Can stable isotope ratios provide for community-wide measures of trophic structure? Comment.

Over the last few decades, the use of stable isotope ratios in ecological studies has increased greatly, most notably in the area of trophic ecology and food webs (Peterson and Fry 1987, Fry 2006). However, applications of stable isotopes in food web studies have been mostly qualitative to date, prompting the recent development of quantitative approaches to investigate different aspects of trophic ecology and food web structure using stable isotope data (e.g., Bearhop et al. 2004, Newsome et al. 2007, Schmidt et al. 2007). One such contribution (Layman et al. 2007a) proposed a series of six metrics that provide community-wide measures of trophic structure using a dual-isotope framework. However, the metrics as proposed by Layman et al. (2007a) will not accurately represent aspects of trophic structure for most ecological systems. Herein we summarize the framework and metrics proposed by Layman et al. (2007a), identify key limitations that prevent their widespread application, and discuss other directions that may facilitate the development of quantitative food web approaches using stable isotope data.