Impacts of beaver disturbance on avian species richness and community composition in the central Adirondack Mountains, NY, USA
In the eastern forests of North America, impacts by beaver (Castor canadensis) represent one of the most ubiquitous but underappreciated sources of natural disturbance across the landscape. Beaver activity in the form of harvesting trees and building dams affects forest ecosystems by consuming or releasing resources and altering available niches, which in turn alters community structure and composition of both terrestrial and aquatic ecosystems. Beaver disturbance is associated with enhanced biodiversity at the landscape scale for multiple taxa, including understory plants, amphibians, and birds.
Our goal was to quantify the links between beaver disturbance and forest bird communities. Specifically, we quantified avian species richness and community composition in beaver-impacted habitats compared to adjacent areas of unaltered forest and riparian habitat and compared bird biodiversity patterns among wetland areas varying in the duration and recentness of beaver occupation, as determined using a 33 year dataset of beaver occupancy. We conducted repeat point counts in the central Adirondacks, NY during summer 2013 and measured vegetation characteristics among the four habitat types: 6 active beaver wetlands, 6 inactive beaver wetlands, 12 riparian areas with no known history of beaver occupancy, and 12 upland forested sites.
Bird species richness was higher at beaver-disturbed sites (34.3±4.4; mean±2 SE) than riparian or forest sites (22.4±3.7 and 24.8±4.0 respectively). Active beaver sites had higher richness (38.7±4.1) than inactive sites (29.8±6.0). Across all sites, bird species richness was positively related to tree species richness. At beaver-disturbed sites, bird species richness was positively related to wetland size. The number of avian taxonomic families was higher at beaver sites (14.1±1.8) than at forest or riparian sites (10.0±1.2 and 8.7±1.2 respectively), which was positively related to the complexity of understory vegetation structure. Community similarity indices calculated between habitat types were not significantly different, though a non-metric multidimensional scaling suggests separation in species composition between beaver sites and forest/riparian sites.
This study provides strong evidence that beaver can provide benefits for avian community diversity. Because the temporally dynamic patterns of beaver occupancy result in a landscape matrix of active and inactive wetlands, the magnitude of impacts of beaver disturbance on avian diversity will likely vary spatially and change over time at specific locations. Because past beaver activity has been shown to have persistent effects for up to 50 years, abandoned impoundments or areas where beaver are extensively harvested may still offer benefits to birds.