Long-Term Regional Shifts in Plant Community Composition Are Largely Explained by Local Deer Impact Experiments

Abstract

The fact that herbivores and predators exert top-down effects to alter community composition and dynamics at lower trophic levels is no longer controversial, yet we still lack evidence of the full nature, extent, and longer-term effects of these impacts. Here, we use results from a set of replicated experiments on the local impacts of white-tailed deer to evaluate the extent to which such impacts could account for half-century shifts in forest plant communities across the upper Midwest, USA. We measured species' responses to deer at four sites using 10–20 year-old deer exclosures. Among common species, eight were more abundant outside the exclosures, seven were commoner inside, and 16 had similar abundances in- and outside. Deer herbivory greatly increased the abundance of ferns and graminoids and doubled the abundance of exotic plants. In contrast, deer greatly reduced tree regeneration, shrub cover (100–200 fold in two species), plant height, plant reproduction, and the abundance of forbs. None of 36 focal species increased in reproduction or grew taller in the presence of deer, contrary to expectations. We compared these results to data on 50-year regional shifts in species abundances across 62 sites. The effects of herbivory by white-tailed deer accurately account for many of the long-term regional shifts observed in species' abundances (R2 = 0.41). These results support the conjecture that deer impacts have driven many of the regional shifts in forest understory cover and composition observed in recent decades. Our ability to link results from shorter-term, local experiments to regional long-term studies of ecological change strengthens the inferences we can draw from both approaches.

Deer‐mediated changes in environment compound the direct impacts of herbivory on understorey plant communities

“…Results from this study and Frerker, Sabo & Waller () support the contention that regional abundances of many native forb, shrub and palatable tree species may be threatened by overabundant deer. Deer impacts are not only due to the direct effects of herbivory on susceptible plants but also the alteration of a broad suite of environmental conditions.…”

“…Frelich & Lorimer ; Tilghman ; Miller, Bratton & Hadidian ; Anderson ; Balgooyen & Waller ; Fletcher et al . ; Horsley, Stout & DeCalesta ; Frerker, Sabo & Waller ; Nuttle, Ristau & Royo ; Bradshaw & Waller ), losses of plant species richness and community diversity (e.g. Rooney & Dress ; Horsley, Stout & DeCalesta ; Bressette, Beck & Beauchamp ; Shelton et al .…”

“…). Previous work with these exclosures (Frerker, Sabo & Waller ) revealed that deer exclusion affects populations of common understorey species, increasing tree regeneration and shrub and forb cover, and decreasing the abundance of graminoids. To extend this study, we have coupled vegetation and environmental data from the same exclosures to test these hypotheses:…”

Interactions between white-tailed deer density and the composition of forest understories in the northern United States

“…Relative to their population size, white-tailed deer (Odocoileus virginianus Zimmerman) have a disproportionately large effect on their surrounding environment (Waller and Alverson, 1997) and can have an immediate impact on forest health and diversity by reducing the presence and abundance of commercially and ecologically important tree species through preferential browsing or can influence forests indirectly by altering habitat availability for other wildlife and forest-dependent organisms (Rooney and Waller, 2003). A number of plant community studies have employed methods including fenced exclosures (e.g., White, 2012;Frerker et al, 2014), enclosures (Horsley et al, 2003;Nuttle et al, 2014), and island studies (e.g., Mudrak et al, 2009;Cardinal et al, 2012); however, studies linking deer density with forest structure across large geographic regions are limited. White-tailed deer have strong negative impacts on forest understory plant communities in North America, but future research should seek to evaluate the potential for plant species shifts in areas with differing deer densities (Habeck and Schultz, 2015).…”

“…White-tailed deer have strong negative impacts on forest understory plant communities in North America, but future research should seek to evaluate the potential for plant species shifts in areas with differing deer densities (Habeck and Schultz, 2015). As deer may account for up to half of the variability in long-term forest vegetation dynamics (Frerker et al, 2014), understanding how deer density may affect future trends in vegetation growth and survival is essential to maintaining the ecosystem services that forests provide.…”