Polynesian rats clog a brown tree snake trap on Cocos Island, Guam. Though the rats were abundant and the snakes were rare on Cocos, both posed a significant threat to island birds. (Credit: Daniel Vice/USDA-APHIS-WS)

Invasive rodent species occupy more than 80 percent of island groups worldwide, with devastating impacts to native flora and fauna. Rodents prey on birds, mammals, reptiles, amphibians, and invertebrates, and indirectly affect native wildlife by destroying plants, competing for food, and transmitting disease. Species such as Norwayrats (Rattus norvegicus), black rats (R. rattus), and Polynesian rats (R. exulans) have caused the extinction of several species of native birds, mammals, and lizards (Atkinson 1985). For example, less than a decade after black rats were accidentally introduced to Big South Cape Island in New Zealand in the early 1960s, one bat species — the greater short-tailed bat (Mystacina robusta) — and five bird species, including the South Island snipe (Coenocorypha iredalei), went extinct (Bell 1978). To protect natural resources from rodents, biologists and landowners have typically applied anticoagulant rodenticides through bait stations. Though such stations can effectively eradicate rodents in small areas, they have not been feasible or effective in larger areas or on inaccessible islands. In 2005, USDA-APHIS Wildlife Services, in collaboration with theU.S. Fish and Wildlife Service (FWS) and two private companies, achieved a breakthrough in large-scale rodent eradication by developing and registering three new anticoagulant rodenticide products for broadcast application, either by hand or by air. To date, these products — Diphacinone 50 Conservation, Brodifacoum 25W Conservation, and Brodifacoum 25D Conservation — have helped successfully eradicate rodents from numerous islands in California, Florida, Alaska, Hawaii, and through the Virgin Islands, American Samoa, and the Pribilof Islands. Once the rodents were gone, the recovery and restoration of wildlife on these islands has been dramatic.

Victory on Cocos Island

One of the most-notable recent success stories in rodent eradication has occurred on tiny Cocos Island, a 34-hectare island immediately south of Guam. There, superabundant invasive Polynesian rats had been impacting native birds and other island species, including nesting black noddies (Anous minutus) and Micronesian starlings (Aplonis opaca). Rats were also limiting the ability of wildlife managers to detect and eliminate a potential population of brown tree snakes (Boiga irregularis) on the island. The reason: When there is an ample supply of rodent prey in the environment, foodbased control tools are less effective against snakes. It was believed that if brown tree snakes became established on Cocos, this would ultimately lead to the loss of all nesting birds on the island.

Rodent eradication and prevention of snake establishment on Cocos were especially critical because, in 2006, the Guam Department of Agriculture’s Division of Aquatic and Wildlife Resources initiated the Ko’ko’ for Cocos Project, an ecosystem restoration effort aimed at establishing a breeding population of endemic Guam rails (Gallirallus owstoni) — locally known as the ko’ko’ — on Cocos Island. The project included the release of captive-bred Guam rails, which are federally listed as endangered and are extinct in the wild onGuam. Partners in the rodent eradication included the Cocos Island Resort, USDA-APHIS Wildlife Services, FWS, and the U.S. Department of the Interior’s Office of Insular Affairs.

Eradicating rodents from Cocos presented a technically challenging situation for several reasons. Terrestrial crabs and native forest birds consumed broadcast rodenticide baits. In addition, daily ferries between  mainland Guam and Cocos Island created the risk of  rodent re-introduction, and a day-use resort on the  island created substantial manmade rodent habitat  (such as trash stations) that required specialized  control approaches. Finally, control activities in close proximity to visitors posed a potential health risk.  To help address these issues, project partners identified key needs in the eradication effort and developed an integrated eradication plan, which involved the following steps:

Biosecurity

The team drafted and implemented a comprehensive biosecurity plan for the island.  This plan involved improving waste management processes; educating resort staff, visitors, and the boating public about the risks of invasive species; installing rodent bait stations on ferry and vendor boats servicing Cocos Island; and inspecting resort-bound commodities for rodents and snakes.

Trapping

Intensive rat trapping was conducted in and around all trash areas and resort buildings including offices, restrooms, locker rooms, and  maintenance buildings. Trapping was also conducted in kitchen and dining areas — where rodenticide  baits were not allowed — thereby providing a safe  alternative to the use of toxicants in areas where  people were cooking and eating.

Crab-Proof Bait Stations

We began to distribute the anticoagulant rodenticide containing brodifacoum  via crab-resistant bait stations throughout the resort  settings of the island. The bait stations were made crab-proof simply by affixing a commercially available,  tamper-resistant bait station to a five-liter plastic pail, which crabs couldn’t climb. These stations were placed on a 10m-by-10m grid across the resort facility. Each morning, the stations were gathered and stored out of  sight, then after the departure of the last tourist ferry,  they were set out again.

Bait Broadcasting

Project managers made two hand-broadcast applications of the anticoagulant Diphacinone 50 Conservation across the forested areas of the island, an area covering approximately  25 square hectares. A rodenticide bait containing diphacinone was chosen because it represented less risk to non-target species. Baits containing other ingredients would likely result in bird mortality. In addition, a hand broadcast was used to place baits more accurately and prevent bait from entering the marine environment. Two one-day broadcast applications  were completed seven days apart, placing adequate  bait — 12.98 kilograms per hectare during the first application  and 10.36 kilograms per hectare during the  second — throughout the island to target all rodents.

Monitoring

Throughout the project, researchers conducted environmental residue testing and intensive monitoring for environmental impacts — including non-target mortality. Monitoring indicated the eradication was completed successfully with no non-target species losses and minimal environmental persistence of anticoagulants. After completion of the baiting program, we conducted post-eradication monitoring using non-toxic baited tracking stations situated around the island. In March 2011 — after two years with no sign of rodents — Cocos Island was officially declared rodent-free.

This victory enabled the release of captive-bred Guam rails onto Cocos Island, and has allowed the ko’ko’ to thrive and reproduce in an environment free of rodents and snakes for the first time since humans colonized Guam. In addition, native forest and seabirds continue thriving across the island, a sure sign that rodent eradication on Cocos Island represents a great step forward for conservation, and a great hope for island ecosystems everywhere.

Author Bios

William Pitt is the Field Station Leader and Project Leader with USDA-APHIS Wildlife Services’ National Wildlife Research Center Hawaii Field Station in Hilo, Hawaii. 

Daniel Vice is the Assistant State Director with USDA-APH IS Wildlife Services in Barrigada, Guam.   

Dana Lujan is a Wildlife Biologist with USDA -APH IS Wildlife Services in Barrigada, Guam.   

Diane Vice is a Wildlife Biologist with the Guam Division of Aquatic and Wildlife Resources in Mangilao, Guam.   

Gary Witmer is a Project Leader with USDA-APHIS Wildlife Services’ National Wildlife Research Center in Fort Collins, Colorado.   

Watch a video about efforts to protect the Guam rail on Cocos Island.