Community-level impacts of white-tailed deer on understorey plants in North American forests: a meta-analysis.

Christopher W Habeck,Alexis K Schultz

doi:10.1093/aobpla/plv119

Abstract

The impacts of introduced or overabundant large herbivores are a concern for the conservation of forest plant communities and the sustainability of ecosystem function. White-tailed deer (Odocoileus virginianus) are considered ecologically overabundant in much of North America. Previous work suggests that impacts of deer overabundance are broadly negative and are consequently degrading forests at multiple ecological and taxonomic levels. However, no quantitative synthesis currently exists to verify the generality or magnitude of these impacts. Here, we report the results of a meta-analysis quantifying the effects of deer exclusion on the diversity, cover and abundance of woody, herbaceous and whole community components of forest understories in North America. In addition, we explore the relationships of environmental and experimental factors on the direction and magnitude of plant community outcomes using meta-regression. Using 119 calculated effect sizes sourced from 25 peer-reviewed articles, we constructed 10 community-specific data sets and found strongly positive diversity, cover and abundance responses of the woody community to deer exclusion, but no significant effects for the herbaceous or whole community components of forest understories. Local deer density and time since exclusion were significant moderators of both whole community and woody community richness. Local deer density also moderated the effects of deer exclusion on whole community cover. Plot area, in contrast, showed no relationship to any of the community response outcomes. We suggest that the use of inadequate diversity indices, non-native species replacement or legacy effects of chronic deer overabundance might explain why the herbaceous and whole community components of forest understories showed no diversity or cover responses to deer exclusion. We also suggest some strategies to increase opportunities for future quantitative syntheses of deer impacts on forests, including providing better access to existing and future data. Ultimately, we show that white-tailed deer have strongly negative impacts on forest understorey plant communities in North America, but these impacts are not ubiquitous for all components of the plant community.

Highlights

At global to regional scales, the composition and functioning of forest ecosystems are being altered by multiplePublished by Oxford University Press on behalf of the Annals of Botany Company
Community, in particular, did not exhibit positive responses to deer removal: (i) the plant community indices reported in the literature are insensitive to the types of negative impacts that white-tailed deer have on herbaceous understorey communities, (ii) native herbs are being replaced by non-native herbs and (iii) chronic deer overabundance has degraded the forest understorey community so severely that recovery of herbaceous species is not probable in the short term
We suggest two potential reasons why the woody community exhibited a strong response to deer removal when the herbaceous community did not: (i) the re-establishment of tree species is more likely to occur over shorter timescales because, even after decades of deer overabundance, there is a relatively constant local rain of propagules sourced from mature individuals in the canopy, whereas re-establishment of locally extirpated herbaceous species depends more strongly on long-distance seed dispersal, and (ii) a variety of vertebrates are effective longdistance dispersers of tree propagules over short timescales, speeding up the process of woody species re-establishment

Summary

Introduction

At global to regional scales, the composition and functioning of forest ecosystems are being altered by multiple. Observational and experimental studies conducted over the past several decades indicate that contemporary white-tailed deer populations can affect the diversity and functioning of forests at levels of biological organization from species (Eschtruth and Battles 2009; Waller and Maas 2013; Thomas-Van Gundy et al 2014) to communities and ecosystems (Ritchie et al 1998; Bressette and Beck 2013; Murray et al 2013), and across taxonomic groups such as plants (Rooney 2009; Dornbush and Hahn 2013), vertebrates (deCalesta 1994; McShea and Rappole 1997) and invertebrate organisms (Ruhren and Handel 2003; Takada et al 2008; Davalos et al 2015) The magnitude of these effects, are neither equal across levels of biological organization or geographical location, nor are they always negative (Royo et al 2010a) nor do they always exist (Kraft et al 2004; Collard et al 2010; Aronson and Handel 2011; Levine et al 2012)

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