Temporal dynamics in the foraging decisions of large herbivores

The foraging decisions involved in acquiring a meal can have an impact on an animal's spatial distribution, as well as affect other animal species and plant communities. Thus, understanding how the foraging process varies over space and time has broad ecological implications, and optimal foraging theory can be used to identify key factors controlling foraging decisions. Optimality models are based on currencies, options and constraints. Using examples from research on free-ranging bison (Bison bison), we show how variations in these model elements can yield strong spatio-temporal variation in expected foraging decisions. First, we present a simple optimal foraging model to investigate the temporal scale of foraging decisions. On the basis of this model, we identify the foraging currency and demonstrate that such a simple model can be successful at predicting animal distribution across ecosystems. We then modify the model by changing (1) the forager's option, from the selection of individual plants to the selection of food bites that may include more than one plant species, (2) its constraints, from being omniscient to having incomplete information of resource quality and distribution and (3) its currency, from the maximisation of energy intake rate (E) to the maximisation of the ratio between E and mortality risk (u). We also show that, where the maximisation of E fails, the maximisation of E/u can explain the circadian rhythm in the diet and movements of bison. Simple optimal foraging-theory models thus can explain changes in dietary choice of bison within a foraging patch and during the course of a day.