Late-successional forest communities of the eastern United States are believed to be highly resistant to invasion by exotic plant species.  Intact forests dominated by eastern hemlock (Tsuga canadensis) once seemed to be good examples of this axiom.  However, the integrity of many hemlock forests is currently threatened by a host of disturbances including the devastating impact of an introduced pest, the hemlock woolly adelgid (Adelges tsugae). Interactions of co-occurring disturbances, including elevated white-tailed deer (Odocoileus virginianus) populations, may strongly influence community and ecosystem response to hemlock mortality. These compounded perturbations may provide opportunities for the establishment of new plants in communities not prone to invasion.

We used repeated censuses of exclosures and paired controls to investigate the role of a generalist herbivore, white-tailed deer, in the invasion of exotic plant species in eastern hemlock forests in New Jersey and Pennsylvania that spanned a gradient in the severity of canopy disturbance caused by hemlock woolly adelgid. We used maximum likelihood estimation and information theoretics to quantify the strength of evidence for alternative models of the influence of canopy disturbance severity and its interaction with deer density on exotic plant abundance. Over the study period, the abundance of all three exotic plants increased significantly more in the control plots than in the paired exclosures. For all species, the inclusion of canopy disturbance parameters resulted in models with substantially greater support.

Results/Conclusions

Our results suggest that hemlock woolly adelgid related canopy decline can accelerate the invasion of exotic plants and that white-tailed deer herbivory can interact with canopy decline to magnify the observed impact. In addition, our results provide compelling evidence of nonlinear relationships between deer density and the impact of herbivory on exotic species abundance. These findings highlight the important role of herbivore density in determining impacts on plant abundance and provide evidence of the operation of multiple mechanisms in exotic plant invasion. Although many forest ecosystems are currently experiencing unprecedented levels of novel and simultaneous disturbances, ecologists do not fully understand the role of disturbances and their interactions in influencing community susceptibility to invasion. These compounded disturbances may have an impact greater than the sum of the individual events and have the potential to fundamentally alter ecosystems.