Estimating Niche Width Using Stable Isotopes in the Face of Habitat Variability: A Modelling Case Study in the Marine Environment

Abstract

Distributions of stable isotopes have been used to infer an organism's trophic niche width, the ‘isotopic niche’, and examine resource partitioning. Spatial variation in the isotopic composition of prey may however confound the interpretation of isotopic signatures especially when foragers exploit resources across numerous locations. In this study the isotopic compositions from marine assemblages are modelled to determine the role of variation in the signature of prey items and the effect of dietary breadth and foraging strategies on predator signatures. Outputs from the models reveal that isotopic niche widths can be greater for populations of dietary specialists rather than for generalists, which contravenes what is generally accepted in the literature. When a range of different mixing models are applied to determine if the conversion from δ to p-space can be used to improve model accuracy, predator signature variation is increased rather than model precision. Furthermore the mixing models applied failed to correctly identify dietary specialists and/or to accurately estimate diet contributions that may identify resource partitioning. The results presented illustrate the need to collect sufficiently large sample sizes, in excess of what is collected under most current studies, across the complete distribution of a species and its prey, before attempts to use stable isotopes to make inferences about niche width can be made.

Use of stable isotope ratios of fish larvae as indicators to assess diets and patterns of anthropogenic nitrogen pollution in estuarine ecosystems

“…To estimate contributions for each dietary source to the diet of P. microps larvae, we adopted a Bayesian stable-isotope mixing model (stable isotope analyses in R: SIAR; Parnell et al, 2010), which allows the inclusion of isotopic signatures, elemental concentrations and fractionation together with the uncertainly of these values within the model. Before running this model, fractionation values of +3.54‰ ± 0.74 for δ 15 N and 1.63‰ ± 0.63 for δ 13 C were used following Cummings et al (2012). The model was run separately for each sampling date, for both estuaries.…”

How adaptable are narwhal? A comparison of foraging patterns among the world's three narwhal populations

“…Heide-Jørgensen, unpublished data). Although it is generally assumed that a larger isotopic niche can be interpreted as larger trophic niche where organisms are typically generalists that feed on array of prey, it has been shown that populations confined to one site may display greater isotopic variances within their population due to individual specialization, whereby individuals exploit different aspects of the niche to reduce intraspecific competition (Cummings et al 2012). This may be the case in EG, however, further evidence, such as that from fatty acids or stomach contents, is needed to confirm this hypothesis.…”

“…In general, animals with very small isotopic niches are often dietary specialists that consume only one or a few types of resources and therefore have relatively low variability in their isotopic values, whereas dietary generalists that forage on a variety of organisms have a much larger isotopic niche. This general rule, however, has caveats and depends on the isotopic variability of the environment in which the organisms live and the prey sources they consume and usually stable isotope niche width is not directly comparable to an animal's trophic niche (Cummings et al 2012). Despite the limitations of stable isotope niche analysis, if isotopic variation in prey is taken into account, niche analysis can provide information on dietary variation at the population level (Newsome et al 2012) In this study we investigated three hypotheses.…”

Niche segregation amongst sympatric species at exposed sandy shores with contrasting wrack availabilities illustrated by stable isotopic analysis

“…Stable isotopes ratios of carbon and nitrogen have been proved to be able to discriminate among various primary producers that did support the food web in several coastal regions (Baeta et al, 2009;Bergamino et al, 2011;Epstein, 1978, 1980). Stable isotope analysis (SIA) is currently among the most powerful tools for the study of trophic relationships and to estimate niche segregation among species in the marine environment (Cummings et al, 2012), and its use in characterising the food web structure of macrofaunal communities or consumers diets is relatively common on sandy beaches (e.g. Bergamino et al, 2011;Colombini et al, 2011;Olabarria et al, 2009;Rossi et al, 2010).…”