Individual niche variation is a widespread phenomenon in nature with clear evolutionary and ecological relevance. Although recent studies have focused on the consequences of individual variations (e.g. species coexistence), we still have no clear understanding of how different ecological factors act independently and interactively to determine the magnitude of individual niche specialization (IS) in natural populations. Theory has proposed four potential main ecological causes of IS: intraspecific competition, interspecific competition, ecological opportunity, and predation. Here, we empirically tested this theory by analyzing natural patterns of individual trophic specialization in four syntopic, trophic generalists, and closely related frog species across a wide range of communities in central Brazil. Specifically we asked: (a) What is the relative importance of each of these ecological factors on the degree of IS; (b) Does the relative importance of ecological factors vary between species and/or size classes within species?
Overall we found that three of the four predicted ecological drivers of individual specialization affected individual specialization to some extent. Only prey diversity did never show a significant relationship with the degree if individual niche variation in the studied frog species. The observed positive effects of intraspecific competition (quantified as conspecific density) matched theoretical predictions and previous experimental evidence. However, interspecific competition (effective population sizes of heterospecifics) also had a positive effect on IS, opposing predictions of theory in quantitative genetics. Predation (median of the distribution of potential intra-guild predators) had a negative effect on individual specialization in the small-sized group within species. We observed significant interaction terms of our predictors with species, but not with size class. These results suggest that idiosyncratic responses of species to different ecological contexts, instead of size-based responses, can underlie the magnitude of interindividual niche variations in natural populations. Considering this finding, the degree of intraspecific niche diversity should respond differently across species along environmental gradients, which can impact ecological dynamics such as competition and species coexistence.