Elk herbivory alters small mammal assemblages in high-elevation drainages

Abstract

Heavy herbivory by ungulates can substantially alter habitat, but the indirect consequences of habitat modification for animal assemblages that rely on that habitat are not well studied. This is a particularly important topic given that climate change can alter plant-herbivore interactions. We explored short-term responses of small mammal communities to recent exclusion of Rocky Mountain elk (Cervus elaphus) in high-elevation riparian drainages in northern Arizona, where elk impacts on vegetation have increased over the past quarter century associated with climate change. We used 10-ha elk exclosures paired with unfenced control drainages to examine how browsing influenced the habitat use, relative abundance, richness and diversity of a small mammal assemblage. We found that the small mammal assemblage changed significantly after 5 years of elk exclusion. Relative abundance of voles (Microtus mexicanus) increased in exclosure drainages, likely due to an increase in habitat quality. The relative abundances of woodrats (Neotoma neomexicana) and two species of mice (Peromyscus maniculatus and P. boylii) decreased in the controls, while remaining stable in exclosures. The decline of mice in control drainages was likely due to the decline in shrub cover that they use. Thus, elk exclusion may have maintained or improved habitat for mice inside the exclosures while habitat quality and mouse abundance both declined outside the fences. Finally, small mammal species richness increased in the exclosures relative to the controls while species diversity showed no significant trends. Together, our results show that relaxation of heavy herbivore pressure by a widespread native ungulate can lead to rapid changes in small mammal assemblages. Moreover, exclusion of large herbivores can yield rapid responses by vegetation that may enhance or maintain habitat quality for small mammal populations.

Keywords: Cervus elaphus; Rocky Mountain elk; deer mice; exclosure; habitat quality; herbivory; indirect effects; northern Arizona; small mammals; voles.