Forage Abundance as the Impetus for Large Ruminant Aggregation: A Modeling Approach
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Two distinct explanations exist for the evolutionary origin of grouping in primary consumers: the reduction of individual predation risk and resource-mediated aggregation. Several studies have found little to no relationship between aggregation of a prey species and predation risk, while grazers may aggregate in response to a heterogeneous distribution of resources, suggesting that the second explanation may be more plausible. However, aggregated consumers will deplete resources more rapidly, thus resource mediated aggregation may not be stable. Using a modeling approach, I examine if forage preference alone can generate aggregation, and if so, what are the circumstances of its emergence and stability. The model is a spatially explicit consumer-resource model using empirically derived parameters to simulate large ruminant foraging in a finite pasture. Model output indicates that grouping can arise in response to forage abundance and population density if grazers exhibit preference for forage of higher nutritional quality, usually associated with intermediate stages of forage growth. In this case, foragers can establish continuous areas of high quality forage wherein they aggregate. However, aggregation is temporary and occurs only within a narrow range of forage characteristics and population density. Findings indicate that resource mediated grouping may provide an alternative explanation for the evolution of sociality in gregarious ruminants during the Miocene epoch, when herbivore habitat transitioned from shrub-dense systems to the open grazing systems observed today. Because no modern analog to the developing Miocene grasslands exists, recent incidence of such grouping is unlikely. Moreover, transiency of grazer-maintained foraging areas occurred across all combinations of model parameters, indicating consumer behavior influenced frequent turnover of foraging areas. Preferential selection of nutritionally favorable (intermediate height) forage permits surrounding unforaged areas to mature while reducing growth rate within the maintained foraging area, suggesting that selection of nutritionally favorable forage may be ecologically unstable.