Escaping captivity: The biological invasion risk from vertebrate species in zoos

Worldwide, invasive alien species increasingly contribute to environmental change and are a massive drain on social and economic resources. In Australia, the detection of new vertebrate incursions (i.e., alien species not currently established) has increased over the last decade. In other parts of the world, zoos have been identified as one of the influential pathways for the establishment of alien vertebrate species. We quantified the number of vertebrate species released (escaped and stolen) from Australian zoos between 1870 and 2010. The majority of reported releases (185 out of 230) have occurred since 1985. Most of the species (77.9%), which have escaped, or been stolen, from Australian zoos have only ever been released once. In sum, escapes were much more common (89%) than thefts. Compared to the other three vertebrate classes (amphibians, birds, mammals) reptiles experienced a significantly greater proportion of thefts than expected by chance. Almost half of all escapes (46%) were bird species. Birds also had the lowest retrieval rate, and therefore posed the greatest potential risk to establishment and subsequent invasion. We used phylogenetic logistic regression models to assess the association of evolutionary traits correlated with the propensity of a bird species for escaping. There was only weak evidence of phylogenetic signal (association among related species) in the tendency of a bird to escape. Bird species were significantly more likely to have escaped if their current total collection size was larger. There was no relationship between escape and the type of holding (aviary versus free-range/open-pond), or life history traits (adult body size and geographic breeding range size). Zoos are a prominent part of our culture and play a valuable role in education and conservation. Captive animals, including those in zoos, are subject to release, through both intentional and unintentional pathways, however, the establishment of alien species associated with Australian zoos is extremely low. We conclude that, in Australia, the risk of introduction by alien species from zoos is low, and substantially less than other ‘backyard’ and illegal sources of private species keeping and trade.