Influence of Refuging Consumers (Pikas: Ochotona Princeps) on Subalpine Meadow Vegetation

Abstract

Talus slopes in western North America frequently are bordered by vegetation that differs in biomass, cover, height, and species composition from vegetation located farther from talus. These areas are grazed by pikas (Ochotona princeps), which nest in that talus. Foraging theory predicts that pikas will produce a gradient of grazing pressure, which could produce the observed vegetational zonation. In a subalpine meadow in west—central Colorado, pikas produced the predicted gradient of grazing pressure, along which plant groups varied clinally in abundance. The effects of pikas on talus—border vegetation were tested by excluding pikas from small plots at several distances from talus. 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.