Mass-Balance Models for Animal Isotopic Ecology

Analysis of natural stable isotope ratios has created a methodological upheaval in animal ecology. Because the distribution of stable isotopes in organisms follows reliable patterns, their analyses have become established useful methods for animal ecologists. However, because animal ecologists have adopted a phenomenological approach to the use of stable isotopes, the mechanisms that create isotope variation patterns remain unexplored. The mass-balance models that can provide a mechanistic, and hence predictive foundation for animal isotopic ecology are presented here. We review and elaborate the current mixing models used to reconstruct animal diets and develop new mathematical models to explain one of the most widely used patterns in animal isotopic ecology: enrichment in 15N observed across trophic levels. Construction of element and isotope budgets is central to testing the mass-balance models described herein. Because the concept of a budget is central to all animal physiological ecology, development of a mechanistic and predictive framework for isotopic animal ecology falls naturally on physiological ecologists. We argue that progress in isotopic animal ecology hinges on laboratory experiments that explore mechanism, documentation of pattern in the field, and theoretical integration of mechanism and pattern.