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Ontogenetic niche shifts and evolutionary branching in size-structured populations

There are many examples of size-structured populations where individuals sequentially exploit several niches in the course of their life history. Efficient exploitation of such ontogenetic niches generally requires specific morphological adaptations. Here, we study the evolutionary implications of the combination of an ontogenetic niche shift and environmental feedback. We present a mechanistic, size-structured model in which we assume that predators exploit one niche when they are small and a second niche when they are big. The niche shift is assumed to be irreversible and determined genetically. Environmental feedback arises from the impact that predation has on the density of the prey populations. Our results show that, initially, the environmental feedback drives evolution towards a generalist strategy that exploits both niches equally. Subsequently, it depends on the size-scaling of the foraging rates on the two prey types whether the generalist is a continuously stable strategy or an evolutionary branching point. In the latter case, divergent selection results in a resource dimorphism, with two specialist subpopulations. We formulate the conditions for evolutionary branching in terms of parameters of the sizedependent functional response. We discuss our results in the context of observed resource polymorphisms and adaptive speciation in freshwater fish species.