Great Lakes forest restoration through gray wolf recolonization

Background/Question/Methods

People have eliminated top predators from much of the world’s habitats over the last century.  In the absence of predators, ungulates became overabundant in many temperate and boreal forests.  Through intensified and selective herbivory on saplings, these large herbivores are now altering forest community structure and inhibiting tree regeneration. If re-introducing top predators can reduce herbivore numbers and/or alter their foraging behavior, then predator reintroduction may benefit plant growth as herbivory pressure is reduced, and a trophic cascade emerges.  My research focuses on whether recently re-established gray wolves (Canis lupus) may be generating such a trophic cascade in a Great Lakes forest by reducing white-tailed deer (Odocoileus virgnianus) herbivory on saplings.  I have constructed deer exclosures located throughout the University of Notre Dame Environmental Research Center (Land O’ Lakes, WI) northern mesic forest.  These exclosures are located evenly among areas of high (6) and low (6) wolf use so that I can quantify the impacts of deer herbivory at these sites and examine how they vary in relation to wolf predation risk.  I also placed camera traps on the exclosure control plots to directly record differences in deer behavior between high and low wolf use areas.

Results/Conclusions

Results suggest wolves benefit regeneration of the dominant maples (Acer spp.) by altering deer herbivory.  Deer were observed less frequently on high wolf use plots than low wolf use plots in 2012 (t-test, p = 0.04), and deer in high wolf use areas switched from foraging-focused behavior to a vigilance-focused strategy (t-test, p = 0.03).  Therefore, deer browse pressure (maple browse index) was substantially reduced in high wolf use areas (t-test, p < 0.01).  As a result, the impacts of deer on maple sapling growth were significantly affected by a wolf*year interaction (ANOVA, p = 0.01), with the predicted negative impacts of deer seen in low wolf use areas (post-hoc ANOVA, p = 0.03) disappearing in high wolf use areas (post-hoc ANOVA, p = 0.14).  Maples in high wolf use controls were averaging well over twice the height of their low wolf use counterparts by 2012 (post-hoc t-test, p < 0.01).  These results taken together suggest reintroduced predators can generate strong trophic cascades by altering the density and behavior of their prey, allowing saplings to grow and enter the forest canopy.  Wolf reintroductions may thus represent a useful management tool for restoring forests damaged by overabundant ungulates.