Callosciurus squirrels: Worldwide introductions, ecological impacts and recommendations to prevent the establishment of new invasive populations

1. 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. 2. Here we review the worldwide introductions, ecology and impacts of two tree squirrel species, Callosciurus erythraeus and C. finlaysonii, and discuss the elements of a strategy to reduce squirrel introductions and settlements. 3. C. erythraeus has established viable populations in Argentina, France, Netherland, 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. 4. The most evident damage produced by these species relates to bark-stripping behavior that can be severe and significantly impact trees and timber plantations. Data on negative impacts to native species are reported but are not yet formally quantified. Both squirrel species carried with them parasites from the native range into the new habitats, causing new species introductions. 5. The ability of tree squirrels to establish successfully often with 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 pro-active approach on preventing new introductions should therefore include trade restrictions and be combined with public education initiatives at the national and the European scale. 6. Tree squirrels represent an ‘alien species conundrum’. Experience from the UK and Italy have shown, that if action is delayed until introductions are recognised as a problem, it is generally too late to effectively control them due to logistic, legal, economic reasons or lack of public support. In case of new populations, a rapid response mechanism is therefore critical. Once established populations may become invasive and difficult or impossible to control.