Quantitative approaches to the analysis of stable isotope food web data

• Published in: Ecology (Durham) Vol. 88; no. 11; pp. 2793 - 2802
• Main Authors: Schmidt, S.N, Olden, J.D, Solomon, C.T, Vander Zanden, J
• Format: Journal Article
• Language: English
• Published: United States 01.11.2007
• Subjects: Animals; aquatic arthropods; Arthropods - growth & development; carbon; Carbon Isotopes; case studies; data analysis; Ecology - methods; Ecosystem; Environmental Monitoring - methods; Fishes - growth & development; Food Chain; food webs; Fresh Water; freshwater fish; invasive species; Isotopes - analysis; lakes; nitrogen; Nitrogen Isotopes; Phragmites; quantitative analysis; salt marshes; Seawater; shrimp; stable isotopes; statistical analysis; New Jersey; California; Nevada
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/07-0121.1

Ecologists use stable isotopes (δ13C, δ15N) to better understand food webs and explore trophic interactions in ecosystems. Traditionally, δ13C vs. δ15N bi-plots have been used to describe food web structure for a single time period or ecosystem. Comparisons of food webs across time and space are increasing, but development of statistical approaches for testing hypotheses regarding food web change has lagged behind. Here we present statistical methodologies for quantitatively comparing stable isotope food web data. We demonstrate the utility of circular statistics and hypothesis tests for quantifying directional food web differences using two case studies: an arthropod salt marsh community across a habitat gradient and a freshwater fish community from Lake Tahoe, USA, over a 120-year time period. We calculated magnitude and mean angle of change (θ) for each species in food web space using mean δ13C and δ15N of each species as the x, y coordinates. In the coastal salt marsh, arthropod consumers exhibited a significant shift toward dependence on Spartina, progressing from a habitat invaded by Phragmites to a restored Spartina habitat. In Lake Tahoe, we found that all species from the freshwater fish community shifted in the same direction in food web space toward more pelagic-based production with the introduction of nonnative Mysis relicta and onset of cultural eutrophication. Using circular statistics to quantitatively analyze stable isotope food web data, we were able to gain significant insight into patterns and changes in food web structure that were not evident from qualitative comparisons. As more ecologists incorporate a food web perspective into ecosystem analysis, these statistical tools can provide a basis for quantifying directional food web differences from standard isotope data.