Wolves, elk, and willows: Spatial variation in landscape configuration on Yellowstone’s Northern Range

Background/Question/Methods

A fundamental challenge in contemporary ecology is to understand how disturbance influences interactions within food webs and how spatial variability in these interactions shapes landscape structure and function. In Yellowstone National Park the absence of an apex predator during most of the 20^th century allowed elk populations to increase dramatically in abundance. As a result, willows (Salix. sp) experienced heavy browsing by elk and were essentially eliminated from the landscape. After wolves were reintroduced to Yellowstone in 1995, many suggested that elk browsing pressure declined and willows recovered on the Northern Range. Our findings continue to show that recovery has been less widespread than originally believed and that some areas are fully recovered while others are still suppressed. In this study, we examine the causes of spatial variation in willow growing conditions across 27 Northern Range study sites. At each site, we measured willow height, annual production, and over-winter offtake during 2008-2010. Landscape covariates were developed using a GIS as potential predictors of spatial variation in all three responses. Specifically, we investigated metrics that described elevation, aspect, and soil moisture. We used Bayesian hierarchical models to investigate the effects of landscape covariates on individual willows and stand-level responses. Competing models were evaluated and model averaging employed to obtain the best-fit model. 

Results/Conclusions

Overall, model results suggest that watershed context and soil moisture metrics describe meaningful variation in willow height and growth patterns. Smaller scale variation in elevation above groundwater also plays a role in annual production. Further, our results show that despite declining elk numbers in recent years, browsing pressure remains high across much of the Northern Range. However, plants with adequate access to water can withstand high levels of herbivory year after year. Together, our results suggest that climate variability, landscape variation, and ecological interactions interact to shape willow growing conditions on the Northern Range.