In this Yukon study, McIntire and Hik looked at an alpine herbivory gradient established by collared pikas (Ochotona collaris), a small central foraging lagomorph, to examine the effects of multiple grazing levels on above-ground live biomass (AGB) and species richness (SR) in alpine tundra. Depending on the plant group and the timing of sampling, McIntire and Hik observed all possible responses (increasing, decreasing, and biphasic) along a gradient extending from virtually no herbivory to a greater than 50% decrease of AGB. Graminoids decreased in abundance along the grazing gradient in June and July, but showed a unimodal peak at an intermediate level of grazing in August. These results show partial support for an herbivore optimization model (Hilbert et al. 1981), Huntly's model (1987), and a strictly deleterious model depending on the temporal and spatial level of examination. They also found that grazing history influences current plant performance. Following two seasons of herbivore exclusion, plants located in patches that have had a history of intensive grazing produced more AGB than plants that had no history of grazing. These results suggest that infrequent herbivore vacancies may provide local short-term release from pika grazing, thereby contributing to the persistence of productive, highly productive vegetation.
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