Quantifying the indirect effect of wolves on aspen in northern Yellowstone National Park: Evidence for a trophic cascade?

Dr. Lainie Brice

Post-doctoral Researcher, Cornell University

Abstract

Research has established that carnivores can indirectly benefit plants through both lethal and trait-mediated effects on their prey, but much of what we know about such processes, or trophic cascades, stems from studies on small-bodied carnivores in controlled environments, and we know less about large carnivores across vast landscapes. The purpose of my research was to address this gap through a comprehensive test of a seminal trophic cascade: wolves (Canis lupus), elk (Cervus canadensis), and aspen (Populus tremuloides) in Yellowstone National Park. I used 20 years of data on these species to: 1) clarify annual trends in browsing and height of young aspen after wolf reintroduction, 2) quantify the strength of trait-mediated indirect effects of wolves (i.e., elk fear) on aspen, and 3) evaluate the strength of density-mediated indirect effects of wolves (i.e., direct killing of elk) on aspen. My research suggests that wolves have benefitted aspen since their reintroduction by helping to maintain the elk population at lower numbers, but that any fear of wolves on the part of elk did not reduce their foraging in a way that improved aspen growth. Furthermore, the role of humans as predators of elk outside of the Park was critical in reducing elk density and causing this trophic cascade. However, despite these forces limiting elk abundance, there is still widespread aspen recruitment failure nearly 30 years after wolf reintroduction, suggesting that chronic herbivory may have pushed aspen past its tipping point, and the pathway to recovery will be slower than that of initial degradation.