Diversity and Invasibility of Southern Appalachian Plant Communities

Abstract

We propose that the relationship between diversity and community invasibility depends on the degree to which community composition is driven by immigration processes. When immigration is enhanced by high propagule pressure or low‐intensity disturbance, the relationship between diversity and exotic species invasion should be positive. Only when such immigration processes are limited should competitive interactions lead to a negative correlation between diversity and invasibility. Moreover, competition should be more apparent at smaller scales where individual plants compete directly for space; thus, diversity and invasibility are more likely to be negatively correlated at small spatial scales. We tested these predictions by comparing exotic and native species diversity of vascular plants across five spatial scales in riparian and upland plant communities in the southern Appalachians. We found a positive relationship between species diversity and exotic invasion in riparian areas at large scales (100 m2), which graded into a negative relationship at small scales (0.01 m2). In uplands, there was a slight positive relationship between native and exotic species diversity at both scales of observation. Overall, riparian areas had more exotic and native species than upland areas, and both native and exotic species diversity increased with flood frequency within the riparian zone. Corresponding Editor: T. J. Stohlgren.

Diversity–invasibility Across an Experimental Disturbance Gradient in Appalachian Forests

“…2. Brown and Peet (2003) observed similar positive relationships between native and nonnative species richness in riparian areas of the southern Appalachians with higher flooding frequency, a pattern attributed to propagule pressure from both native and nonnatives species.…”

“…Several hypotheses have been proposed to account for the paradox between experimental and observational studies (reviewed by Fridley et al 2007), including the scale-dependent nature of diversity-invasibility relationships (Stohlgren et al 1999, Levine 2000, Brown and Peet 2003, Fridley et al 2004, Knight and Reich 2005, covarying extrinsic factors or favorable conditions that promote higher levels of both native and nonnative diversity (Levine and D'Antonio 1999, Naeem et al 2000, Shea and Chesson 2002Fig. 1A), and resource heterogeneity (Davies et al 2005).…”

“…Resource supply and uptake is often altered following disturbance (Huston 2004) because individual mortality leads to decreased uptake and increased availability of resources (Sher and Hyatt 1999). Moreover, disturbance is often proposed as an extrinsic factor contributing to diversity-invasibility relationships (Stohlgren et al 1999, Levine 2000, Naeem et al 2000, Brown and Peet 2003, Gilbert and Lechowicz 2005, Von Holle 2005. However, to our knowledge no studies have experimentally manipulated disturbance and compared pre-disturbance with post-disturbance diversity-invasibility patterns across various spatial scales, although the need for such studies is well recognized (Stohlgren 2002, Fridley et al 2007.…”

Patterns of exotic plant invasions in Pennsylvania’s Allegheny National Forest using intensive Forest Inventory and Analysis plots

“…Secondly, this relationship suggests that ANF's invasibility is currently being driven by immigration processes rather than competitive interactions among native and exotic species (Brown and Peet, 2003). At this stage, locating propagule sources may be the most efficient way of reducing invasive species spread in ANF.…”

“…Several studies support a positive relationship between native plant richness and invasibility by exotic invasive species (Higgins et al, 1999;Stohlgren et al, 1999;Huebner and Tobin, 2006;Belote et al, 2008). Such a relationship suggests that species saturation does not exist but that resources and propagules are abundant causing invasion to be driven by immigration processes rather than competitive interactions among the native and non-native plants (Brown and Peet, 2003). Species-rich sites tend to be resource-rich sites and disturbed sites often show an increase in or a re-distribution of site resources; thus, variables associated with each of these three categories (biotic, abiotic, and disturbance) need to be addressed when attempting to predict vulnerability to invasion.…”