11.l Evolution’s many effects on interspecific competition

‘Evolutionary effect' is a phrase that encompasses a wide range of potential initi­ating factors and responding variables in interacting species. Species can influence each other's evolution via pathways of effect involving changes in population size or changes in any trait affecting the interaction.

Either a change in population size or an evolutionary change in traits related to resource consumption in one consumer can have a wide range of potential effects on the population sizes and resource-use traits of other consumers. This generaliza­tion might have been expected because competition is an indirect interaction that has many types of transmitting entities (resources), and species differ widely in their con­sumption processes. The chain of effects from an initial evolutionary change in one consumer almost always involves alterations in the abundances and in some of the characteristics of its resources. The evolutionary changes in other consumers pro­duced by an altered population size or mean trait value of a focal consumer may depend in magnitude, and possibly sign, on the size of the initial change in the focal consumer. This should not be surprising because we know that the ecological effect on the other consumers usually differs depending on the magnitude of the change.

In contrast to the previous paragraph, most thinking about competitive coevolu­tion has been based on systems without explicit resources.

Competition Theory in Ecology. Peter A. Abrams, Oxford University Press. © Peter A. Abrams (2022).

DOI: 10.1093∕oso∕9780192895523.003.0011 that do not alter per capita consumption rates can also be affected. Part of the rea­son for the narrow range of predictions in previous works has been their reliance on theory in which the population dynamics of the competitors are described by a Lotka-Volterra (LV) model. Models that explicitly included resources have raised the possibility that evolution of biotic resources can play a major role in determining the evolutionary impact of one consumer species on another. They suggest that the popu­lation dynamical effects of adaptive evolutionary responses to competition may also be quite diverse, and that this type of adaptive change can decrease the population size of the adapting species.

In analysing the evolution of competitors, it is important to realize that competing species in the real world are parts of larger communities. The most common food web model of competition has two consumers and two resources, and most models of this type have assumed that there is no interaction between the two resources. The lack of interaction is reasonable (although not always justified) if the resources are abi­otic. However, if the resources themselves are consumers, logical consistency would seem to require consideration of between-resource competition. If the two resources compete with each other, the ecological effects of the two consumers (in the absence of evolution) are often mutually positive, at least over some range of perturbations (Levine 1976; Vandermeer 1980; Abrams and Nakajima 2007).

One of the factors that can influence the evolutionary response to competition is the presence of behaviour or other forms of adaptive plasticity. In many cases, behaviour alters the selective regime to which an individual is exposed. The inter­action of rapid adaptation with both evolution and population dynamics may lead to qualitatively different outcomes (e.g., Abrams 2006b), both ecologically and evo­lutionary. Behaviour is particularly likely to be important in spatially structured environments with heterogeneous patches. In this scenario, the behaviour determines the selective regime experienced by an individual. Rapid adaptive processes are likely to operate in biotic resources as well as in the consumers, and this could also cre­ate opportunities for qualitatively different evolutionary responses of the consumer species to each other's abundances or trait values.

Like many other parts of competition theory, the evolutionary subset has been unduly influenced by both the models and empirical work that appeared early in the history of the field. Empirical studies have concentrated on traits related to food item size in animals. The theoretical literature has followed MacArthur's lead in focusing on the LV model and evolution of a single trait that determines the relative capture rates of many resources. The tradition (again established by MacArthur) has been to assume a ‘utilization curve' of a fixed shape (generally Gaussian), whose mean value is the only trait that can evolve.

Whether or not resources have been included in the model of coevolution, most previous theory has been based on a scenario in which the introduction of a new consumer species and its growth to an ecological equilibrium population size initiate evolutionary change in a resident consumer species. The resident species is assumed to initially be at ecological and evolutionary equilibrium. An example where this sce­nario may be appropriate is when a new consumer species colonizes an island that has a long existing resident community. This way of conceptualizing the evolutionary response assumes a separation of ecological and evolutionary timescales. However, it is possible for evolution to be rapid enough that both the new and resident species will have evolved during the period required to reach their new ‘ecological equilib­rium' population sizes (Fussman et al. 2007; Hendry 2017). To use the concept of an ecological equilibrium, it is generally necessary to have a separation of ecological and evolutionary timescales. In the case of purely exploitative competition, both the (one or more) residents' and the invader's responses are affected by the relative and abso­lute changes in resource abundances that are produced by the invader. Even when the invading consumer's evolution is slow relative to population dynamics, there are often comparably rapid evolutionary changes in the resource populations that may affect the evolutionary change in the focal consumer. These complications have usu­ally been ignored in past field work on coevolving competitors, because the typical study is a natural experiment involving the comparison of one consumer population that occurs ‘alone', and a second, spatially distinct population of the same consumer that has existed for a long time in the presence of a second consumer species.

The properties of the resources involved are bound to play an important role in both ecological and evolutionary system dynamics. Even if the resources are abiotic (so cannot evolve), coevolution of the two consumers does not cease after some spec­ified time period, and their evolution will usually affect resource abundances, often altering any quantitative measure of the interaction. When two or more competitors coexist for a significant time span, novel traits are likely to arise at some point in one or both species. If this possibility is realized, it produces longer-term evolutionary change after the initial evolutionary equilibrium, based on pre-existing genetic vari­ation, is attained. It is also possible for either the population interaction itself or the evolutionary change it produces to result in sustained fluctuations in both popula­tions and traits. When this occurs, it usually produces different mean values of traits, as well as different population sizes.

The relative importance of the exploitation and interference components of com­petition has received little empirical study. This has allowed most of the theoretical work on the evolution of competition to focus on the exploitation component, which is usually easier to quantify. Evolution of intraspecific interference has been the subject of some theoretical work (Abrams and Matsuda 1994; Kisdi 1999), which predicts the existence of equilibria with both a low- and a high-competitive ability phenotype. This could also occur with inter- rather than intraspecific competition, but it has received little attention.

Coevolutionary theory for predator-prey interactions should have important implications for any competitive system with biotic resources. However, the trade­offs assumed in resource-based models of competition are fundamentally different from those assumed in predator-prey models.

The importance of evolution and/or adaptive behavioural plasticity in the effect­transmitting species has received more attention in systems with other types of indirect interactions. For example, the impact of evolution in one or more species has been examined in models of three-species food chains (e.g., Abrams and Vos 2003; Loeuille and Loreau 2004), and these models have shown that there are many pos­sible indirect evolutionary effects between top- and bottom-level species that arise from the adaptive change in the middle species. The possibility of coevolution of two prey species that share a common predator was examined in Abrams (2000a) and Abrams and Chen (2002a, b). Here, adaptive foraging by the predator and adap­tive evolution in the predator’s capture traits are both important determinants of the nature of the indirect interaction between the shared prey species. Given the much larger body of research that has been devoted to interspecific competition than to these two comparably indirect interactions, it is quite surprising how seldom adap­tive change in resources is considered in analyses of competitive coevolution. In any interaction chain involving three dynamical entities, change in a property of the mid­dle entity (e.g., adaptive evolution) is expected to be important in determining the population-level effects of one of the indirectly interacting species on the other.

The remainder of this chapter will look at the questions addressed by past work on the evolution of competitors, and what additional questions are most in need of theoretical exploration in the future. It will begin (Section 11.2) with a quick sum­mary of empirical work and a review of early theoretical work. Section 11.3 is a short description of the modelling approach to evolution used here, which is then employed to study several non-standard sets of ecological assumptions (all involv­ing explicit resources) in Section 11.4. Section 11.5 briefly discusses the evolution of apparent competitors. Section 11.6 examines contest/interference traits, while Sec­tion 11.7 considers the food web context of evolution. The final section (Section 11.8) looks at evolution and coexistence.

11.2