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.