Road verges: Corridors for plant invasions — A spatial hierarchical approach

Abstract

Inhabited areas are often sources from which invasive species disperse. Road verges have been suggested as corridors facilitating this dispersal, as disturbed habitats are swiftly colonised by problem species. We therefore hypothesized that (i) houses and urban areas are sources from which problem plants disperse, and that (ii) verges act as corridors for the dispersal of problem plant species. To test these hypotheses a spatial hierarchical approach was used as ecological processes vary across spatial scales. We sampled presence and cover of problem plants in 20 plots per road at 5-km intervals for four roads, nested within three city-centred localities (n = 240 plots). Roads started off from Beaufort West, Prieska, and Middelburg; South African transit cities with no other major urban areas within a radius of 200 km. We also mapped permanent structures (e.g., houses and train stations; n = 3349) in a 5-km radius around plots from topographical maps. Environmental processes as predictors of problem plants differed across spatial scales. At the regional scale, Canonical Correspondence Analysis showed that annual precipitation, elevation, bare soil and indigenous plant cover were the strongest predictors of problem species composition. At the locality scale, environmental variables predicting problem species composition differed strongly between localities. At the road scale, correlation analysis of problem species richness with housing showed that richness was significantly positively correlated with housing density, particularly within 1500 m from plots. Environmental variables as predictors of problem species composition differed across spatial levels, confirming the need for a spatial hierarchical approach in such studies. Distance of plots from urban areas was never a significant predictor for problem species richness or cover at any of the spatial levels. Density and proximity of structures predicted problem species richness but never cover, suggesting that propagule pressure affects successful colonisation while local variables affect population growth.