Evolution of competitors in a food web context
Consider a food web in which a consumer of two substitutable resources is itself eaten by a specialist predator or is limited by resources other than food, such as nesting sites.
What is the ecological and evolutionary effect of introducing a second consumer of the same two resources? If they can coexist, the second species will have no effect on the first, whose population size is set by its predator. After the introduction, changes in the resource-independent mortality rate of either consumer will not have any effect on the equilibrium abundance of the other. These generalizations hold provided the changes are not so great as to eliminate coexistence or cause extinction of the specialist predator. Regardless of this lack of effect on the equilibrium abundance of species 1, consumer 2 will alter the relative abundances of the resources when it has a different set of consumption rates of the resources than does consumer 1. If consumer 1 has genetic variation for traits influencing the relative capture rates of resources that have changed in relative abundance, then it will evolve. With substitutable resources, this evolution will usually be divergence, and will result in reduced overlap in resource use. If the resources have nutritional interactions, non-divergent displacement may occur. More details are provided in Abrams (2012). The important point in this example is that the evolutionary change occurs independently of the change in population size of the first consumer that is caused by addition of the second.Chapter 7 discussed a model having two species at each of three trophic levels. This example, with strict resource limitation at each level, exhibited a complete lack of population response of species on the middle level to changes in any parameter that did not directly affect their interaction with the top two predators (e.g., the densityindependent mortality rate). However, such changes do alter the abundances of the predators, and are therefore likely to alter traits that affect vulnerabilities to these two predators.
In this case, evolutionary responses to an altered mortality in one of the mid-level consumers will reflect apparent competition, but not resource competition.In larger food webs, and with age- or size- structured populations, it seems likely that a much wider range of evolutionary responses to competition is possible. However, it is also possible that some of the varieties of evolutionary responses observed in models with only a few species are rare in cases with many more species. It remains to be seen how far current results based on simple cases will allow us to understand the evolutionary responses in naturally occurring food webs.
The above treatment of competitive coevolution has been based on an underlying conceptualization that is more appropriate for animals than for plants. It has also concentrated on consumers and resources that have a stable equilibrium. One of the non-equilibrium situations that has been studied theoretically is temporal variation in seedling establishment, which is based on plant competition during the seedling stage in a temporally variable environment (Snyder and Adler 2011; Abrams et al. 2013). The latter work suggests that divergence in environmental sensitivities is likely to evolve under conditions similar to those under which divergence evolves in traditional resource-based models. The pattern of seasonal variation in resource use (as in Chapter 8) is likely to evolve, but this has yet to be explored (although Yamamichi and Letten (2021) consider a related scenario).
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