Beaver (Castor canadensis) can help to restore wetlands and mitigate the effects of climate change on hydrological regimes in some areas. Therefore, ongoing resource management efforts seek to promote the presence and persistence of beaver populations. The long-term success of such endeavors requires an understanding of what conditions are conducive to sustaining beaver populations, often in landscapes with degraded forest resources. Available information about beaver habitat suitability is largely based on short-term studies in stream habitats that do not consider aquatic plant suitability, even though aquatic plants may comprise over half of beavers' annual diets. In the present study, we assess whether the availability of woody plants, conditions conducive to the availability of aquatic vascular plants (macrophytes), and/or other features of basin morphology are associated with the persistence and density of beaver occupancy in 23 lakes over a 50-y period. We incorporate field-based study that includes all species of macrophytes, to extend previous work that focused specifically on water lily species (Nymphaea spp., Nuphar spp.). Lake perimeter (a function of sinuosity and surface area) and total macrophyte cover were associated with the persistence and density of beaver occupancy in lakes. Each factor independently explained ∼70% (R2adj) of the variation in the persistence of beaver occupancy. Percent cover of floating-leaved macrophytes was a leading predictor of beaver colony density in lakes, independently explaining 72% (R2adj) of the variation. Brasenia schreberi appeared particularly valuable to sustaining beaver in smaller lakes. Several lakes with abundant Brasenia supported high colony densities and long-term colony occupancy. Where feasible, beaver restoration efforts may increase the probability of success by facilitating beaver access to lakes that host key rhizomatous macrophyte species.
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Acknowledgments—We thank Philip Shelton, Doug Smith, and Rolf Peterson for their contributions to beaver surveys on Isle Royale National Park over the past 50 years. We are grateful to colleagues at Isle Royale National Park for their collegiality and assistance with field logistics; to Luis Veríssimo for his expertise and guidance in geospatial analyses; and to Luke Obermeyer, Angela Dow, Shawn O’Neil, Ruth Bennett, Linda Kartano, and Luis Veríssimo for helping the lead author with field data collection in wilderness conditions under demanding schedules. We appreciate the careful review of anonymous reviewers, whose contributions improved this manuscript. This research was funded in part by the National Science Foundation (DGE 0841073) and the Ecosystem Science Center of Michigan Tech University.
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