Road verges as invasion corridors? A spatial hierarchical test in an arid ecosystem
Disturbed habitats are often swiftly colonized by alien plant species.\nHuman inhabited areas may act as sources from which such aliens\ndisperse, while road verges have been suggested as corridors\nfacilitating their dispersal. We therefore hypothesized that (i) houses\nand urban areas are propagule sources from which aliens disperse, and\nthat (ii) road verges act as corridors for their dispersal. We sampled\npresence and cover of aliens in 20 plots (6 x 25 m) per road at 5-km\nintervals for four roads, nested within three localities around cities\n(n = 240). Plots consisted of three adjacent nested transects. Houses (n\n= 3,349) were mapped within a 5-km radius from plots using topographical\nmaps. Environmental processes as predictors of alien composition\ndiffered across spatial levels. At the broadest scale road-surface type,\nsoil type, and competition from indigenous plants were the strongest\npredictors of alien composition. Within localities disturbance-related\nvariables such as distance from dwellings and urban areas were\nassociated with alien composition, but their effect differed between\nlocalities. Within roads, density and proximity of houses was related to\nhigher alien species richness. Plot distance from urban areas, however,\nwas not a significant predictor of alien richness or cover at any of the\nspatial levels, refuting the corridor hypothesis. Verges hosted but did\nnot facilitate the spread of alien species. The scale dependence and\nmultiplicity of mechanisms explaining alien plant communities found here\nhighlight the importance of considering regional climatic gradients,\nlandscape context and road-verge properties themselves when managing\nverges.