A critical evaluation of intrapopulation variation of δ13C and isotopic evidence of individual specialization

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Abstract
Individual variation in the diet of consumers is common in many ecological systems and has important implications for the study of population dynamics, animal behavior, and evolutionary or ecological interactions. Ecologists frequently quantify the niche of a population by intensive analyses of gut contents and feeding behaviors of consumers. Inter-individual differences in δ13C signature can indicate long term differences in feeding behavior, often unattainable by a single snapshot analysis of gut contents. If a consumer’s food sources have unique δ13C signatures, then the intrapopulation variation in δ13C may be useful for quantifying diet variation and detecting isotopic evidence of individual specialization. However, intrapopulation variation in δ13C can underestimate or overestimate dietary variation, and therefore is not directly equivalent to a dietary based niche. In this paper we show that intrapopulation variability of δ13C in consumers critically depends on the isotopic range and distribution of food sources. Our analyses fundamentally challenge how we interpret the intrapopulation isotopic variance of δ13C, and how we evaluate isotopic evidence of individual specialization.