North American pikas (Ochotona princeps) are central-place foragers that are restricted to talus slopes in the mountains of western North America. They graze meadows surrounding talus producing a gradient of grazing pressure extending out from the talus edge. We asked: 1) how does the physical structure of the talus slope affect pika foraging behavior? and 2) how do differences in foraging behavior affect the relative abundance of different plant functional groups? Pikas at three talus-meadow interfaces differed in the distances they traveled for forage, depending upon the structure of the talus slope. Pikas fed preferentially on vegetation within the talus, ventured into the surrounding meadow when intra-talus vegetation was rare, and ranged farther when haying than when grazing. They ventured into the surrounding meadow less when talus fields were more fragmented and intra-talus vegetation was more abundant. We tested the effects of pikas on meadow vegetation by excluding pikas from small plots located at various distances from talus edges. When habitat structure forced pikas to venture out from the talus to forage, they decreased total vegetative biomass by as much as 80% and increased the relative importance of cushion plants and sedges. These effects were undetectable at distances beyond one meter. These patterns appear to result from a change in the relative performance of different plant functional groups caused by herbivory.