CAN STABLE ISOTOPE RATIOS PROVIDE FOR COMMUNITY-WIDE MEASURES OF TROPHIC STRUCTURE?

TLDR

Stable isotope ratios of carbon and nitrogen are commonly used to describe trophic niches, yet they have not been applied to quantify community‑wide trophic structure. The authors propose quantitative metrics to characterize community‑wide trophic structure, inspired by ecomorphology approaches. They use convex hull area and mean nearest‑neighbor distance in δ¹³C–δ¹⁵N niche space, illustrated with empirical and conceptual examples from Bahamian tidal creek food webs. The examples show that these metrics quantify trophic diversity and redundancy, link species to food‑web characteristics, and offer a new perspective on food‑web structure, function, and dynamics.

Abstract

Stable isotope ratios (typically of carbon and nitrogen) provide one representation of an organism's trophic niche and are widely used to examine aspects of food web structure. Yet stable isotopes have not been applied to quantitatively characterize community-wide aspects of trophic structure (i.e., at the level of an entire food web). We propose quantitative metrics that can be used to this end, drawing on similar approaches from ecomorphology research. For example, the convex hull area occupied by species in δ13C–δ15N niche space is a representation of the total extent of trophic diversity within a food web, whereas mean nearest neighbor distance among all species pairs is a measure of species packing within trophic niche space. To facilitate discussion of opportunities and limitations of the metrics, we provide empirical and conceptual examples drawn from Bahamian tidal creek food webs. These examples illustrate how this methodology can be used to quantify trophic diversity and trophic redundancy in food webs, as well as to link individual species to characteristics of the food web in which they are embedded. Building from extensive applications of stable isotope ratios by ecologists, the community-wide metrics may provide a new perspective on food web structure, function, and dynamics.