Document: REB-3-35-22

Trends in community structure along the immigration-extinction gradient.

BROWN, R.L.*

University of North Carolina, Chapel Hill, NC 27599-3275 USA ¹

Abstract:
Ecologists often invoke one of two contrasting groups of processes to explain the composition and structure of communities. One group includes density-dependent, extinction-driven processes, such as resource competition and assembly rules, which limit community membership to a particular subset of the available species pool that contains well-adapted, compatible, competitive species. The second group includes processes such as immigration (especially propagule pressure) and disturbance (which reduces competitive intensity) that allow a broader subset of species to establish. I propose ordering communities along a gradient from those organized largely by extinction processes to those organized largely by immigration as a means of understanding previously confusing patterns in the species diversity, compositional predictability, and invasibility of communities. Plant communities of southern Appalachian riparian zones span a broad range of disturbance and propagule pressure and, therefore, provide an ideal system to illustrate the utility of ordering plant communities along an immigration-extinction gradient. I determined species composition, richness and predictability across upland, scour bar, floodplain, and terrace positions. Species richness and exotic invasion is extremely high in floodplain and scour bar communities (immigration-driven communities). In terrace and upland communities (extinction-driven communities) there were 50% fewer species and substantially fewer plots with exotic plants. In addition, the vegetation of scour bars and floodplains is more unpredictable in species composition compared to terraces and uplands. The juxtaposition of high species richness, high frequency of exotic species, and lack of compositional predictability found in floodplain and scour bar communities can be explained by the high rates propagule pressure provided by flood water, combined with periodic disturbances from flood events. We suggest that similar patterns in other plant communities can be understood when viewed in the context of the dominance of immigration and establishment processes.

Keywords: immigration, disturbance, propagule pressure, riparian plant communities, community assembly, immigration-extinction gradient, invasion, exotic species, species richness

This abstract is being presented at: 10:15 AM in session:
Oral Session #59: Plant Communities: Vegetative Analysis.