Comparison of beaver density and foraging preferences between urban and rural riparian forests along the South Saskatchewan River, Canada

Beavers have recolonized much of their historic range throughout the northern hemisphere and numerous studies have documented their habitat preferences and foraging behavior in rural riparian areas. Beavers, however, are also recolonizing waterways in cities, yet there has been little study of habitat use and foraging practices in these managed, urban systems. We studied beaver lodge distribution and riparian foraging preferences along the South Saskatchewan River, Canada, comparing a reach (24 km) passing through the City of Saskatoon where beavers and trees are managed with an upstream reach (29 km) passing through a conservation area where neither beavers nor trees are managed. In a canoe-based census at low flow, we found that beaver density in the conservation area was twice that in the city. Lodges were dispersed in the city with longer water-based distances between them. We found both differences and similarities in beaver foraging behavior. Riparian tree sampling along transects revealed that while diversity in the city and conservation area is markedly different, beavers preferentially cut green ash, Manitoba maple, paper birch and three poplars in both places. Beavers also cut six other tree species in the city, including three that are introduced, but the diversity was higher. A least-squares general linear model showed greater probability of cutting of trees further from the river in the city than conservation area, but of smaller diameter. Study results will be useful to urban planners in managing urban riparian forests and in developing beaver management plans.