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.