Interactive effects between canopy gaps and deer on invasive earthworms

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Samuel P. Reed, Lee E. Frelich and Peter B. Reich, Department of Forest Resources, University of Minnesota, St. Paul, MN, Dustin R. Bronson, Northern Research Station, United States Forest Service, Rhinelander, Leah M. Prudent, Sustainable Resources Management, he State University of New York College of Environmental Science and Forestry, Syracuse, NY, Jodi A. Forrester, Forestry and Environmental Resources, North Carolina State University, Raleigh, NC

Background/Question/Methods
Canopy gaps, uncontrolled deer populations, and invasive European earthworms are three of the most common drivers of change in North American temperate forests, each influencing species composition and ecological function. Although these disturbances co-occur on the landscape, they are often studied separately. This presents a gap in our understanding of forest dynamics, as certain disturbances may be linked and have unforeseen ecological impacts. Research has shown that deer presence may lead to increased earthworm biomass and density, but this relationship lacks evidence. Further, whether canopy gaps influence earthworm populations is unexplored and could play an important role in gap regenerative dynamics. Our objective was to determine whether individual and combined deer and canopy gap treatments influence invasive earthworm biomass, density, and species assemblages.
We use two long-term, multi-disturbance experiments in the northern hardwoods of Wisconsin that manipulate canopy gaps and deer exclosures. We sampled earthworms from 0.07-m2 subplots across experimental treatments with 1) deer fencing, 2) 24-m canopy gaps, 3) both 24-m canopy gaps and fencing, 4) and unmanipulated controls. The 3,000+ earthworms collected were then weighed, measured, identified to species, and binned by ecotype (anecic, epigeic, endogeic, or epi-endogeic).
Results/Conclusions
Our preliminary results show that deer fencing and canopy gaps have varying effects on invasive earthworms. Average earthworm biomass and density did not vary significantly between fenced and unfenced control plots, indicating that deer presence alone does not change earthworm’s whole-community biomass or density. However, the average density of epigeic and endogeic earthworms was greater outside of fencing, potentially due to deer primarily effecting earthworms that dwell in the leaf litter or in shallow soil. In addition, within one experimental site the average earthworm density was significantly higher (77 earthworms/m2) in canopy gaps without deer fencing than in canopy gaps with fencing (61 earthworms/m2) (p < 0.04, F=4.24). These results show that the combined influence of canopy gaps and deer may increase invasive European earthworm density to a greater degree than either disturbance alone. Further, our work highlights the unexplored interactive effects between disturbances and how they may impact communities.