32 REPORTS Ecology, 84(1), 2003, pp. 32–39  2003 by the Ecological Society of America DIVERSITY AND INVASIBILITY OF SOUTHERN APPALACHIAN PLANT COMMUNITIES REBECCA L. BROWN 1,3 AND ROBERT K. PEET 2 1 Curriculum in Ecology, CB#3275, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3275 USA 2 Department of Biology, CB#3280, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3280 USA Abstract. We propose that the relationship between diversity and community invasi- bility 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 pos- itive. 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 m 2 ), which graded into a negative relationship at small scales (0.01 m 2 ). 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. Key words: disturbance; diversity; exotic species; immigration–extinction gradient; invasibility; propagule pressure; riparian plant communities; spatial scale; species richness. INTRODUCTION Traditional theory suggests that species-rich com- munities should be more difficult to invade than spe- cies-poor communities owing to more complete use of resources (i.e., resource complementarity; Trenbath 1974) and more intense competition (Elton 1958, Le- vine and D’Antonio 1999). However, available data do not consistently support this prediction. Relatively spe- cies-rich communities have been variously reported to experience high and low levels of exotic species in- vasion, with experimental studies often reporting a neg- ative relationship (Tilman 1997, Knops et al. 1999, Naeem et al. 2000, Prieur-Richard et al. 2000) and ob- servational studies often reporting a positive relation- ship (Stohlgren et al. 1998, Wiser et al. 1998, Lonsdale 1999, Stohlgren et al. 1999, Kalkhan and Stohlgren Manuscript received 8 November 2001; revised 22 May 2002; accepted 5 June 2002; final version received 1 July 2002. Cor- responding Editor: T. J. Stohlgren. 3 Present address: Patrick Center for Environmental Re- search, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103 USA. E-mail: brown@acnatsci.org 2000). The growing problem of exotic species inva- sions and the uncertainty over the functional signifi- cance of species diversity make clarification of the re- lationship between species richness and invasibility particularly important. Levine (2000) has suggested that the inconsistency observed in exotic plant invasion into species-rich communities is a consequence of the scale of obser- vation. When Levine examined the occurrence of native and exotic species on tiny islands along a 7-km stream reach in California, he found a positive correlation, which he attributed to propagule pressure (the abun- dance and diversity of propagules entering a commu- nity). However, when he experimentally added prop- agules to microcosms in a single large riffle, he found that, at the small spatial scale of his microcosms (0.035 m 2 ), high richness was correlated with greater resis- tance to invasion. These results are consistent with Wil- son and Watkins (1994) and Wilson et al. (1995), who found plant niche limitation in grasslands to operate only at small scales where herbs compete directly and where there is no room for substitution among ecolog- ical equivalents.