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 three years after simulated beaver cutting on paired plants with and without intense elk browsing (ϳ85% 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 higher 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 lower percentage of dead biomass. Total stem biomass after three years was 10 times greater on unbrowsed plants than on browsed plants. Unbrowsed plants recovered 84% of their pre-cut biomass after only two 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 that 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.

Willows are dominant woody plants of many high-elevation riparian areas of the western USA, and constitute an important food resource for various ungulates, which tend to concentrate in riparian areas. The response of willow to browsing was analyzed in the elk winter range of Rocky Mountain National Park, by considering the effect of elk browsing on Salix monticola Bebb, one of the most common willow species in this area. Unbrowsed and browsed treatments were established during the 1997 growing season (May to October), using eight long-term exclosures built in the fall of 1994. Plants in the browsed treatment were in the areas open to browsing, but they were protected from browsing by small exclosures during the experimental period. Winter browsing by elk induced the following measured responses in plant morphology and development: (1) higher shoot biomass production but similar leaf biomass and leaf area per plant, (2) a lower number of and bigger shoots, (3) a lower number of and bigger leaves, and (4) flower inhibition. In addition, we infer that browsing induces (5) lower belowground allocation and (6) a more negative N balance but a higher soil N uptake. We conclude that elk browsing negatively affects willow even though willow compensate for aboveground biomass removal. Continuous browsing produces long-term changes in willow morphology which constrain plant growth and development. High browsing utilization, as occurred in this experiment, could therefore reduce the competitive ability and survivorship of willow, in particular under drier environmental conditions.

Abstract. Since the 1800s managed grasslands and shrublands of the arid American Southwest have been grazed predominantly by cattle originally bred for temperate climates in northern Europe. A heritage breed, the criollo cattle, has survived in northern Mexico for more than 400 years under desert-like conditions of low and variable rainfall, hot temperatures in the growing season, and both spatially and temporally scarce levels of primary production. We tested the hypothesis that the heritage breed has a broader spatial foraging distribution under harsh environmental conditions, and that its distribution is driven by environmental variables which differ from those that control the distribution of the introduced European breed. Movements of individual criollo and Angus breed animals were monitored autonomously in the northern Chihuahuan desert of southern New Mexico, USA. Georeferenced foraging locations acquired at 5-minute intervals for each animal were fit to a logistic regression using environmental factors as predictors. In the spring, when forage availability was high and more uniformly distributed across the landscape, animal foraging patterns were similar for both breeds. In the fall when forage availability was low and non-uniformly distributed, the two breeds exhibited very different foraging patterns: heritage animals foraged across a much larger spatial extent whereas their domestic counterparts remained in close proximity to the permanent source of water. These differences in foraging behavior driven by environmental variables have important implications for sustainability of rangelands in spatially and temporally variable environments. Heritage breeds of animals that are generalist foragers during unfavorable conditions can reduce environmental impacts compared to more recently introduced breeds.Key words: Chihuahuan desert; criollo cattle; GPS monitoring; herbivory; landscape foraging patterns; resource selection functions. Ecosphere 2(5):art57.