Animal trade and non-indigenous species introduction: the world-wide spread of squirrels

Aim In this study, a dataset on world-wide squirrel introductions has been used to locate the relative pathways and to determine the factors correlated with species establishment. Location The world. Methods The analysis includes a chronological table of introductions, a biogeographical analysis and an assessment of the likelihood of establishment according to species, propagule pressure, area of origin and characteristics of the recipient area. Results The main vector of such introductions was the intentional importation of live animals. Introductions increased in developed countries and proportionately to the volume of imported mammals. Moreover, areas characterized by higher numbers of native squirrels were more invaded. Squirrels were often introduced deliberately and only to a smaller extent escaped from captivity. The likelihood of their establishment increased proportionately to the number of animals released and decreased proportionately to the increase of the latitudinal distance between the recipient area and the native range of the species. The likelihood that the release of one pair of either Sciurus or Callosciurus species would establish a new population was higher than 50%. Main conclusion Squirrels proved to be successful invaders and their importation should be restricted so as to prevent further introductions.