USDA National Wildlife Research Center Symposia


Date of this Version

August 2007


Published in: Witmer, G. W., W. C. Pitt, and K. A. Fagerstone, editors. 2007. Managing vertebrate invasive species: proceedings of an international symposium. USDA/APHIS Wildlife Services, National Wildlife Research Center, Fort Collins, Colorado, USA. Also available online at


Invasive rats continue to colonize rat-free islands around the world. To prevent rats from establishing on rat-free islands, especially following their eradication, biosecurity actions are required to enable early detection and elimination. Rats arrive at islands by both human transportation and by swimming. There are very little data on the rates of rat transportation by humans, although it is known that they are not negligible. There are better data on the distances rats can swim, allowing estimates to be made of the risk of reinvasion of islands close to source populations. Biosecurity prioritization must take place across all rat-free islands, balancing the likelihood and impact of rat establishment. Dense grids of poison bait stations are not preferable for preventing rat invasion. Instead, surveillance systems that integrate multiple device types appear to be best for intercepting invading rats, but must be tested to ensure they are effective. This can be done by releasing a controlled number of monitored rats onto a rat-free island. Islands can now be maintained rat-free despite non-negligible reinvasion rates; however, in some cases islands must be managed within a larger meta-population context and eradication may never be achieved.