The road to invasion: fine-grained distribution and suitability model for Carpobrotus sp. pl., a plant invader on a small Mediterranean island

Carpobrotus is a genus of succulent Aizoaceae originating from South Africa that has become invasive in the Mediterranean and represents a serious threat in coastal ecosystems. On the small island of San Pietro (Sardinia), Carpobrotus sp. pl. is invading also habitats far from the sea. We surveyed the distribution of Carpobrotus on the island using the transect method and defined two models of potential suitability for this taxon based on a set of variables predicting its occurrence at local scale. Our data corroborate previous evidence about the ecological synanthropy of Carpobrotus sp. pl. Using a representative sample of transects (70,720 m in total length), we obtained evidence of 39 sites of occurrence with a higher linear density along paved roads (with high traffic levels) when compared to unpaved dirt roads (with low traffic levels). When compared to other sites, in sites of occurrence we observed a higher density of urbanization and road infrastructure, both at 100-m and 500-m scales. However, after performing a logistic regression analysis using a set of selected non-autocorrelated variables as covariates, and the occurrence of Carpobrotus sp. pl. as the dependent variable, we observed that distance from the coastline was the main predictor. Suitability models suggest that the actual rate of invasion is likely to increase if disturbance (along particular roads) is not effectively managed and that this island can be at risk from a future wider invasion of this taxon, caused by progressive urbanization.