Acceleration of Exotic Plant Invasion in a Forested Ecosystem by a Generalist Herbivore

Abstract

The successful invasion of exotic plants is often attributed to the absence of coevolved enemies in the introduced range (i.e., the enemy release hypothesis). Nevertheless, several components of this hypothesis, including the role of generalist herbivores, remain relatively unexplored. We used repeated censuses of exclosures and paired controls to investigate the role of a generalist herbivore, white-tailed deer (Odocoileus virginianus), in the invasion of 3 exotic plant species (Microstegium vimineum, Alliaria petiolata, and Berberis thunbergii) in eastern hemlock (Tsuga canadensis) forests in New Jersey and Pennsylvania (U.S.A.). This work was conducted in 10 eastern hemlock (T. canadensis) forests that spanned gradients in deer density and in the severity of canopy disturbance caused by an introduced insect pest, the hemlock woolly adelgid (Adelges tsugae). We used maximum likelihood estimation and information theoretics to quantify the strength of evidence for alternative models of the influence of deer density and its interaction with the severity of canopy disturbance on exotic plant abundance. Our results were consistent with the enemy release hypothesis in that exotic plants gained a competitive advantage in the presence of generalist herbivores in the introduced range. The abundance of all 3 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 than the deer density only models. Our results suggest that white-tailed deer herbivory can accelerate the invasion of exotic plants and that canopy disturbance can interact with herbivory to magnify the 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.

Performance and reproduction of an exotic invader across temperate forest gradients

“…5). These results are consistent with other researchers that report decreased M. vimineum growth and reproduction in drought and shade (Claridge and Franklin 2002, Cole and Weltzin 2004, 2005, Glasgow and Matlack 2007, Marshall and Buckley 2008a, Eschtruth and Battles 2009a, Flory and Clay 2009a v www.esajournals.org 2010, Droste et al 2010, Flory 2010, Huebner 2010a, Schramm and Ehrenfeld 2010). Yet, as noted, these associations cannot fully decouple cause and effect in M. vimineum establishment.…”

“…Moreover, our results, along with additional recent work (Cheplick 2010, Miller andMatlack 2010), confirm earlier observations that dispersal distances increase considerably via floodwaters (Mehrhoff 2000, Eschtruth andBattles 2009a). As noted by Miller and Matlack (2010), however, much of the landscape is exempt from overland waterflow, including habitat occupied by M. vimineum.…”

“…2), suggests these habitats are suitable for M. vimineum persistence, in addition to providing potential dispersal corridors (Mehrhoff 2000, Christen and Matlack 2009, Eschtruth and Battles 2009a). Yet, habitat associations cannot disentangle dispersal from niche limitations, and multiple environmental variables shift similarly along the transition from forest interior to edge to exterior.…”

In a long-term experimental demography study, excluding ungulates reversed invader's explosive population growth rate and restored natives

“…Because Alliaria cannot flourish in this forest without deer, our results suggest that previous hypotheses for Alliaria's invasion success (48) may be context dependent. In contrast to studies that demonstrate how native herbivores can increase biotic resistance by consuming invaders (18), our findings show that deer, a native generalist ungulate, find the invasive Alliaria completely inedible, which undoubtedly bolstered its fitness where deer had access (39,41). Thus, our data lend support to the hypothesis that a lack of palatability may be a general trait of highly successful invaders (16,53), particularly in communities with overabundant ungulates (54, 55).…”

Predicting Microstegium vimineum invasion in natural plant communities of the southern Blue Ridge Mountains, USA

“…Flooding and water flow paths that intersect a source population also have the potential to move propagules (Mehrhoff 2000;Miller and Matlack 2010;Warren et al 2011a). While increased deer density accelerates M. vimineum invasion by enhancing local site conditions through herbivory and litter disturbance (Eschtruth and Battles 2009a;Warren et al 2011a), strong evidence of animal dispersal does not exist (Mehrhoff 2000). The 3-km scale represents the average size of sub-populations and is likely to change over time as the invasive finds its way via intermediate-distance dispersal events to available and suitable locations.…”