Are Wolf-Mediated Trophic Cascades Boosting Biodiversity in the Great Lakes Region?

Abstract

In recent years, conservation biologists broadened their efforts beyond genes, species, and ecosystems to include the conservation of species interactions such as mutualisms and predation (Kearns et al. 1998 ; Soule et al. 2003, 2005) . Ecologists generally agree that predators generate top-down effects in food webs, but the consensus ends there. Considerable disagreement remains over the strength of top-down effects, the relative importance of top-down versus bottom-up effects, and how the relative importance of these effects differs among systems, seasons and across scales (Polis 1999 ; Polis et al. 2000 ; Shurin et al. 2002 ; Schmitz et al. 2004) . The question is further complicated because many top predators were significantly reduced in abundance or eliminated from temperate zone ecosystems decades or centuries before ecologists formally conceptualized trophic cascades (cf. Jackson 1997 ; Jackson et al. 2001) . Still, many conservation biologists view the recovery of the gray wolf ( Canis lupus ) in the Great Lakes region as more than a conservation success story. This recovery carries with it the hope and expectation that the top-down effects generated by gray wolves will aid in the maintenance of regional biodiversity (McShea 2005 ; Ray 2005) . High densities of white-tailed deer ( Odocoileus virginianus ) throughout much of the upper Great Lakes region pose a challenge to conservation efforts. Densities are so great that deer harvests have set state records within the last 10 years (e.g., Michigan in 1998, Wisconsin in 2000, and Minnesota in 2003). High densities of deer come at an ecological cost: browsing contributed to the loss of plant diversity over the last few decades (Rooney et al. 2004) , and, in turn, these losses might be generating additional indirect effects on insects, birds, and other species (Rooney and Waller 2003 ; McShea 2005) . Several studies from western North America suggest that recovery of gray wolves generated strong top-down effects on vegetation and lateral effects on assemblages of scavengers (Ripple et al. 2001 ; Wilmers et al. 2003 ; Hebblewhite et al. 2005) . Are wolves having a similar effect in the Great Lakes states? While it might be tempting to simply extrapolate findings from western North America and apply them