Why aren't there more beaver in Rocky Mountain National Park?

Baker, Bruce; Mitchell, David; Ducharme, Heather; Stanley, Thomas; Peinetti, H

Beaver populations have declined dramatically in Rocky Mountain National Park since 1940. Declines were initiated by trapping in 1941 – 1949. Blood tests of 39 livetrapped beaver indicated disease is not present in the population. Current beaver populations occur in areas with low willow use by elk. We compared use of woody stems in a winter food cache with nearby unused stems and found that beaver selected stems that had a lower percentage of leaders browsed by elk. We experimentally tested the effects of elk browsing on regrowth of simulated beaver-cut willow inside and outside 5 elk exclosures and found intense elk browsing produced plants of low vigor that were small, short, and hedged with a high percentage of dead stems. In contrast, regrowth of protected plants was large, tall, highly branched, and leafy with a low percentage of dead stems. We conclude that if beaver cut tall willow, and intense elk browsing suppresses regrowth, then the interaction of beaver and elk may function as a mechanism of declining riparian willow. Further, we speculate that intense elk browsing may decrease the suitability of willow as winter beaver food, increase willow mortality (directly or indirectly through loss of beaver), and increase elk carrying capacity by conversion of beaver-wetland to upland-meadow.