http://klamathconservation.org/docs/blogdocs/Callanetal2013.pdf
Recolonizing wolves trigger a trophic cascade
in Wisconsin (USA)
Ramana Callan1
*, Nathan P. Nibbelink2
, Thomas P. Rooney3
, Jane E. Wiedenhoeft4 and
Adrian P. Wydeven4
1
State University of New York College of Environmental Science and Forestry, Box 37, Wanakena, NY 13695, USA;
2
Warnell School of Forestry and Natural Resources, The University of Georgia, 180 E. Green Street, Athens, GA
30602, USA; 3
Department of Biological Sciences, Wright State University, 3640 Colonel Glenn Hwy, Dayton, OH
45435, USA; and 4
Wisconsin Department of Natural Resources, 875 S 4th Street, Park Falls, WI 54552, USA
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.
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