Callosciurus squirrels: worldwide introductions, ecological impacts and recommendations to prevent the establishment of new invasive populations.
Rodents are traded as pet species, a practice that frequently results in new introduced populations. This is particularly true for tree squirrels where, often, only a few founders can establish viable colonies. Here, we review the worldwide introductions, ecology and impacts of two tree squirrel species, Callosciurus erythraeus and Callosciurus finlaysonii, and discuss the elements of a strategy to reduce squirrel introductions and settlements. C. erythraeus has established viable populations in Argentina, France, The Netherlands, Hong Kong and Japan. An introduction to Belgium may have been stopped successfully. C. finlaysonii has been introduced to Italy, Singapore and Japan. After 1950, the mean number of introduction events was one every two years. The most evident damage caused by these species is bark stripping that can be severe and may significantly impact trees and timber plantations. Data on negative impacts to native species are reported but have not yet been formally quantified. Both squirrel species carried with them parasites from the native range into the new habitats, leading to the introduction of other species. The ability of tree squirrels to establish themselves successfully, often from only a few founders, combined with their human appeal make them high-risk species, and the pet trade should be considered as a high-risk pathway for new introductions. A proactive approach to preventing new introductions should therefore include trade restrictions, and should be combined with public education initiatives at national and European scales. Tree squirrels represent an 'alien species conundrum'. Experience from the UK and Italy has shown that if action is delayed until introductions are recognized as a problem, it is generally too late to control populations effectively, due to logistic, legal or economic reasons, or due to lack of public support. In the case of new populations, a rapid response mechanism is therefore critical. Once established, populations may become invasive and difficult or impossible to control.