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 at 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. Rapid population declines and species extinctions have been reported following the widespread introduction of nonindigenous species in Hawai‘i (Burney et al. 2001, Athens et al. 2002). Human intervention is then often required for short-term recovery or maintenance of native biodiversity (Burney and Burney 2007). Unfortunately, insufficient understanding of both the magnitude of the threat that nonindigenous species pose to native biodiversity and the potentially complex interactions among the introduced species can lead to unexpected outcomes (Novacek and Cleland 2001, Doak et al. 2008). Given the large number of nonindigenous species that have altered Hawaiian ecosystems, understanding the interactions among nonindigenous species is crucial to effectively preserve the remaining native biodiversity. Introductions of rats (Rattus exulans Peale, R. norvegicus Berkenhout, R. rattus L.) and terrestrial snails have been implicated in the decline of native Hawaiian flora and fauna (Hadfield 1986, Burney et al. 2001, Athens et al. 2002, Joe and Daehler 2008). All three rat species were introduced to the Hawaiian Islands by people and are among the most noxious invasive species on islands worldwide (Lowe et al. 2000, Russell and Clout 2004, Pacific Science (2009), vol. 63, no. 3:339–347 : 2009 by University of Hawai‘i Press All rights reserved 1 Funding and support was provided by U.S. Army Environmental, O‘ahu; the University of Hawai‘i Ecology, Evolution, and Conservation Biology program (NSF grant DGE05-38550 to K. Y. Kaneshiro); and the Achievement Rewards for College Scientists program. Manuscript accepted 23 November 2008. 2 Corresponding author: Department of Zoology, University of Hawai‘i at Mānoa, 2538 McCarthy Mall, Edmonson 152, Honolulu, Hawai‘i 96822 (e-mail: [email protected]). 3 Center for Conservation Research and Training, Pacific Biosciences Research Center, University of Hawai‘i at Mānoa, 3050 Maile Way, Gilmore 408, Honolulu, Hawai‘i 96822. 4 Department of Botany, University of Hawai‘i at Mānoa, 3190 Maile Way, Room 101, Honolulu, Hawai‘i