American Society of Naturalists

Athmanathan Senthilnathan and Sergey Gavrilets (Jan 2021)

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Evolution helps species coexist

How species coexist is a fundamental question in ecology. Athmanathan Senthilnathan and Sergey Gavrilets attempt to answer this question using mathematical models of two-species interactions between competitors, mutualists, or a victim species and an exploiter species. In their model, species interactions depend on the similarity in traits like beak shape, body size, coloration, etc. These traits contribute to an individual’s fitness as they interact with individuals from the same species and the other species. The authors also assume that the traits have their own optimum values dependent on the environment. The authors show that strong interactions between similar individuals promote stable coexistence of competitors and exploiter-victim pair. In contrast, mutualists require weak interactions for stable coexistence. This study thus highlights the important role of individual differences for coexistence. Moreover, it provides an eco-evolutionary explanation for why we do not observe “the orgy of mutual benefaction” – the paradoxical situation where mutualism leads to an indefinite increase in population size.

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Abstract

The patterns and outcomes of coevolution are expected to depend on intraspecific trait variation. Various evolutionary factors can change this variation in time. As a result, modeling coevolutionary processes solely in terms of mean trait values may not be sufficient; one may need to study the dynamics of the whole trait distribution. Here, we develop a theoretical framework for studying the effects of evolving intraspecific variation in two-species coevolutionary systems. In particular, we build and study mathematical models of competition, exploiter-victim interactions, and mutualism in which the strength of within- and between-species interactions depends on the difference in continuously varying traits between individuals reproducing asexually. We use analytical approximations based on the invasion analysis and supplement them with numerical results. We find that intraspecific variation can be maintained if stabilizing selection is weak in at least one species. When intraspecific variation is maintained under competition or mutualism, coexistence in a stable equilibrium is promoted when between-species interactions mostly happen between individuals similar in trait values. In contrast, in exploiter-victim systems coexistence typically requires strong interactions between dissimilar exploiters and victims. We show that trait distributions can become multimodal. Our approach and results contribute to the understanding of the ecological consequences of intraspecific variation in coevolutionary systems by exploring its effects on population densities and trait distributions.