Ecological Monographs, 79(2), 2009, pp. 265–280 Ó 2009 by the Ecological Society of America Assessing the relative importance of disturbance, herbivory, diversity, and propagule pressure in exotic plant invasion ANNE K. ESCHTRUTH 1 AND JOHN J. BATTLES Environmental Science, Policy, and Management, 137 Mulford Hall, University of California, Berkeley, California 94720-3114 USA Abstract. The current rate of invasive species introductions is unprecedented, and the dramatic impacts of exotic invasive plants on community and ecosystem properties have been well documented. Despite the pressing management implications, the mechanisms that control exotic plant invasion remain poorly understood. Several factors, such as disturbance, propagule pressure, species diversity, and herbivory, are widely believed to play a critical role in exotic plant invasions. However, few studies have examined the relative importance of these factors, and little is known about how propagule pressure interacts with various mechanisms of ecological resistance to determine invasion success. We quantified the relative importance of canopy disturbance, propagule pressure, species diversity, and herbivory in determining exotic plant invasion in 10 eastern hemlock forests in Pennsylvania and New Jersey (USA). Use of a maximum-likelihood estimation framework and information theoretics allowed us to quantify the strength of evidence for alternative models of the influence of these factors on changes in exotic plant abundance. In addition, we developed models to determine the importance of interactions between ecosystem properties and propagule pressure. These analyses were conducted for three abundant, aggressive exotic species that represent a range of life histories: Alliaria petiolata, Berberis thunbergii, and Microstegium vimineum. Of the four hypothesized determinants of exotic plant invasion considered in this study, canopy disturbance and propagule pressure appear to be the most important predictors of A. petiolata, B. thunbergii, and M. vimineum invasion. Herbivory was also found to be important in contributing to the invasion of some species. In addition, we found compelling evidence of an important interaction between propagule pressure and canopy disturbance. This is the first study to demonstrate the dominant role of the interaction between canopy disturbance and propagule pressure in determining forest invasibility relative to other potential controlling factors. The importance of the disturbance–propagule supply interaction, and its nonlinear functional form, has profound implications for the management of exotic plant species populations. Improving our ability to predict exotic plant invasions will require enhanced understanding of the interaction between propagule pressure and ecological resistance mechanisms. Key words: Alliaria petiolata; Berberis thunbergii; canopy disturbance; Delaware Water Gap National Recreation Area, USA; exotic plants; hemlock woolly adelgid; herbivory; invasibility; invasive plants; Microstegium vimineum; propagule pressure; relative variable importance. INTRODUCTION The ecological threats posed by exotic invasive plant species have intensified the need to better understand the factors determining invasion success. The current rate of invasive species introductions is unprecedented, and the dramatic impacts of invasive plant species on commu- nity and ecosystem function have been well documented (Vitousek and Walker 1989, D’Antonio and Vitousek 1992, Gordon 1998, Mack et al. 2000). Despite the pressing management implications, ecologists do not fully understand the mechanisms that control exotic plant invasion, and general theories explaining commu- nity susceptibility to invasion remain elusive. The extent of exotic plant invasion varies widely among ecosystems. However, it is unclear to what degree these differences result from properties of the invading species, the number and distribution of arriving prop- agules (i.e., propagule pressure), or the inherent susceptibility of an ecosystem to invasion (i.e., invasi- bility; Lonsdale 1999). Invasibility is defined as the probability of establishment and subsequent survival of individual plants per arriving propagule or the increase in biomass or percent cover of the plant species over a specified time at a given propagule pressure (Davis et al. 2000). Our ability to attribute exotic plant invasion to differences in ecosystem invasibility requires an im- proved understanding of the factors that control ‘‘ecological resistance,’’ the community properties that Manuscript received 5 February 2008; revised 23 April 2008; accepted 24 April 2008; final version received 5 June 2008. Corresponding Editor: L. M. Wolfe. 1 E-mail: eschtruth@nature.berkeley.edu 265