Black Rat (Rattus rattus) Predation on Nonindigenous Snails in Hawai'i: Complex Management Implications

Understanding interactions among nonindigenous species that pose a threat to native species Is crucial to effectively preserve native biodiversity. Captive feeding trials demonstrated that the black rat, Rattus rattus, will readily consume two of the most destructive nonindigenous snails, the giant African snail, Achatina fulica (100% predation), and the predatory snail Euglandina rosea (80% predation). Rats consumed snails from the entire size range offered (11.5 to 59.0 mm shell length), suggesting that there is no size refuge above which snails can escape rat predation. Damaged E. rosea shells from the captive feeding trials were compared with shells collected in the Wai'anae Mountains, O'ahu. This revealed evidence that R. rattus is responsible for it least 7%-20% of E. rosea mortality. However, this is likely a substantial underestimate because 67% of E. rosea shells in the captive feeding trials were damaged in such a way that they would not have been collected in the field. Therefore, we hypothesize that reduction or eradication of R. rattus populations may cause an ecological release of some nonindigenous snail species where these groups coexist. As such, effective restoration for native snails and plants may not be realized after R. rattus removal in forest ecosystems as a consequence of the complex interactions that currently exist among rats, nonindigenous snails, and the remaining food web.