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

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 delta13C 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 delta13C signatures, then the intrapopulation variation in delta13C may be useful for quantifying diet variation and detecting isotopic evidence of individual specialization. However, intrapopulation variation in delta13C 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 delta13C 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 delta13C, and how we evaluate isotopic evidence of individual specialization.

Resource partitioning among top predators in a Miocene food web

“…However, the use of variance as a measure of dietary variation has been questioned, because dispersion of the isotopic values of a population depends not only on the number of species consumed but also on variation in the isotopic values of the food sources [56,57]. Consequently, consumers feeding on only two resources that exhibit highly divergent isotopic composition could have broader isotopic values than consumers feeding on a greater number of diet sources with less divergent isotopic composition.…”

Consequences of large temporal variability of zooplankton δ15 N for modeling fish trophic position and variation

“…Temporal, spatial, and compositional variability of the consumer's food sources affect how we choose ␦ 15 N base (Cabana and Rasmussen 1996;Post 2002;Matthews and Mazumder 2003). For example, measuring the trophic position of fish in lakes may require a baseline for both the pelagic (e.g., mussels, bulk zooplankton, Daphnia) and benthic (snails, Chironomids) food chains (Post et al 2000;Matthews and Mazumder 2003;Vadeboncoeur et al 2003) because fish use both pelagic and benthic resources.…”

“…The most reliable method for estimating trophic position is to measure the elevation of ␦ 15 N from a baseline (␦ 15 N base ), divide by an average enrichment per trophic level (⌬ ␦N ), and add the trophic position of the baseline ( base ) (Post 2002). Using a baseline helps account for large intersystem variation in ␦ 15 N that is unrelated to trophic variation (Vander Zanden and Rasmussen 1999;Post 2002;Matthews and Mazumder 2003). However, residual variation of ␦ 15 N (i.e., intrapopulation variation) may not be synonymous with trophic variation because of large temporal, compositional, and spatial isotopic variability of food sources (Maruyama et al 2001;Melville and Connolly 2003;Needoba et al 2003) and large variance in ⌬ ␦N (Post 2002).…”

Site specialists, diet generalists? Isotopic variation, site fidelity, and foraging by loggerhead turtles in Shark Bay, Western Australia

“…skin and muscle) have been used to infer amongindividual foraging variation and the dynamics of generalist populations (Bolnick et al 2003, Bearhop et al 2004, but see Matthews & Mazumder 2004). Within a generalist population, a wide dietary niche width may reflect a gradient of foraging behaviors from individual specialists, which consume different subsets of the overall population's resources, to individual generalists, which consume the same broad diet (Van Valen 1965, Bolnick et al 2003, Bearhop et al 2004.…”

“…N values of primary producers may also contribute to the isotopic signatures of consumers (Matthews & Mazumder 2004). In Shark Bay, small but significant seasonal differences have been found in the isotopic signatures of some primary producer groups (gelatinous macroplankton and benthic macroalgae), but not others (seagrass), which may in-fluence turtle isotopic distributions (Burkholder et al 2011).…”