Bison distribution under conflicting foraging strategies: site fidelity vs. energy maximization

Foraging strategies based on site fidelity and maximization of energy intake rate are two adaptive forces shaping animal behavior. Whereas these strategies can both be evolutionarily stable, they predict conflicting optimal behaviors when population abundance is in decline. In such a case, foragers employing an energy-maximizing strategy should reduce their use of low-quality patches as interference competition becomes less intense for high-quality patches. Foragers using a site fidelity strategy, however, should continue to use familiar patches. Because natural fluctuations in population abundance provide the only non-manipulative opportunity to evaluate adaptation to these evolutionary forces, few studies have examined these foraging strategies simultaneously. Using abundance and space use data from a free-ranging bison (Bison bison) population living in a meadow-forest matrix in Prince Albert National Park, Canada, we determined how individuals balance the trade-off between site fidelity and energy-maximizing patch choice strategies with respect to changes in population abundance. From 1996 to 2005, bison abundance increased from 225 to 475 and then decreased to 225 by 2013. During the period of population increase, population range size increased. This expansion involved the addition of relatively less profitable areas and patches, leading to a decrease in the mean expected profitability of the range. Yet, during the period of population decline, we detected neither a subsequent retraction in population range size nor an increase in mean expected profitability of the range. Further, patch selection models. during the population decline indicated that, as density decreased, bison portrayed stronger fidelity to previously visited meadows, but no increase in selection strength for profitable meadows. Our analysis reveals that an energy-maximizing patch choice strategy alone cannot explain the distribution ofindividuals and populations, and site fidelity is an important evolutionary force shaping animal distribution. Animals may not always forage in the richest patches available, as ecological theory would often predict, but their use of profitable patches is dependent on population dynamics and the strength of site fidelity. Our findings are likewise relevant to applied inquiries such as forecasting species range shifts and reducing human-wildlife conflicts.