TROPHIC CASCADES: LINKING HYPER-ABUNDANT UNGULATES TO SHRUB-DEPENDENT BIRDS AND BUTTERFLIES
Interactions within food webs are complex. Given that some ecosystems today are rapidly changing due, in part, to the direct and indirect effects of humans, research is needed to better understand and predict anthropogenic influences on ecosystems. For my MSc work (University of Alberta, supervisors: Dr. Scott Nielsen & Dr. Jens Roland I investigated the direct effects of high ungulate density (moose, deer, elk, bison) on plants and the indirect effects of ungulates on shrub-dependent bird and butterfly species.
This work was carried out in east-central Alberta provincial nature reserves and Elk Island National Park, a fenced area with some of the highest ungulate densities in Canada. Due to the loss of top predators (wolves, cougars and grizzly bears) and park management practices geared towards maintaining high ungulate density, Elk Island provided an ideal setting to test the impact of high ungulate numbers on lower trophic levels.
With the help of field technicians and volunteers, we carried out vegetation surveys, bird point counts and butterfly pollard walks at sites of varying ungulate density. The results of this work are similar to other studies addressing trophic cascades initiated by the loss of top predators including wolves and cougars. Even in a productive and resilient system, we demonstrated a cascading effect of high ungulate density on shrub-nesting yellow warblers (bird) through reductions in shrub cover and Canadian tiger swallowtail (butterfly) numbers through reductions in larval feeding plant density, a relevant finding to consider when managing ungulate populations. Resource management decisions should consider predator reintroduction for ecosystem restoration.