Are wolves in Wisconsin affecting the biodiversity of understory plant communities via a trophic cascade

Background/Question/Methods

The expansion of the Midwest wolf population presents a natural experiment in the long term ecological impacts of a keystone predator recovering from local extinction. We use this opportunity to address two questions: (1) are plant species richness and seedling density higher in white cedar wetlands occupied by wolves?; and (2) are top-down processes primarily responsible for observed differences in these variables?  To accomplish this, we used a spatially explicit hierarchical vegetation survey to evaluate if, and at what spatial scales, species composition and structure differ between areas continuously occupied by wolf packs and areas unoccupied by wolves.  

Results/Conclusions

Results from summer 2008 indicate that a positive relationship exists between wolf pack occupation and understory plant diversity in white cedar wetlands. As predicted, forb species richness was significantly higher in wolf areas (Wolf: 32.6 ± 3.4, N=7, p = 0.04; Non-wolf: 23.9 ± 2.9 N=7), as was shrub species richness (Wolf: 22.7 ± 1.3, N=7, p = 0.02; Non-wolf: 18.142± 1.550 N=7). Furthermore, species area relationships for shrubs and forbs show that diversity is higher at all scales in wolf-occupied areas. Also, grass and fern cover was lower in wolf areas (only ferns were significant). Several species of forbs considered to be in decline and sensitive to deer browsing (“looser species”), showed higher abundance, cover and/or reproductive effort in wolf areas ( Aralia nudicaulis, Circea alpina, Huperzia lucidula, Mitchella repens, and Pyrola secunda). In addition, several orchid species were found only in wolf areas (Platanthera sp., Malaxis monophyllos, and Corallorhiza trifida). This study provides strong evidence of trophic effects but more research oriented at ruling out confounding factors and identifying top-down vs. bottom-up effects is needed.