FORAGE QUALITY AND AGGREGATION BY LARGE HERBIVORES

Three general hypotheses have been proposed to explain why many large herbivores have highly aggregated patterns of distribution: dilution of predation risk, maintenance of forage in an immature but highly nutritious growth stage, and ideal free distribution in relation to spatial variation in either forage quality or primary productivity. Evaluation of the latter two hypotheses has been hampered by lack of a formal theoretical framework for assessing the effects of forage quality on resource acquisition by herbivores. A model is proposed that demonstrates how trade-offs between instantaneous intake and forage digestibility might lead to optimal rates of energy intake at low to intermediate levels of forage abundance. Simulations based on the energy-intake model suggest a number of key environmental factors that should determine the impact of forage maturation and spatial variation on herbivore distribution patterns. All of the proposed advantages of aggregation may have general applicability, but the relative importance of each probably varies according to the specific circumstances facing a given herbivore population.