Ecologists use stable isotopes (delta13C, delta15N) to better understand food webs and explore trophic interactions in ecosystems. Traditionally, delta13C vs. delta15N 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 (theta) for each species in food web space using mean delta13C and delta15N 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.

Resource polymorphisms are widely observed in fishes; however, ontogenetic contributions to morphological and ecological differences are poorly understood. This study examined whether ontogenetic changes in niche partitioning could explain morphological and buoyancy differences between lake trout ( Salvelinus namaycush ) morphotypes in Great Slave Lake (Northwest Territories, Canada). Morphometric analysis, buoyancy, capture depth, diet, and stable isotope data were used in concert to determine whether (i) differences occur in small, as well as large, lake trout, (ii) ontogenetic changes in morphology and buoyancy correlate with shifts in depth or diet, and (iii) a subset of small trout, putatively identified as “humpers”, are distinct from other morphotypes. Ontogenetic changes in lake trout morphology were associated with an ecological shift between benthic and pelagic feeding. Resource partitioning between lean and siscowet-like trout occurred within benthic (small trout) and pelagic (large trout) habitats. The humper subset did not differ from small siscowet-like trout. By combining multiple methods and an ontogenetic perspective, our study provides novel perspectives on resource polymorphisms in large, deep lakes and on existing interpretations of stable isotope data from large lakes in general.