"The impacts of hyperabundant white-tailed deer (Odocoileus virginianus) populations on understorey plant community structure and composition are well-established"................ However, few studies have examined how the recovery of wolves might moderate these effects"........... "Recent studies of species interactions in Yellowstone National Park (YNP) suggest that the recovery of wolf populations can naturally ameliorate ungulate-caused ecosystem simplification"............. "In this study, we examine whether a similar trophic cascade was triggered by the recovery of the Great Lakes wolf population in northern Wisconsin"........... "In addition, by assessing community-level responses as opposed to species-level responses and by measuring across several spatial scales of observation, we hope to inform future research by identifying the ideal response variable and spatial scale for detecting effects of top predators in similar terrestrial systems"............ "As predicted for a trophic cascade response, forb species richness at local scales (10 m2) was significantly higher in high wolf areas (high wolf areas: 10.7 ± 0.9, N = 16, low wolf areas: 7.5 ± 0.9, N = 16, P < 0.001), as was shrub species richness (high wolf areas: 4.4 ± 0.4, N = 16, low wolf areas: 3.2 ± 0.5, N = 16, P < 0.001)"................... "Also as predicted, percentage cover of ferns was lower in high wolf areas (high wolf areas: 6.2 ± 2.1, N = 16, low wolf areas: 11.6 ± 5.3, N = 16, P < 0.05)"

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Recolonizing wolves trigger a trophic cascade in Wisconsin (USA)

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• First published: 6 June 2013

1. Summary

   1. We tested the hypothesis that wolves are reducing local browse intensity by white-tailed deer, thus indirectly mitigating the biotic impoverishment of understorey plant communities in northern Wisconsin.
   2. To assess the potential for such a top-down trophic cascade response, we developed a spatially and temporally explicit model of wolf territory occupancy based on three decades of wolf monitoring data. Using a nested multiscale vegetation survey protocol, we compared the understorey plant communities of northern white cedar wetlands found in high wolf areas with control sites found in low wolf areas.
   3. We fit species–area curves for plant species grouped by vegetation growth form (based on their predicted response to release from herbivory, i.e. tree, seedling, shrub, forb, grass, sedge or fern) and duration of wolf territory occupancy.
   4. As predicted for a trophic cascade response, forb species richness at local scales (10 m2) was significantly higher in high wolf areas (high wolf areas: 10.7 ± 0.9, N = 16, low wolf areas: 7.5 ± 0.9, N = 16, P < 0.001), as was shrub species richness (high wolf areas: 4.4 ± 0.4, N = 16, low wolf areas: 3.2 ± 0.5, N = 16, P < 0.001). Also as predicted, percentage cover of ferns was lower in high wolf areas (high wolf areas: 6.2 ± 2.1, N = 16, low wolf areas: 11.6 ± 5.3, N = 16, P < 0.05).
   5. Beta richness was similar between high and low wolf areas, supporting earlier assumptions that deer herbivory impacts plant species richness primarily at local scales. Sampling at multiple spatial scales revealed that changes in species richness were not consistent across scales nor among vegetation growth forms: forbs showed a stronger response at finer scales (1–100 m2), while shrubs showed a response across relatively broader scales (10–1000 m2).
   6. Synthesis. Our results are consistent with hypothesized trophic effects on understorey plant communities triggered by a keystone predator recovering from regional extinction. In addition, we identified the response variables and spatial scales appropriate for detecting such differences in plant species composition. This study represents the first published evidence of a trophic cascade triggered by wolf recovery in the Great Lakes region.
   7. Photo pair of understorey vegetation within the Chequamegon-Nicolet National Forest, WI.( top picture below) Shows a high wolf area (within the Bootjack Lake pack territory) and (bottom pictue below) shows the paired low wolf area (in the buffer zone between the Bootjack Lake pack and the Miles Lake pack).

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Diagram of hypothesized tri-trophic interactions in northern Wisconsin forests. Solid arrows represent direct positive and negative interactions. Dashed arrows represent hypothesized indirect interactions. Dotted line represents competitive interactions.

intensity of wolf impact based on 10 years (1998–2008) of wolf pack territory data (WiDNR). Years of occupancy represent the duration of wolf pack tenure. High wolf areas = 8–10 years of occupancy, low wolf areas = 0–3 years of occupancy