Quantitative patterns of individual specialization: More generalized populations are also more heterogeneous

Ecologists often assume that individuals within a given population are ecologically interchangeable, using the same prey and exposed to the same competitors, predators, and parasites. This typological view overlooks substantial phenotypic and behavioral variation that can exist even among members of a single population. Many apparently generalized species are in fact composed of a heterogeneous collection of relatively specialized individuals. While this ‘individual specialization’ is widely documented, studies generally take a simple hypothesis testing approach, testing the null hypothesis that individuals sample randomly from a common pool of resources. To better understand the incidence and implications of individual specialization, it is instead necessary to quantify the degree of niche variation. We illustrate the insights that can be obtained by more quantitative approaches. First, we used network theory and empirical diet data to demonstrate the existence of ‘microguilds’ within lake populations of three-spine stickleback. These guilds are composed of morphologically and isotopically distinct individuals within genetically panmictic populations. Second, by quantifying individual specialization, it is possible to identify environmental factors that promote or constrain niche variation. We present experimental results demonstrating that individual specialization is magnified by resource scarcity, consistent with a recent model. We also use comparative surveys of multiple populations to show that more generalized populations actually tend to be composed of more heterogeneous individuals, a trend that holds across such disparate taxa as fish, frogs, lizards, and intertidal snails. Our results highlight the potential generality of individual specialization, and how it might qualitatively alter ecological interactions.