Reliability of delta C-13 and delta N-15 in faeces for reconstructing savanna herbivore diet

We tested the reliability of herbivore faecal delta C-13 and delta N-15 values for reconstructing diet through review of an extensive database derived from a 3-year study of ungulates in South Africa's Kruger National Park. Faeces are a useful material for stable isotope studies of diet because they record dietary turnover at very short time scales, and because sampling is non-invasive. However, the validity of faecal isotope proxies may be questioned because they represent only undigested food remains. Results from Kruger Park confirm that free-ranging browsers have faecal delta C-13 consistent with C-3 feeding, grazer faeces are C-4, and mixed-feeder faeces intermediate. Although the respective ranges do not overlap, there is significant variation in faecal delta C-13 of browsers and grazers (similar to 2.0-4.0%o) across space and through time. We demonstrate that most (similar to 70%) of this variation can be ascribed to corresponding patterns of variation in the delta C-13 of C-3 and C-4 plants, respectively.. re-enforcing the fidelity of faecal isotope proxies for diet but highlighting a need for mixing models that control for variations in plant delta C-13 in order to achieve accurate diet reconstructions. Predictions for the effects of climate (rainfall) and ecophysiology on delta N-15-abundance variations in mammals do not persist in faeces. Rather, faecal delta N-15 tracks changes in plant delta N-15, with further fractionation occurring primarily due to variations in dietary protein (reflected by %N). Controlling for these effects, we show that a dual-isotope multiple source mixing model (Isosource) can extend diet reconstructions for African savanna herbivores beyond simplified C-3/C-4 distinctions, although further understanding of variations in mammal delta N-15 are needed for greater confidence in this approach. (C) 2008 Deutsche Gesellschaft fur Saugetierkunde. Published by Elsevier GmbH. All rights reserved.