RESEARCH ARTICLE Road verges as invasion corridors? A spatial hierarchical test in an arid ecosystem Jesse M. Kalwij �� Suzanne J. Milton �� Melodie A. McGeoch Received: 9 November 2007 / Accepted: 24 January 2008 / Published online: 14 February 2008 �� Springer Science+Business Media B.V. 2008 Abstract Disturbed habitats are often swiftly col- onized by alien plant species. Human inhabited areas may act as sources from which such aliens disperse, while road verges have been suggested as corridors facilitating their dispersal. We therefore hypothesized that (i) houses and urban areas are propagule sources from which aliens disperse, and that (ii) road verges act as corridors for their dispersal. We sampled presence and cover of aliens in 20 plots (6 9 25 m) per road at 5-km intervals for four roads, nested within three localities around cities (n = 240). Plots consisted of three adjacent nested transects. Houses (n = 3,349) were mapped within a 5-km radius from plots using topographical maps. Environmental pro- cesses as predictors of alien composition differed across spatial levels. At the broadest scale road- surface type, soil type, and competition from indig- enous plants were the strongest predictors of alien composition. Within localities disturbance-related variables such as distance from dwellings and urban areas were associated with alien composition, but their effect differed between localities. Within roads, density and proximity of houses was related to higher alien species richness. Plot distance from urban areas, however, was not a significant predictor of alien richness or cover at any of the spatial levels, refuting the corridor hypothesis. Verges hosted but did not facilitate the spread of alien species. The scale dependence and multiplicity of mechanisms explain- ing alien plant communities found here highlight the importance of considering regional climatic gradi- ents, landscape context and road-verge properties themselves when managing verges. Keywords Anthropogenic disturbance Exotics Invasion Invasive species Karoo Habitat conduits Road ecology Seed dispersal South Africa Introduction Habitat corridors may improve population persistence by facilitating the exchange between populations and the colonization of suitable habitat (Beier and Noss 1998 Gonzalez et al. 1998), and by directing the movement of propagules to habitats with a relatively high survival probability (Wenny 2001). Conserva- tion measures, therefore, often aim at connecting isolated populations through habitats corridors, reducing the effect of local extinctions (e.g., Hazell Electronic supplementary material The online version of this article (doi:10.1007/s10980-008-9201-3) contains supple- mentary material, which is available to authorized users. J. M. Kalwij (&) S. J. Milton M. A. McGeoch Department of Conservation Ecology and Entomology/ DST-NRF Center of Excellence for Invasion Biology, Stellenbosch University, Private Bag X1, Matieland ZA-7602, South Africa e-mail: kalwij@sun.ac.za 123 Landscape Ecol (2008) 23:439���451 DOI 10.1007/s10980-008-9201-3

and Gustafsson 1999). However alien plants, such as invasive and alien species or noxious weeds, may also profit from such corridors. This potentially compromises the conservation value of these habitats and the adjacent matrix (Proches �� et al. 2005). The most widely available linear habitats in human- influenced ecosystems are roadside verges, i.e., strips of habitat situated between the road surface and adjacent land. The corridor function of road verges is thus of growing interest to ecologists and conserva- tion practitioners (Spellerberg 1998 Saarinen et al. 2005 Sykora �� et al. 2002). Road verges may play a constructive role as habitat reserves for native species (Esler and Milton 2006 Forman 2003), particularly if less affected by anthropogenic disturbance than adjacent land (Spooner and Lunt 2004). They may also act as effective habitat corridors, as has been shown for various mobile species in observational studies (e.g., Ries et al. 2001 Saarinen et al. 2005). However, verges are commonly affected by various anthropo- genic disturbances and edge effects. This may result in colonization and eventually dominance of verges by disturbance-adapted alien species, which establish readily in disturbed habitats and have high seed production (Pauchard and Alaback 2004 Hansen and Clevenger 2005). Once established, populations of alien species may spread rapidly along the continuous habitat provided by the road verge, particularly along those verges that are disturbed as a result of road construction and management practices (Christen and Matlack 2006). High propagule pressure from alien- dominated verges may also lead to invasion of adjacent land, and spread of alien plant species across the landscape (Richardson et al. 2000 Von Holle and Simberloff 2005). The role of road verges as either habitat reserve or invasive species reservoir and conduit is likely to be ecosystem and management dependent (Von Holle and Simberloff 2005). In arid ecosystems, where water resources are limited and plants have a relatively low growth and reproduction rate, verges are especially susceptible to invasion due to road construction and verge management related distur- bances (Spellerberg 1998), to increased fire frequency and intensity (Milberg and Lamont 1995), and to increased water and nutrient input as a result of road surface run-off (Milton and Dean 1998 O���Farrell and Milton 2005). While it has been well documented that verges in arid to semi-arid ecosystems are commonly colonized by alien species (e.g., Gelbard and Belnap 2003 Pauchard and Alaback 2004), the demographic processes of road- verge invasion are still little studied (Christen and Matlack 2006). Urban, agricultural, and other areas with high levels of human activity often serve as sources of alien and invasive species propagules (Pauchard and Alaback 2004 Milton and Dean 1998 Larson 2003 Qian and Ricklefs 2006). The invasion process may thus result in a spatial distribution of alien species with highest occurrences and abundances near urban centers (Arevalo �� et al. 2005 Grapow and Blasi 1998), farm houses (Dauer et al. 2007), or adjacent to agroecosystems (Gelbard and Belnap 2003 O���Farrell and Milton 2005 Johnston and Johnston 2004 Wester and Juvik 1983 Rentch et al. 2005). The process of dispersal and colonization is likely to follow a Poisson or negative binomial distribution from source to sink, reflected by an advancing wave of high to low richness and abundance of alien species (Levin et al. 2003 Christen and Matlack 2006). However, both the pattern of richness and abundance and the processes underlying them are difficult to observe directly or to quantify in field experiments as they have various distinct spatial and temporal components (Christen and Matlack 2006 Nathan 2006). Bullock and Clarke (2000), for example, showed that seed dispersal of Erica species was best described as a combination of two functions: one describing a short distance seed rain and the other describing the long distance dispersal. Although long distance dispersal events are rare and difficult to quantify in field experiments, their effect on species dispersal rate may be disproportionally important (Nathan 2006 Von der Lippe and Kowarik 2007). Spatially explicit analysis of colonization patterns is therefore required to control for the spatiotemporal limitations of observational and experimental studies on long distance seed dispersal (Crawley and Brown 2004 Spooner et al. 2004). Here we test the hypothesis that road verges in an arid ecosystem act as conduits for the dispersal of alien plant species originating from human-inhabited areas. To determine if human population density affects the distribution pattern of alien species communities we distinguished between potential propagule dispersal from urban areas (highly 440 Landscape Ecol (2008) 23:439���451 123