The Biology of Introduced Norway Rats on Kiska Island, Alaska, and an Evaluation of an Eradication Approach

Introduced, invasive rats can cause substantial damage to native flora and fauna, including ground-nesting seabirds, when they become established on islands. We tested a control method for introduced Norway rats on Kiska Island, Alaska, during April-May, 2004, by hand-broadcasting rodenticide pellets (0.005% diaphacinone) over a 4-ha area at the rate of 28 kg/ha. We also gathered data on aspects of rat ecology and distribution, although rats were difficult to detect and capture. The rodenticide bait pellets seemed to have been effective in reducing the Norway rat population, however, this is based on a limited observation of rat sign and captures. Four rats were captured on elevational transects on the northside of the island, all below 20 m elevation. Twelve rats captured in other aspects of the study also came from lower elevations. Rat stomach contents revealed that vegetation and seabirds were important components of the diet at the north end of Kiska Island, but stomach contents varied by location depending upon the type of food most readily available. All eight females captured were pregnant and bore an average of 10 embryos. Although the control or eradication of rats at remote locations such as the Aleutian Islands is theoretically possible, there are many challenges posed to resource managers. This field study has provided insight into the ecology and management of Norway rats at Kiska Island, but also points out some of the challenges that remain. Author to whom correspondence should be addressed. E-mail: [email protected] Introduction Introduced, invasive rats pose a serious threat to the native flora and fauna of islands. Rats can be very prolific on islands where there is little mammalian predation and competition for resources. Their omnivorous foraging has led to the endangerment or extinction of many native island species (Moors and Atkinson 1984, Ebbert and Byrd 2002). Most seabirds that nest on islands have not evolved to deal with predation and are very vulnerable to introduced predators, including rats. There has been substantial documentation of the negative impact of introduced Norway rats (Rattus norvegicus) on nesting seabirds of the Aleutian Islands of Alaska (Sowls and Rauzon 2001, Ebbert and Byrd 2002, Major and Jones 2004). There has been a concerted worldwide effort to eradicate introduced rats from islands with numerous successes (Veitch and Clout 2002). USDA Wildlife Services (WS) conducted a successful eradication of roof rats (Rattus rattus) from Buck Island in the U.S. Virgin Islands for the National Park Service (Witmer et al. 1998, Witmer et al. 2001), using a grid of elevated bait stations across the 80-ha island. In recent years, WS National Wildlife Research Center (NWRC) personnel in Fort Collins, Colorado, and at the Hilo, Hawaii, Field Station have been providing data sets for the U.S. EPA for the registration of a 0.005% diphacinone bait pellet to be used for aerially broadcast baiting of conservation areas to manage or eliminate rats (John Eisemann, NWRC, personal communication). The USDI Fish and Wildlife Service (FWS) has proposed to use 0.005% diphacinone bait pellets on the Aleutian Islands of the Alaska Maritime National Wildlife Refuge, but studies are needed to assess the bait efficacy, durability, and non-target hazards. These studies are needed because of the vastly different climate, soils, fauna and flora than what occurs in more tropical settings. The main objective of this study was to assess the efficacy of a broadcast rodenticide bait pellet (0.005% diphacinone) to reduce or eliminate the presence of Norway rats in the test area on Kiska Island, Alaska. Secondary objectives included an evaluation of bait uptake (acceptability/palatability), an assessment of the elevational distribution of rats at the north end of Kiska Island, and a qualitative assessment of reproduction and food