THE INFLUENCE OF HERBIVORES ON PLANT COMMUNITIES: EXPERIMENTAL STUDIES OF A SUBALPINE MEADOW ECOSYSTEM (MICROTUS, THOMOMYS, OCHOTONA).

Abstract

The effects on vegetation of three small mammalian herbivores were determined experimentally in a subalpine meadow in southwestern Colorado. Pikas (Ochotona princeps) inhabit talus and forage on surrounding vegetation. Their foraging was consistent with predictions of central place foraging theory. Amount of foraging decreased with distance from talus, whereas selectivity increased. These patterns are expected to influence the resources used by pikas, and plant abundance and species richness increased with distance from talus, as predicted. Experimental exclusion of pikas demonstrated that a significant portion of the lower vegetational cover and species richness was due to pikas. A model postulating that the effects of pikas on vegetation resulted from a gradient in herbivory, and a corresponding gradient in the relative importance of herbivory and competition among plants, was generally supported. Total vegetational abundance and species richness increased in the absence of pikas. The increase in pika exclosures compared to controls was greatest near talus and decreased with distance. The initial effect of pika exclusion on cushion plants (assumed to be the poorest competitors among the plants in the absence of herbivory) was positive near talus, but was insignificant and tended to be negative at greater distance from talus. The prediction of eventual decline of cushion plant populations in the absence of pikas was supported in two of three sites after three years. The effects on vegetation of two co-occurring herbivores, pocket gophers (Thomomys talpoides) and voles (Microtus montanus), were evaluated using a factorial exclusion experiment. Non-independence of effects was assessed as presence of a significant interaction term in 2-way ANOVA. Gopher x vole interaction terms were at least marginally significant for many analyses. Results only weakly suggested direct foraging interactions between gophers and voles, but showed considerable importance of indirect, plant-plant interactions. Exclusion of either gophers or voles significantly increased vegetational abundance, however, no additional change in abundance resulted when both were excluded simultaneously. Exclusion of either increased forb abundance, and exclusion of gophers increased grass abundance. Only simultaneous exclusion of both, however, altered proportional abundances of plant groups.