Variations in nitrogen isotope ratios in terrestrial foodwebs are described, and alternative models for variation in the enrichment between trophic levels are evaluated. Nitrogen isotope ratios in bone collagen have been used to determine trophic levels and differentiate marine from terrestrial resource consumption among prehistoric humans. However, recent research in terrestrial ecosystems has revealed significant variation in nitrogen isotope ratios between habitats, and within trophic levels in the same environment. Foodwebs in hot, arid environments tend to have higher nitrogen isotope ratios than in cool, wet ones. The stepwise enrichment between trophic levels is often greater in hot, arid environments. Herbivore species with physiological adaptations to water conservation have higher nitrogen isotope ratios than water-dependent species. The nitrogen isotope ratios of human bones may be affected by climate and physiology and thus cannot be directly compared between different types of ecosystems without first determining the isotopic composition of the local foodweb and the stepwise enrichment between trophic levels.