Modelling chorotypes of invasive vertebrates in mainland Spain

We investigated the existence of chorotypes - assemblages of species with similar geographical ranges - of invasive species in a host territory, and their potential use to advocate similar control or management strategies for species in the same chorotype. We analysed the distribution of 13 exotic terrestrial vertebrate species (six birds, six mammals, and one reptile) with well-known distributions in mainland Spain. We used the presence/absence data on a grid of 10 km x 10 km UTM cells from the Atlases of terrestrial vertebrates of Spain. These data were aggregated to a grid of 50 km x 50 km UTM cells, because it entailed no loss of meaningful information and allowed dealing with a much lower number of cells. Using cluster analysis and a probabilistic assessment of the classification, we identified seven significant chorotypes: four multispecific and three monospecific. The compound chorotypes grouped together species that tended to share certain characteristics about their introduction, release cause, establishment, and spread. We modelled the chorotypes using a favourability function based on a generalized linear model and 31 variables related to spatial situation, topography, lithology, climatic stability, energy availability, water availability, disturbances, productivity, and human activity. Climatic factors affected the favourability for every chorotype, whereas human variables had a high influence in the distribution of three chorotypes involving eight species. On the basis of these variables, we identified favourable areas for all the chorotypes in mainland Spain. The favourability for a chorotype in an area may be a useful criterion for evaluating the local conservation concern due to the whole set of species. Favourable but unoccupied areas can be used to infer possible colonization areas for each chorotype. We recommend using chorotypes to optimize broad-scale surveillance of invasive species.