Ten years of experimental animal isotopic ecology

1 Ten years ago Gannes et al. (1997, Stable isotopes in animal ecology: assumptions, caveats, and a call for laboratory experiments. Ecology, 78, 1271–1276, 1998) identified four major areas requiring further research in experimental animal isotopic ecology: (i) the dynamics of isotopic incorporation, (ii) mixing models, (iii) the problem of routing, and (iv) trophic discrimination factors. 2 Differences in isotopic incorporation rates among tissues seem to be explained by variation in protein turnover. The application of multi-compartment models to isotopic incorporation data has revealed that different inferences can be derived between these and one-compartment models. 3 A variety of mixing models of varying degrees of complexity and realism are used to find the contribution of isotopic sources to the elements in an organism's tissues. The use of these models demands the use of tissue to diet discrimination factors that are rarely measured experimentally. 4 Mixing models assume that assimilated nutrients are disassembled into their elemental components and that these elements are reassembled into biomolecules. This assumption is unrealistic as macromolecules are routed differentially into tissues. Isotopic routing is an area that isotopic ecologists have neglected in their experimental and modelling research. 5 Isotopic ecologists are just beginning to understand why 15N biomagnifies along trophic chains, and to explore the factors that determine the degree of 15N biomagnification. We review the hypotheses that explain why 15N biomagnifies up trophic chains. 6 The use of compound-specific isotopic analyses is opening new fruitful areas of research at the intersection of nutritional and isotopic ecology.