Spatial Heterogeneity and Individual Variation In Diet of an Aquatic Top Predator

Since habitats are generally spatially heterogeneous, and important component of niche variation may be determined by the relationship between individual variation in feeding among and within habitats. We characterized variation in diet among and within individual tiger salamander larvae (Ambystoma tigrinum nebulosum) among and within sites in natural ponds. We also analyzed prey variation among sites. There was significant variation in diet among sites, controlling for variation in larval size among sites. Prey taxa varied significantly among sites. There were also significant size—associated differences in diet within sites. Controlling for larval size we found no further evidence of significant interindividual differences in diet within sites. Intrapopulation variation in diet among sites is likely a consequence of adaptive phenotypic plasticity in feeding behavior of individuals on a spatially variable prey assemblage. This result supports Van Valen's hypothesis that selection should favor multiple phenotypes within populations. Variation in diet among sites is directly analogous to the within— and between—phenotype components of niche variation defined by Roughgarden. Size—associated differences in diet represent a significant ontogenetic component to niche variation in feeding within populations of larval salamanders. Studies of food web structure that ignore predator diet and prey differences among sites are an incomplete summary of predator—prey linkages. In addition, phenotypic plasticity in feeding behavior as a source of niche variation has implications for traditional ecological theory of intra— and interspecific interactions, which has rarely included variation other than morphological size differences among animals.