A comparison of riparian plant community response to herbivory by beavers ( Castor canadensis) and ungulates in Canada's boreal mixed-wood forest

Herbivores, such as beavers ( Castor canadensis) and ungulates, can dramatically alter the species composition and structure of riparian vegetation communities. We examined how four different levels of beaver presence and ungulate density influenced the structure and composition of riparian vegetation communities in the mixed-wood boreal region of east-central Alberta, Canada. Beavers, as central-place foragers, focus on similar species utilized by the six ungulate species in our study area. However, we found that vegetation responded differently relative to the percent cover and heights of emergent, shrub, herbaceous dicots, and tree species depending on the varying combinations of beaver presence and ungulate densities. There was an obvious negative effect of ungulate herbivory on shrub heights and cover; however, we were unable to find any significant effects on the overall composition and structure of vegetation communities that were exclusively related to beaver activities. In areas with no beavers and lower ungulate densities there was a positive growth response in preferred species (e.g., Salix spp., Prunus spp.), which clearly demonstrated the degree of combined influence that these herbivores have on riparian communities. In these areas, Salix spp. was four times shorter than in areas where beavers were not present and ungulate densities were lower. In addition percent cover was lowest for emergent vegetation and herbaceous dicots in areas with higher ungulate densities. Many studies focus on single-species effects on forage resources, yet our research shows that the study of combined effects of multiple herbivores can provide a new insight into these complex systems.