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Rodent eradications undertaken on tropical islands have had a lower success rate than those attempted in temperate regions. A recent project undertaken to eradicate Rattus tanezumi and R. exulans from the 3 islands comprising Wake Atoll is illustrative. R. tanezumi was successfully removed from all 3 islands. R. exulans was permanently eradicated on Peale Island (95 ha) and temporarily on Wilkes Island (76 ha). R. exulans eradication on Wake Island (525 ha) was unsuccessful and the species has since repopulated Wake Island and recolonized Wilkes Island. We completed a detailed review of the project in an attempt to isolate potential causes of eradication failure. Based on the evidence available, we were not able to positively identify a single factor to explain why R. exulans survived on Wake Island. However, monitoring after the operation points to a sequence of events that comprised delayed mortality amongst a subset of breeding females and the emergence of young rats after bait was no longer readily available. Such an event was likely influenced by an abundance of natural food resources throughout the treatment area, a high density of rats, interspecific competition for toxic bait, and rapid disappearance of bait because of consumption by non-target consumers (land crabs). These factors are common to many tropical islands. We provide recommendations for addressing these factors in a future attempt to remove rats from Wake Atoll.

Indicator blocks, also called “chew-tag-cards,” made from a small square of corrugated plastic partially filled with an attractant, are an established rodent detection tool used in many different settings. Here, we discuss the utility of indicator blocks in detecting the presence of rats at Palmyra Atoll National Wildlife Refuge (Palmyra) in the Northern Line Islands. The detection of invasive rodents, in this case black rats, can be challenging in the presence of non-target species that may interfere with detection devices. Palmyra supports a robust community of land crabs. Five of the 7 species of land crabs found at Palmyra routinely interfere with commonly-used rodent detection devices: snap traps, live-capture traps, tracking tunnels, motion-sensing cameras, and gnaw sticks. Interference by crabs renders some of the detection methods useless (gnaw sticks) and reduces the sensitivity of others through false triggering (traps and cameras). Coconut crabs, which can exceed 7 kg and are found throughout Palmyra Atoll, can easily destroy tracking tunnels, traps, and even ruggedized motion-sensing cameras. Prior to the successful eradication of rats from the atoll in 2011, we compared the rate of detecting rats using indicator blocks with that of live-capture traps and tracking tunnels in paired, independent samples (90 chew block versus trap samples, 20 chew block versus tracking tunnel samples). Sampling occurred in September and October 2010 and measures were collected consecutively for 19 days. The frequency at which the indicator blocks detected rats was significantly higher (P < 0.001) than that for traps or tracking tunnels, even when interference by land-crabs was minimized by placing the devices on overturned 5-gallon buckets. The results from this study suggest that indicator blocks are an effective and efficient tool for detecting rodents in the presence of non-target species that interfere with rodent detection devices.

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 introduced black rats from Palmyra Atoll National Wildlife Refuge. Prior to the rat eradication, we assessed several environmental risk factors associated with the application of an anticoagulant rodenticide to Palmyra’s emergent land area. Here, we present the findings from a study of toxicant migration from bait pellets into topsoil. Topsoil from plots characterized as “sandy” or “humus” was collected after exposure to bait pellets containing 50 ppm of diphacinone or 25 ppm of brodifacoum. Brodifacoum and diphacinone were detected in samples of both sandy and organic topsoil while control samples collected outside of the study plots tested negative for both toxicants. With both toxicants, residue concentrations decreased with time and neither toxicant was detected in most of the 28, 36, and 50-day samples; trace amounts (=0.2 ppm for brodifacoum and =2 ppm for diphacinone) of the toxicants were detected in a few samples from these groupings. We did not find a significant difference in toxicant concentrations between the two types of topsoil. The results from this study suggest that following a broadcast of rodenticide across Palmyra’s emergent land area, small amounts of brodifacoum or diphacinone would migrate to, and remain in, Palmyra’s topsoil for a short period of time.

The removal of introduced rodents from islands can be a highly successful, and thus, important conservation tool for land managers. Non-native rodents have adverse impacts to island ecosystems. Rats alone have been implicated in approximately 40-60% of all bird and reptile extinctions globally. Introduced rodents have been removed from more than 330 islands worldwide using techniques adapted over a ~30-year time period – the use of a bait containing a rodenticide delivered by hand into bait stations or broadcast by hand or helicopter, or a combination of these techniques. Island Conservation has applied this basic approach to remove rodents from islands spanning from the deep tropics to north temperate/sub-Arctic latitudes. Across these regions, the same principle has been applied but the details customized to the local biological and sociopolitical environment. The fundamental considerations during rodent eradication planning have included: 1) delivery of a bait containing a rodenticide into every potential rat territory; 2) delivery at a time of year when the rodent population is food stressed and more likely to eat the bait; 3) minimizing or eliminating, wherever possible, risk to non-target species prone to disturbance and/or toxicant exposure; and 4) ensuring appropriate legal or regulatory compliance and acceptance by local communities. In most cases, the balance between the probability of eradication success and these considerations, particularly risk to non-target species and safe workplace seasonal time periods, are not compatible and may conflict. Thus, adjustments to the eradication plans are sometimes necessary, and are done with careful evaluation of counter-risks to eradication efficacy, non-target species, logistics, or workplace safety. We will use our projects as case studies to demonstrate the approach we take to balance the risks and mitigation, with decisions made in conjunction with our project partners. Comprehensive and transparent planning, with the involvement of experts in the eradication field, land managers, local biologists, regulators, and local community are imperative to overall project success.

Invasive marine species are well documented but options to manage them are limited. At Palmyra Atoll National Wildlife Refuge (Central North Pacific), native invasive corallimorpharians, Rhodactis howesii, have smothered live native corals since 2007. Laboratory and field trials were conducted evaluating two control methods to remove R. howesii overgrowing the benthos at Palmyra Atoll (Palmyra): 1) paste mixed with chlorine, citric acid, or sodium hydroxide (NaOH), and 2) hot water. Paste mixed with NaOH had the most efficacious kill in mesocosm trials and resulted in > 90% kill over a 98 m2 area three days after treatment. Hot water at 82°C was most effective in mesocosms; in the field hot water was less effective than paste but still resulted in a kill of ca. 75% over 100 m2 three days after treatment. Costs of paste and heat (excluding capital equipment and costs of regulatory approval should this method be deployed large scale) were $70/m2 and $59/m2 respectively. Invasive R. howesii currently occupy ca 5,800,000 m2 of reef at Palmyra with ca. 276,000 m2 comprising heavily infested areas. Several potential management strategies are discussed based on costs of treatment, area covered, and the biology of the invasion. The methods described here expand the set of tools available to manage invasive species in complex marine habitats. [ABSTRACT FROM AUTHOR]