A damming history: an analysis on the legacy effects of beaver dams on the composition and structure of a montane riparian ecosystem

Abstract

Landscape heterogeneity is widely recognized for its positive influence on ecosystem diversity and stability. As an "ecosystem engineer," the American beaver (Castor canadensis) increases heterogeneity in riparian systems through dam-building activities, resulting in diverse wetland complexes. This study explored the impact that historic, off-channel beaver dams have on the structure and composition of the surrounding vegetative communities. Specifically, I explored how vegetative communities differed in areas of beaver occupancy history, land management impacts, and distance from the river. This study was conducted in the Kawuneeche Valley of Rocky Mountain National Park, Colorado. Field methods focused on vegetative surveys around historic, off-channel beaver dams, the Colorado River, and its major tributaries. Observed beaver sign was classified by age (historic, recent/current, absent), and statistical analysis assessed compositional differences among variables of interest. I found that historic beaver dams supported more wetland-adapted and less disturbance-tolerant plant communities. Notably, shrub cover at historic dams was similar to that of exclosed sites. In contrast, upland, disturbance-adapted species dominated areas of historic beaver activity where dams were not present. Impacts of historic agriculture are still present on the landscape, with higher intensity farming areas dominated by herbaceous species. Finally, the effects of river incision reduced shrub cover within the valley, restricting communities to the river corridor. This study demonstrated that off-channel, historic beaver ponds function as a refuge for wetland plant species in disturbed riparian systems. Incorporating these underutilized structures already present on the landscape could enhance restoration success by increasing heterogeneity and wetland health.