricha provides a defense mechanism to avoid predation by an array of vertebrate predators, not just a single species of predator.
In the chapters that follow, we will explore an array of examples of co-evolution. Some represent highly specialized co-adaptations between two species in which the interaction has become obligate (essential to the survival of the two species involved), whereas others represent the result of generalized relationships between groups of species—diffuse relationships between competitors, predator and prey, or mutualists.
12.6 Species Interactions Influence the Species’ Niche
The diversity of species inhabiting our planet reflect different evolutionary solutions to the same basic processes of assimilation and reproduction, and that the characteristics that distinguish each species often reflect adaptations (products of natural selection) that allow individuals of that species to survive, grow, and reproduce under a particular set of environmental conditions (see Part Two). As such, each species may be described in terms of the range of physical and chemical conditions under which it persists (survives and reproduces) and the array of essential resources it uses. This characterization of a species is referred to as its ecological niche .
The concept of the ecological niche was originally developed independently by ecologists Joseph Grinnell (1917, 1924) and Charles Elton (1927), who proposed slightly different meanings for the term. Grinnell’s definition centered on the concept of habitat (see Section 7.14, Figure 7.25) and the limitations imposed by the physical environment (as discussed in Chapters 6 and 7), whereas Elton emphasized the role of the species in the context of the community (species interactions). The limnologist G. Evelyn Hutchinson (1957) later expanded the concept of the niche by proposing the idea of the niche as a multidimensional space called a hypervolume, in which each axis (dimension) is defined by a variable relating to the specific resource need or environmental factor that is essential for a species’ survival and successful reproduction. We can begin to visualize this concept of a multidimensional niche by modeling a three-dimensional one—a niche defined by three resources or environmental variables: temperature, salinity, and pH (Figure 12.11). For each axis there is a range of values (conditions) that permit a species to survive and reproduce (or in Hutchinson’s own words, “for the population to persist indefinitely”). For example, in Chapters 6 and 7 we presented numerous examples of the response of plant (Figures 5.19– 5.22) and animal (Figures 7.14 and 7.18) species to variation in environmental temperature. Each of these figures represents a description of the species’ niche for the single dimension (variable) of environmental temperature. Likewise, the distribution of seed sizes used by the three species of Darwin’s ground finch inhabiting the Galapagos Islands presented in Figure 5.20 represents a description of the species’ niches for the single dimension of food resource size.
Hutchinson referred to this hypervolume that defines the environmental conditions under which a species can survive and reproduce as the fundamental niche . The fundamental niche, sometimes referred to as the physiological niche, provides a description of the set of environmental conditions under which a species can persist. As we have discussed in the previous sections, however, a population’s response to the environment may be modified by interactions with other species. Hutchinson recognized that interactions such as competition may restrict the environment in which a species can persist and referred to the portion of the fundamental niche that a species actually exploits as a result of interactions with other species as the realized niche (Figure 12.12).
An illustration of the difference between a species’ fundamental and realized niche is provided in the work of J. B. Grace and R. G. Wetzel of the University of Michigan. Two species of cattail (Typha) occur along the shorelines of ponds in Michigan. One species, Typha latifolia (wide-leaved cattail), dominates in the shallower water, whereas Typha angustifolia (narrow-leaved cattail) occupies the deeper water farther from shore. When these two species grew along the water depth gradient in the absence of the other species, a comparison of the results with their natural distributions revealed how competition influences their realized niche (Figure 12.13). Both species can survive in shallow waters, but only the narrow-leaved cattail, T. angustifolia, can grow in water deeper than 80 centimeters (cm). When the two species grow together along the same gradient of water depth, their distributions, or realized niches, change. Even though T. angustifolia can grow in shallow waters (0–20 cm depth) and above the shoreline (−20 to 0 cm depth), in the presence of T. latifolia it is limited to depths of 20 cm or deeper. Individuals of T. latifolia outcompete individuals of T. angustifolia for the resources of nutrients, light, and space, limiting the distribution of T. angustifolia to the deeper waters. Note that the maximum abundance of T. angustifolia occurs in the deeper waters, where T. latifolia is not able to survive.
As originally proposed, the concept of realized niche focused on how the fundamental niche of a species is restricted as a result of negative interactions with other species. Competition can function to restrict the range of resources or environmental conditions used by a species, as in the example of the distribution of T. angustifolia along the gradient of water depth presented in the previous example. In other cases, the presence of predators or pathogens may restrict the range of behaviors exhibited by a potential prey species, the resources it uses, or ultimately the habitats in which it can persist (see Chapter 14, Section 14.8 for an example of changes in foraging behavior under the risk of predation). As such, the realized niche of a species was seen as a subset of the broader, more inclusive range of conditions and resources that the species could use in the absence of interactions with other species. In more modern times, however, ecologists have come to appreciate the importance of positive interactions, particularly mutually beneficial interactions, and how this class of interactions can modify the species’ fundamental niche. By either directly or indirectly enhancing the probabilities of survival and reproduction of individuals in the participating populations, interactions that are either beneficial to one species and neutral to the other (commensalism), or mutually beneficial to both (mutualism), can function to expand the range of environmental conditions or resources under which a species can persist. In this case, the realized niche of the species is greater (more expansive) than that of its fundamental niche. For example, nitrogen-fixing Rhizobium bacteria associated with the root systems of certain plant species provide a direct source of mineral nitrogen to the plant, enabling it to persist in soils that have low mineral nitrogen content (see Section 15.11 for a detailed discussion of this mutualistic interaction). In the absence of interaction with the bacteria, the plants are restricted to a narrower range of soils that have higher availability of mineral nitrogen.
Although the realized niche is by definition a product of species interactions, over evolutionary time, biotic interactions can play a critical role in defining a species’ fundamental niche. The previous discussion of species’ adaptation to the environment focused almost exclusively on the role of the physical and chemical environments as agents of natural selection (see Part Two). We now have seen, however, that species interactions also function as agents of natural selection, and phenotypic characteristics often reflect adaptations to these selective pressures. As such, over evolutionary timescales, species interactions can have a major role in determining the range of physical and chemical conditions under which species can persist (survive and reproduce) and the array of essential resources they use, that is, the species’ ecological niches.
12.7 Species Interactions Can Drive Adaptive Radiation
Adaptive radiation is the process by which one species gives rise to multiple species that exploit different features of the environment, such as food resources or habitats (see Section 5.9, Figure 5.22). Different features of the environment exert the selective pressures that push populations in various directions (phenotypic divergence); reproductive isolation, the necessary condition for speciation to occur, is often a by-product of the changes in morphology, behavior, or habitat preferences that are the actual objects of selection. Likewise, variations among local populations in biotic interactions can result in phenotypic divergence and therefore have the potential to function as mechanisms of adaptive radiation. Resource competition is often inferred as a primary factor driving phenotypic divergence. For example, species of the globeflower fly Chiastocheta present a unique case of adaptive radiation as a result of resource competition. At least six sister species of the genus Chiastocheta lay their eggs (oviposition) on the fruits of the globeflower, Trollius europaeus (Figure 12.14); however, the different species of globeflower flies differ in the timing of their egg laying. One species lays its eggs in 1-day-old flowers, whereas all the other species sequentially deposit their eggs throughout the flower life span. In a series of field experiments, Laurence Despres and Mehdi Cherif of Université Joseph Fourier (Grenoble, France) found evidence that supports the hypothesis that the evolutionary divergence of species of Chiastocheta was a result of disruptive selection on the timing of egg laying (reproduction). The researchers established that intense intraspecific competition occurs within each of the species, but differences in the timing of egg laying and larval development functions to minimize competition among species (the concept of resource partitioning will be examined in Chapter 13).
Although numerous studies have illustrated the role of competitive interactions in adaptive radiation, the importance of other interactions, such as mutualism or predation, remain largely unexplored. The research of Patrik Nosil and Bernard Crespi of Simon Fraser University (British Columbia, Canada), however, has shown that adaptive radiation can result from divergent adaptations to avoid predators. Nosil and Crespi’s research focused on two ecotypes (populations of the same species adapted to their local environments) of the stick insect Timema cristinae (see Section 5.8 and Chapter 5, Field Studies: Hopi Hoekstra for discussion of ecotypes). Timema walking sticks are wingless insects inhabiting southwestern North America. Individuals feed and mate on the host plants on which they reside. The two distinct ecotypes of Timema are adapted to feeding on different host plants, Ceanothus and Adenostoma. The two host plants differ strikingly in foliage form, with Ceanothus plants being relatively large and tree-like with broad leaves and Adenostoma plants being small and shrub-like with thin, needle-like leaves (Figure 12.15).
The two Timema ecotypes differ in 11 quantitative traits (see Figure 12.15), comprising aspects of color, color pattern, body size, and body shape. These differences between the two ecotypes appears to be a result of divergent selection. The different traits exhibited by each of the ecotypes appear to provide crypsis (avoidance of observation) from avian predators on the respective host-plant species. Field experiments were conducted to determine how differences in phenotypic traits influenced the survival rates of the two ecotypes on the two plant species. Each of the two Timema ecotypes was placed on each of the two host-plant species. The results of the experiment clearly indicated that the direction and magnitude of divergence in traits represent adaptations that function to reduce rates of predation on Timema on their respective host-plant species. The ecotypes of T. cristinae, like the example of the limnetic and benthic ecotypes of sticklebacks examined in Chapter 5, can be considered to represent an early stage of adaptive radiation because studies indicate that reproductive isolation is not complete (see Section 5.6, Figure 5.15).
Ecological Issues & Application Urbanization Has Negatively Impacted Most Species while Favoring a Few
As we will see in the chapters that follow, species interactions are ubiquitous in nature and play a fundamental role in the structuring of ecological communities. Perhaps no other interaction, however, has as great an impact on the diverse array of plants and animals that inhabit our planet as their interaction with the human species.
As we first presented in Chapter 9 (Ecological Issues & Applications), the primary cause of population declines and recent species extinctions is habitat loss as a result of human activities—namely, changing land-use patterns. There are two major land-use changes that are responsible for habitat loss in terrestrial environments: expanding agriculture and urbanization.
According to the Food and Agricultural Organization (FAO) United Nations’ statistics, at present some 11 percent (1.5 billion hectares) of the globe’s land surface (13.4 billion ha) is used in crop production (arable land and land under permanent crops), and even more land (3.2 to 3.6 billion ha) is used to raise livestock. Together, agricultural lands account for almost 40 percent of Earth’s land surface. The negative impacts of the expansion of agriculture to meet the needs of the growing human population have been central to the discussion of the decline of biological diversity on our planet, a topic we will examine in more detail in Chapter 26. The increasing urbanization of the human population over the past century (Figure 12.16), however, has led to the emergence of a new field of ecology—urban ecology—to study the ecology of organisms in the context of the urban environment.
Ecology has historically focused on “pristine” natural environments; however, by as early as the 1970s, many ecologists began turning their attention toward ecological interactions taking place in urban environments. What has emerged is a picture of species interactions dominated by humans, which negatively impacts most species and benefits only a few.
Estimates of urban land area vary widely from 0.5 to slightly more than 2.0 percent of the world’s land, depending on the criteria used to define urban development. Historically, cities have been compact areas with high population densities that grew slowly in their physical extent. Today, however, urban areas are expanding twice as fast as their populations. According to the United States Census Bureau, about 30 percent of the U.S. population currently lives in cities, whereas another 50 percent lives in the suburbs. More than 5 percent of the total surface area of the United States is covered by urban and other developed areas; this is more than the land covered by the combined totals of national and state parks.
The expansion of urbanization produces some of the greatest local extinction rates and frequently eliminates the large majority of native species. Eyal Shochat of Arizona State University’s Global Institute of Sustainability and colleagues used data from Phoenix, Arizona, and Baltimore, Maryland, to contrast the distribution of species in these two urban areas as compared to the surrounding natural ecosystems. Their findings show a general pattern of decline in the number of species in urban environments as compared to both surrounding agricultural and natural ecosystems (Figure 12.17).
Species vary in their ability to adapt to the often drastic physical changes along the gradient from rural to urban habitat. Moving from the rural landscape of natural ecosystems and cultivated lands into the suburban landscape, one moves through a heterogeneous mixture of residential areas, commercial centers, and the managed vegetation of parks and cemeteries. The main cause for the loss of species in these suburban environments is habitat alteration. Yet in contrast to the decline in the number of species, both suburban areas and urban centers are usually characterized by higher population densities of resident species as compared to adjacent natural lands. For example, in a study of population of northern cardinals (Cardinalis cardinalis) in the metropolitan area of Columbus, Ohio, and surrounding forested landscape of central Ohio, Lionel Leston and Amanda Rodewald of Ohio State University found that birds were four times more abundant in urban than rural forests. Their research showed that food abundance was as much as four times greater in the urban habitat as compared to the forests of the surrounding region because exotic vegetation, refuse, and bird feeders may all provide food sources for birds in these urban environments.
Some mammals, such as raccoons (Procyon lotor), skunks (Mephitis mephitis), and rabbits (Sylvilagus spp.) have also benefited from the spread of the suburban landscape, finding shelter beneath sheds and porches, and an abundance of food—for raccoons, garbage; for skunks, insects and larvae on lawns and in gardens; and for rabbits, an abundance of high quality food plants in gardens and flowerbeds. Larger species, rapidly adapting to human presence, are moving into the suburban landscape and dramatically increasing in number. White-tailed deer (Odocoileus virginiaus), carriers of Lyme disease, find an abundance of forage on grass, shrubs, and gardens. Resident Canada geese (Branta canadensis), attracted to large open areas of grass—including golf courses and parks—create both a nuisance and health problems. In recent years, coyotes (Canis latrans), attracted by garbage and small prey including rodents and pets (cats and small dogs), are becoming more common in suburban areas. Even black bears (Ursus americanus) are attracted to backyard bird feeders and dumpsters in suburban areas adjacent to forested, rural landscapes.
In addition to increased abundance and predictability of food resources, recent research indicates that a reduction in predator populations in urban environments favors resident species. Evidence has been gathered that supports the idea that urban environments are safer for some species than are rural habitats. Both birds and squirrels in urban environments benefit from reduced nest predation and are able to spend a greater proportion of their time foraging compared with individuals in the surrounding natural ecosystems, indicating that the urban habitat is less risky than the surrounding rural habitats.
Species adapted to habitats along the suburban gradient drop out as they come to urban centers where habitat changes sharply. Vegetation is limited to scattered parks, some tree-lined streets, and vacant lots. Species that benefit from the habitat provided by these core urban centers are often referred to as “urban exploiters.” Among plants, urban exploiters tend to be ruderal species (see discussion of plant life history classification in Section 10.13) that can tolerate high levels of disturbance. Examples include wind-dispersed weeds (grasses and annuals) that colonize abandoned lots and properties, and plants that can grow in and around pavement.
Bird species that thrive in urban habitats are often adapted to nesting in environments that are similar to the cityscape. For example, species that use cliff-like rocky areas, such as the rock dove (pigeons, Columba livia) and peregrine falcon (Falco peregrinus), are “pre-adapted” to using the barren concrete edifices of urban buildings, whereas cavity-nesting species, such as the house sparrow (Passer domesticus), house finch (Haemorhous mexicanus), and European starling (Sturnus vulgaris) are able to inhabit human dwellings.
Mammalian urban exploiters consist of species that are able to find shelter in human dwellings and exploit the rich food source provided by refuse, such as the house mouse (Mus musculus), the black rat (Rattus rattus), and brown rat (Norway rat: Rattus norvegicus).
Urban environments typically have more in common with other cities than with adjacent natural ecosystems, so species that flourish in urban habitats are often not native to the region. Rather, these species tend to disperse from city to city, typically with assistance—either intentionally or unintentionally—from humans (see Chapter 8, Ecological Issues & Applications). Species such as rock doves, starlings, house sparrows, Norway rats, and the house mouse are found in all cities in Europe and North America. As a result, many studies have found that the number (and proportion) of non-native species tends to increase as you move from rural habitats toward urban centers. In general, the proportion of species that is non-native goes from less than a few percent in rural areas to more than 50 percent at the urban core.
This combination of negative interactions with the majority of native species—while enhancing a small subset of often non-native species, which we have manipulated to serve our needs, facilitated through dispersal, or created urban environments in which their populations flourish—is resulting in what urban and conservation ecologists refer to as biotic homogenization, which is the gradual replacement of regionally distinct ecological communities with cosmopolitan communities that reflect the increasing global activity of humans.
By designating the positive effect of one species on another as +, a detrimental effect as −, and no effect as 0, we can develop a classification of possible interactions between two co-occurring species: (00) neutral; (0+) commensalism; (++) mutualism; (0−) amensalism; (−−) competition; (+−) predation, parasitism, or parasitoidism.
Population Dynamics 12.2
Species interactions typically involve the interaction of individual organisms within the respective populations. By influencing individuals’ probabilities of survival or reproduction, interactions influence the collective properties of birth and death at the population level, and in doing so, influence the dynamics of the respective populations.
Natural Selection 12.3
Phenotypic variations among individuals within the populations can result in differences in the nature and degree of interactions that occur. These phenotypic differences may influence the relative fitness of individuals within the populations in the degree of interaction, resulting in the process of natural selection. The process in which two species undergo reciprocal evolutionary change through natural selection is called coevolution. Mutually beneficial interactions typically serve to reinforce the phenotypic changes that result from the species interaction, and mutually detrimental interactions typically result in phenotypic changes that function to reduce the intensity of interaction.
Geographic Variation 12.4
Species with wide geographic distributions are more likely to encounter a broader range of biotic interactions. Changes in the nature of biotic interactions across a species’ geographic range can result in different selective pressures and adaptations to the local biotic environment. Ultimately, differences in the types of species interactions encountered by different local populations can result in genetic differentiation and the evolution of local ecotypes.
Diffuse Interactions 12.5
Most interactions are not exclusive involving only two species but rather involve a number of species that form diffuse associations.
The range of physical and chemical conditions under which a species can persist and the array of essential resources it uses define its ecological niche. The ecological niche of a species in the absence of interactions with other species is referred to as the fundamental niche. The species’ realized niche is its ecological niche as modified by its interactions with other species within the community. Species interactions can function to either restrict or expand the fundamental niche of a species dependent on whether the interaction is detrimental or beneficial.
Adaptive Radiation 12.7
Variations among local populations in biotic interactions can result in phenotypic divergence and therefore have the potential to function as mechanisms of adaptive radiation, if the divergence in phenotypic characteristics results in reproductive isolation.
Urban Ecology Ecological Issues & Applications
Urban ecology is the study of the ecology of organisms in the context of the urban environment. Increased urbanization has led to a decline in habitat and loss of many native species, while providing habitat for other, often non-native species.