Pushing the Envelope in Paradise: A Novel Approach to Rat Eradication at Palmyra Atoll

Abstract

Palmyra Atoll National Wildlife Refuge is a moist Central Pacific atoll that supports one of the best remaining tropical forest ecosystems in the region, including 10 species of breeding seabirds and a robust population of the world's largest terrestrial invertebrate, the coconut crab. Despite these riches, the atoll's ecosystem has been modified by introduced black rats that were inadvertently brought to Palmyra during WWII. Between June 1 and 30, 2011, a partnership between the U.S. Fish and Wildlife Service, The Nature Conservancy, and Island Conservation successfully implemented a project to remove rats from Palmyra. Independent monitoring of bait application and its environmental effects was undertaken by the USDA. Over the 28-day operation, a team of 41 people from 5 countries utilized 2 helicopters, 10 slingshots, 148 bait stations, and hand spreading to strategically apply 38,561 kg of rodent bait containing the anticoagulant brodifacoum (25 ppm) to Palmyra's 235 hectares of emergent land. Palmyra's challenging eradication environment demanded the development of a novel approach, such as broadcast application rates between 75 and 85 kg/ha and the use of "bolas" to bait coastal forest canopy to minimize bait drift into the marine environment. Initial findings show minimal non-target impacts as a result of the project, and post-eradication monitoring has failed to detect rats. Increased recruitment by at least 2 native tree species has been observed. By removing rats from Palmyra, the partnership aims to safeguard the atoll's indigenous flora and fauna, encourage the reestablishment of extirpated seabird species, and create an ecological refuge for species within the Central Pacific region that are at risk of extinction. �his project is a conservation milestone for the Refuge, and it has established a benchmark �his project is a conservation milestone for the Refuge, and it has established a benchmark for eradication campaigns on other tropical islands.

Best practice guidelines for rat eradication on tropical islands

“…Wet tropical islands can support large numbers of land crabs and baiting rates may need to be significantly higher to allow for sufficient bait to be available to rats, even in the presence of efforts to reduce bait exposure to non-target species. On Palmyra Atoll extensive bait uptake trials were conducted and two applications of 84 kg/ha and 79 kg/ha were used to successfully eradicate rats in 2011 (Wegmann et al, 2012;Engeman et al, 2013). It can also be harder to identify the ideal timing for an eradication on a wet tropical island, and indeed there may not be much variation in natural resource availability or rat breeding activity within and between years (Osborne, 2000;Kricher, 2011).…”

“…Strecker (1962) showed that R. rattus spend significant periods of time in the crowns of coconut palms (Cocos nucifera) on a tropical island. Radio-telemetry tracking of R. rattus on Palymra Atoll documented significant use of the forest canopy, with a preference for coconut palms (Wegmann et al, 2012). An added benefit of this approach is that bait in the canopy will remain available to rats but not to ground-based non-target species such as land crabs.…”

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

“…In particular, it allowed 4 to 8-fold increases in the broadcast application rates from the parent label. This modification was justified based on evidence of high bait competition from land crabs at Palmyra (Wegmann et al, 2012). The parent label allows two broadcast applications made at a rate of 18 kg/ha for the first application and if needed, 9 kg/ha for the second application.…”

Stable isotope analysis as an early monitoring tool for community‐scale effects of rat eradication

“…competition and predation, especially as juveniles (Wegmann 2009;Samaniego-Herrera & Bedolla-Guzman 2012;Harper & Bunbury 2015). These carnivorous species represent the highest trophic level post-eradication species, and although we do not know their trophic levels prior to eradication, their post-eradication trophic levels likely increased due to increased abundance and consumption of conspecifics (Finke & Denno 2005).…”

“…However, the niche space of the suite of crabs present both pre-and post-eradication contracted, and their individual species niches had greater overlap following rat eradication. Rat populations were estimated to be 90 rats per hectare prior to eradication, and the release of resources due to their eradication likely allowed the crab community as a whole to alter its foraging behavior toward more optimal resources (Wegmann 2009). Given that rats are homeothermic with higher metabolisms than crabs, which are poikilothermic, their feeding rates were presumably higher than those of crabs (Gillooly et al 2001).…”