Interaction of beaver and elk herbivory suppresses standing crop in willow

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

Study shows that intense herbivory by ungulates or livestock can disrupt beaver-willow mutualisms that naturally occur in less competitive environments. Populations of beaver and willow have not thrived in riparian environments that are heavily browsed by livestock or ungulates, such as elk. The interaction of beaver and elk herbivory may be an important mechanism underlying beaver and willow declines in this competitive environment. We conducted a field experiment that compared the standing crop of willow 3 years after simulated beaver cutting on paired plants with and without intense elk browsing (~85% utilization rate). Simulated beaver cutting with intense elk browsing produced willow that was small (biomass and diameter) and short with far fewer but longer shoots and a high percentage of dead biomass. In contrast, simulated beaver cutting without elk browsing produced willow that was large, tall, and leafy with many more but shorter shoots (highly branched) and a low percentage of dead biomass. Total stem biomass after 3 years was 10 times greater on unbrowsed plants than on browsed plants. Unbrowsed plants recovered 84% of their pre-cut biomass after only 2 growing seasons, whereas browsed plants recovered only 6%. Thus, the interaction of beaver cutting and elk browsing strongly suppressed the standing crop of willow. We predict a lack of willow suitable as winter food for beaver can cause beaver populations to decline, creating a feedback mechanism that reduces beaver and willow populations. Thus, intense herbivory by ungulates or livestock can disrupt beaver-willow mutualisms that naturally occur in less competitive environments.

Interaction of beaver and elk herbivory suppresses standing crop in willow

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