Natural selection on mate-finding Allee effects
Although compelling as a concept, demographic Allee effects have received limited empirical support. This has led some to question utility of the concept as such, pointing towards evidence for evolutionary adaptations that allow their carriers to efficiently locate mates, avoid sibs, repel natural enemies, or simply cooperate. On the other hand, component Allee effects are frequently reported, of which those due to difficulties of finding mates in low-density populations (mate-finding Allee effects) are most commonly observed. Our broad aim is to develop an appropriate modeling framework to study the evolution of Allee effects and to apply that framework to address several related questions with ramifications for both basic and applied ecology: How are Allee effects maintained in populations? Can evolution cause an Allee effect to vanish? How commonly should one expect to find strong Allee effects in nature? We start by considering a species with the mate-finding Allee effect in which movement rate can evolve. With faster movement decreasing survival, we hypothesize that faster, short-lived individuals will thrive better at high densities (i.e. more competitive environment), whereas slower, longer-lived individuals will thrive at low densities where endurance in mate search is likely to increase fitness (i.e. less competitive environment).
We synthesize the current evidence for the evolution of Allee effects, and identify the fundamental ingredients an appropriate modeling framework for its study should have. Several processes may promote Allee effects. First, traits having an evolutionary advantage in rare populations may bear costs when populations are abundant. Second, large or dense populations may acquire traits that strengthen or even generate Allee effects, and thus impose costs only as populations decline. Finally, if two Allee effects counteract in a species, at most one can be avoided by evolution. Any appropriate evolutionary framework should consider evolution under non-equilibrium, transient dynamics and agent-based simulations coupled with quantitative genetics appear to be a promising approach. Our preliminary simulations address the question of whether the movement rate will evolutionarily stabilize at a single value, a large-small dimorphism will emerge, or even evolutionary cycles will occur. We demonstrate two classes of outcomes: evolutionary suicide whereby costs of diminishing the mate-finding Allee effect are eventually excessive, and evolution towards a stable movement rate. The implications of these results for selection on populations to contain individuals that are either more stand-alone or more dependent on conspecifics will be discussed.