Evaluating mechanisms of plant-mediated effects on herbivore persistence and occupancy across an ecoregion.
Contemporary climate change is rapidly creating one of the greatest challenges for management and conservation during the 21st century. Mountain ecosystems, which have a high degree of spatial heterogeneity and contain numerous habitat specialists, have been identified as particularly vulnerable. We used data from multiple years across sites spanning a >40 million ha ecoregion to test hypotheses regarding how community-level characteristics of vegetation may affect a mammalian generalist herbivore, the American pika (Ochotona princeps). We examined patterns of pika persistence across sites in the hydrographic Great Basin, and occupancy within a subset of these sites. We used mixed-effects logistic regression models to compare evidence in support of competing explanations for each pattern within an information-theoretic framework (using Akaike's information criterion). Models reflected four hypothesized classes of mechanisms related to nutritional ecology, ecosystem function, indirect indication of climatic effects, and (synergistic) combinations of these three classes. At the site level, models reflecting synergistic effects received the most support. At the within-site level, support appeared to be split equally among hypotheses containing predictors related to either nutritional ecology or indirect climate effects. Well-supported predictors included cover of invasive plant species, cover of more-xeric plant species, species evenness, and proportion of graminoid species. Our results both (1) identify important aspects of vegetation communities that may influence herbivore distribution in mountainous areas across a large, diverse geographic region, and (2) contribute to an improved understanding of how mountain ecosystems may be affected by ongoing climate change, more broadly.