Rodent Eradication on Cocos Island, Guam: Integrating Wildlife Damage Management, Resort Operations, and Non-Target Concerns

Abstract

Introduced Polynesian rats threatened native bird populations and forest habitat on Cocos Island, Guam. To eliminate the threat posed by rat populations, a rodent eradication was conducted on the 33.6-hectare off-shore island in March and April 2009. An integrated approach to eradication was implemented that included trapping, bait stations, and hand broadcast of rodenticide bait. Trapping was conducted within the resort buildings, where human activity precluded the use of rodenticides. Bait stations, employed in commensal resort settings, were designed to prevent terrestrial crabs from accessing the bait. In addition, bait station deployment and retraction methods were used to reduce impacts on daily resort operations. Non-target concerns, primarily with native forest birds during broadcast operations, supported a decision to use diphacinone, a rodenticide with low avian toxicity risk. Bait consumption by the locally threatened Micronesian starling was evident during the broadcast application, but substantial monitoring for non-target impacts revealed no mortality or sublethal effects for starlings or other potential non-targets. Eradication operations on Cocos Island present a prime example of integrated wildlife damage management, combining traditional eradication methods with novel approaches to address site-specific challenges.

Nontarget Risk and Environmental Fate of the Broadcast Application of a Diphacinone Rodenticide at Mōkapu and Lehua Islands, Hawai'i

“…Although the 2009 eradication attempt on Lehua was not successful, the success of Mōkapu, along with numerous ground-based eradications that have used diphacinone around the world (e.g., Lujan et al 2010, Bell et al 2007 demonstrate that there is no intrinsic deficiency associated with using diphacinone for rat eradications. Rather, methods and regulatory restrictions need to be adjusted to accommodate an appropriate exposure profile for this compound and application method.…”

Non-target species mortality and the measurement of brodifacoum rodenticide residues after a rat (Rattus rattus) eradication on Palmyra Atoll, tropical Pacific

“…However, it must be noted that diphacinone is a first generation anticoagulant with lower toxicity and persistence than brodifacoum, which could explain the lack of residues on Mokapu and Lehua. Lujan et al (2010) reported that on Cocos Island, hermit crabs were sampled 7, 30, 60, and 90 days post-bait application. Brodifacoum residues were detected in one of 12 hermit crab samples, and diphacinone residues were not detected in any of the 12 hermit crabs.…”

Diverse examples from managing invasive vertebrate species on inhabited islands of the United States

“…Reintroduction attempts for one, the Guam rail or ko'ko' (Gallirallus owstoni), were attempted on Guam in areas depopulated of B. irregularis, but rails were extirpated by feral cats (Felis catus) (US Fish and Wildlife Service 2009). Cocos Island was a potential haven for Guam rail reintroduction due to absence of B. irregularis, feral cats, feral ungulates (Lujan et al 2010), and potential for removing other possible rail predators.…”