Small mammals and ungulates respond to and interact with revegetation processes following dam removal

Terrestrial wildlife communities are often overlooked as components of ecosystem restoration following dam removal. However, a diverse mammalian fauna colonizes habitat on dewatered reservoirs and may influence restoration processes. We studied mammalian colonization and ungulate herbivory from 2014 to 2018 following the removal of two large dams on the Elwha River in Washington, USA. Specifically, we examined (1) small mammal colonization and (2) Roosevelt elk (Cervus elaphus roosevelti) and Columbian black-tailed deer (Odocoileus hemionus columbianus) distribution and browsing pressure in association with revegetation efforts. We live-trapped small mammals on two former reservoir beds and monitored deer and elk use by conducting pellet group counts and quantifying ungulate herbivory at plots throughout the former reservoir beds and an adjacent reference site. We found that the proportion of deciduous riparian species on plots best predicted occupancy for mice, shrews, and voles. Small mammal species diversity was best explained by the proportion of logs and conifers on study plots. Roosevelt elk presence and browsing intensity on willows (Salix spp.) and black cottonwood (Populus balsamifera ssp. trichocarpa) varied both spatially and temporally, affecting the stature and potentially the growth trajectories of these species. Early seral restoration of these terrestrial habitats has included the presence of a diversity of granivores, insectivores, and herbivores, with elk demonstrating the strongest influence over portions of the study area. Small mammal colonization complements revegetation succession and demonstrates restoration of ecological processes, while large ungulates may be playing a more substantial role in shaping revegetation.