Abstract
Excessive herbivory can have transformative effects on forest understory vegetation, converting diverse communities into depauperate ones, often with increased abundance of non-native plants. White-tailed deer are a problematic herbivore throughout much of eastern North America and alter forest understory community structure. Reducing (by culling) or eliminating (by fencing) deer herbivory is expected to return understory vegetation to a previously diverse condition. We examined this assumption from 1992 to 2006 at Fermilab (Batavia, IL) where a cull reduced white-tailed deer (Odocoileus virginianus) abundance in 1998/1999 by 90 % from 24.6 to 2.5/km2, and at West Point, NY, where we assessed interactive effects of deer, earthworms, and invasive plants using 30 × 30m paired fenced and open plots in 12 different forests from 2009 to 2012. We recorded not only plant community responses (species presence and cover) within 1m2 quadrats, but also responses of select individual species (growth, reproduction). At Fermilab, introduced Alliaria petiolata abundance initially increased as deer density increased, but then declined after deer reduction. The understory community responded to the deer cull by increased cover, species richness and height, and community composition changed but was dominated by early successional native forbs. At West Point plant community composition was affected by introduced earthworm density but not deer exclusion. Native plant cover increased and non-native plant cover decreased in fenced plots, thus keeping overall plant cover similar. At both sites native forb cover increased in response to deer reduction, but the anticipated response of understory vegetation failed to materialize at the community level. Deer-favoured forbs (Eurybia divaricata, Maianthemum racemosum, Polygonatum pubescens and Trillium recurvatum) grew taller and flowering probability increased in the absence of deer. Plant community monitoring fails to capture initial and subtle effects of reduced or even cessation of deer browse on browse sensitive species. Measuring responses of individual plants (growth, flowering and reproductive success) provides a more sensitive and powerful assessment of forest understory responses to deer management.
Highlights
Conservation successes should be celebrated, but the acclaim is muted when recovery of a species is so successful that its abundance threatens other species
We evaluated effect of deer reduction on understory plant communities at Fermilab by comparing cover, species richness and vegetation height values pre and post deer reduction [a fixed factor with two levels: GLMMs with study year as a random factor to account for temporal correlation within years]
While nativeannual cover slightly increased after deer reduction, non-nativeannual cover peaked the year after deer reduction and steadily decreased over time (Fig. 2; significant deer reduction  year interaction; v2 1⁄4 17.18, df 1⁄4 1, P < 0.001)
Summary
Conservation successes should be celebrated, but the acclaim is muted when recovery of a species is so successful that its abundance threatens other species. The success of white-tailed deer (Odocoileus virginianus) in North America illustrates a conservation success gone awry. Hunted to near extinction by the late 1800s, the species made such a remarkable comeback that large populations quickly created economic and ecological problems (Leopold et al 1947; Severinghouse and Brown 1956; Halls 1984). While some browsing can have beneficial effects on plant diversity by reducing the competitive ability of some fast growing species (Leonardsson et al 2015), large ungulate populations often create simplified and less diverse ecosystems
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