Independent Effects of Invasive Shrubs and Deer Herbivory on Plant Community Dynamics

Jeffrey Ward,Scott Williams,Megan Linske

doi:10.3390/f8010002

Abstract

Both invasive species and deer herbivory are recognized as locally important drivers of plant community dynamics. However, few studies have examined whether their effects are synergistic, additive, or antagonistic. At three study areas in southern New England, we examined the interaction of white-tailed deer (Odocoileus virginianus Zimmermann) herbivory and three levels of invasive shrub control over seven growing seasons on the dynamics of nine herbaceous and shrub guilds. Although evidence of synergistic interactions was minimal, the separate effects of invasive shrub control and deer herbivory on plant community composition and dynamics were profound. Plant communities remained relatively unchanged where invasive shrubs were not treated, regardless if deer herbivory was excluded or not. With increasing intensity of invasive shrub control, native shrubs and forbs became more dominant where deer herbivory was excluded, and native graminoids became progressively more dominant where deer herbivory remained severe. While deer exclusion and intensive invasive shrub control increased native shrubs and forbs, it also increased invasive vines. Restoring native plant communities in areas with both established invasive shrub thickets and severe deer browsing will require an integrated management plan to eliminate recalcitrant invasive shrubs, reduce deer browsing intensity, and quickly treat other opportunistic invasive species.

Highlights

Many plant communities are currently under unprecedented stress from a changing climate, and from the abrupt, nearly simultaneous landscape-scale changes in disturbance regimes [1], functional extirpation of major species following the introduction of non-native insects and diseases [2], wide-spread establishment of invasive plants [3], and severe herbivory by historically high deer densities [4,5]
We investigated the effects of differing combinations of deer herbivory and invasive shrub control to address several questions: (1) What are the individual effects of two stressors on distinct guilds—vegetation layers or growth forms including graminoids, forbs, shrubs, and vines? (2) For each guild, are the effects of these two stressors additive, synergistic, or antagonistic? (3) What are the short-term net effects of these two stressors on plant community structure and dynamics? (4) How do these findings help natural resource managers decide which, if any, actions to implement?
Our study found that plant community composition and dynamics were influenced by both historically high deer anthropogenically‐dominated disturbance regimes, and a changing deer herbivory and densities, invasive shrubs

Summary

Introduction

Many plant communities are currently under unprecedented stress from a changing climate, and from the abrupt (in geological time-scales), nearly simultaneous landscape-scale changes in disturbance regimes [1], functional extirpation of major species following the introduction of non-native insects and diseases [2], wide-spread establishment of invasive plants [3], and severe herbivory by historically high deer densities [4,5]. Excessive deer herbivory and invasive plants are problematic stressors of deciduous forests in eastern North America. We examined their relative importance as drivers of plant community composition and dynamics to assist natural resources managers when deciding how to allocate limited resources available for active management. While several studies reported deer herbivory did not affect total herbaceous cover [8,9,10,11], these studies did not separately examine cover of forbs, graminoids, and ferns. As deer density and resulting herbivory intensity increases, the relative dominance of graminoids and ferns increases [12,13,14]

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