Comparative support for the niche variation hypothesis that more generalized populations also are more heterogeneous

Abstract

There is extensive evidence that some species of ecological generalists, which use a wide diversity of resources, are in fact heterogeneous collections of relatively specialized individuals. This within-population variation, or "individual specialization," is a key requirement for frequency-dependent interactions that may drive a variety of types of evolutionary diversification and may influence the population dynamics and ecological interactions of species. Consequently, it is important to understand when individual specialization is likely to be strong or weak. The niche variation hypothesis (NVH) suggests that populations tend to become more generalized when they are released from interspecific competition. This niche expansion was proposed to arise via increased variation among individuals rather than increased individual niche breadth. Consequently, we expect ecological generalists to exhibit stronger individual specialization, but this correlation has been repeatedly rejected by empiricists. The drawback with previous empirical tests of the NVH is that they use morphological variation as a proxy for niche variation, ignoring the role of behavior and complex phenotype-function relationships. Here, we used diet data to directly estimate niche variation among individuals. Consistent with the NVH, we show that more generalized populations also exhibit more niche variation. This trend is quite general, appearing in all five case studies examined: three-spine stickleback, Eurasian perch, Anolis lizards, intertidal gastropods, and a community of neotropical frogs. Our results suggest that generalist populations may tend to be more ecologically variable. Whether this translates into greater genetic variation, evolvability, or ecological stability remains to be determined.

Fig. 1.

Illustration of two alternative patterns of population niche expansion. A population that uses a narrow range of prey sizes can increase its population niche breadth (bold lines) in two ways. (a) All individual niche breadths (thin lines) can expand resulting in no increased niche variation among individuals. (b) Individual niche breadths can remain limited, whereas individuals diverge from each other to increase among-individual variation.

Fig. 2.

Correlation between diet variation among individuals (V) and the TNW of the population (see Materials and Methods for details). The empirical results are shown with filled circles. Crosses (and the dotted regression line) indicate the expected trend under a null model in which diet variation arises solely by individuals randomly sampling a limited set of prey from a shared prey distribution. Diet variation for Anolis lizards was measured based on variances of prey sizes (BIC/TNW) rather than the Shannon–Weaver diversity index. Hence, the scale for both diet variation and niche breadth are different from the other taxa.