Huntly tested the effects of pikas (Ochotona princeps) on talus-border vegetation by excluding pikas from small plots at several distances from talus on a site in Colorado. Vegetational cover and species richness increased in exclosures relative to control plots. These changes in vegetation following exclusion of pikas were greatest near talus and decreased with distance, paralleling the gradient of foraging by pikas. The effect of pika exclusion on cushion plants changed quantitatively and qualitatively with distance from talus and with time since grazing was stopped. These patterns were predicted from a model of the interaction of competition and predation along a gradient of grazing pressure. Mechanisms similar to those proposed for pikas are also expected for other small herbivores that forage from fixed nest sites.
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