Cheating and the evolutionary stability of mutualisms

TLDR

Interspecific mutualisms have long shaped ecosystems, yet coevolutionary theory for them is lacking, leaving unresolved why they persist despite the selective advantage of cheating, which is thought to erode mutualistic interactions. The study aims to demonstrate that asymmetrical intraspecific competition over mutualistic commodities can explain the long‑term persistence of mutualisms. The authors model asymmetrical competition within species for the commodities supplied by mutualistic partners as the key ecological mechanism. Empirical evidence shows that cheating can coexist with mutualists, creating a background that allows superior mutualists to outcompete, leading to coexistence and divergence of mutualist and cheater phenotypes and even unrelated mutualists and cheaters.

Abstract

Interspecific mutualisms have been playing a central role in the functioning of all ecosystems since the early history of life. Yet the theory of coevolution of mutualists is virtually nonexistent, by contrast with well–developed coevolutionary theories of competition, predator–prey and host–parasite interactions. This has prevented resolution of a basic puzzle posed by mutualisms: their persistence in spite of apparent evolutionary instability. The selective advantage of'cheatin', that is, reaping mutualistic benefits while providing fewer commodities to the partner species, is commonly believed to erode a mutualistic interaction, leading to its dissolution or reciprocal extinction. However, recent empirical findings indicate that stable associations of mutualists and cheaters have existed over long evolutionary periods. Here, we show that asymmetrical competition within species for the commodities offered by mutualistic partners provides a simple and testable ecological mechanism that can account for the long–term persistence of mutualisms. Cheating, in effect, establishes a background against which better mutualists can display any competitive superiority. This can lead to the coexistence and divergence of mutualist and cheater phenotypes, as well as to the coexistence of ecologically similar, but unrelated mutualists and cheaters.