Toward a quantitative nutritional ecology: the right-angled mixture triangle

A recent area of progress in nutritional ecology is a growing awareness that nutritional phenotypes are best understood in a multidimensional context, where foraging is viewed as a process of balancing the intake and use of multiple nutrients to satisfy complex and dynamic nutrient needs. Numerous laboratory studies have shown that this view can yield novel insights into unresolved questions and provide a framework for generating new hypotheses. By contrast, progress with this multidimensional view has been slow in the arena of ultimate interest to functional biologists, the field. One reason for this is that the Geometric Framework for nutrition that has been extensively used in laboratory experiments focuses on amounts of nutrients (e.g., required, eaten, or retained), and such data are typically very difficult or impossible to collect for most free-ranging animals. Further, many problems in field-based nutritional ecology involve comparisons of mixtures that are expressed as proportions (e.g., food, diet, body, or fecal compositions), rather than absolute amounts. As yet, however, no geometric framework has been established in nutritional ecology for this. Here I recommend an approach for the geometric analysis of nutritional mixtures, and illustrate its use in a variety of contexts by reanalyzing published data. Despite its simplicity, this approach holds considerable promise for furthering the study of field-based nutritional ecology.