Wolves, elk, and willows: alternate states and transition thresholds on Yellowstone's northern range

Abstract

The detection and prediction of alternate states of ecosystem configuration is of increasing importance in our changing world. Ecosystems may be perturbed by shifts in climate, or by human activity. Many perturbations to ecosystems can be reversed by reducing the initiating stressor. Sometimes shifts in ecosystem states are irreversible, and alternate configurations persist long after the initiating stressor is reduced. The reintroduction of wolves to Yellowstone National Park 17 years ago provided a rare opportunity to study whether the effects of predation could restore an ecosystem degraded by herbivory. Wolves were absent from the Yellowstone ecosystem for approximately 70 years. When wolves were absent, elk numbers increased and heavy herbivory degraded vegetation communities, particularly in riparian areas. Herbivory induced an alternate state in riparian vegetation, where willows, once dominant, were rare on the landscape and short in stature. My dissertation research describes how the top-down effects of predation and herbivory interact with the bottom-up effects of resource availability in northern range riparian areas. My research addressed three questions: 1) How do water table depth and browsing intensity constrain willow height and annual production? 2) What is the role of landscape heterogeneity in determining spatial variation in the configuration of alternate states? 3) How have climate patterns interacted with trophic effects of ungulates and wolves over the last 40 years to shape willow canopy cover, growth, and establishment? My work provides broad understanding of limitations to willow growth on the northern range, and revealed that wolf reintroduction has not restored riparian areas. A decade-long experiment showed that the effects of removing herbivory on willow height and production depend on water table depth. My second study showed that topography and temporal variation in water table depth influence willow height and growth more strongly than does herbivory. My third study found that bottom-up effects of growing season length and precipitation drive patterns in willow height over four decades. Far less support existed for the effects of elk and wolves on willows through time. All of these studies led to the conclusion that bottom-up effects of resource limitation influence northern range willows more strongly than top-down effects of top predators or herbivores. Results from my research show that wolf reintroduction has not uniformly restored riparian areas along small streams on the northern range. Instead, water table depth, topography, and climate drivers influence willows more strongly than herbivory or wolves.