Midway Atoll eradicated invasive rats after Bonin petrels declined to fewer than 5,000 nesting pairs; By 2008, the seabird colony had recovered to more than 135,000 pairs

The successful removal of invasive black rats from Midway Atoll led to an extraordinary recovery of the local Bonin petrel population, which grew from fewer than 5,000 nesting pairs to more than 135,000 pairs. According to long-term conservation research and field reports from the U.S. Fish and Wildlife Service, eradication of the rats eliminated a major threat that had brought the island’s seabird colonies to the brink of collapse.

Invasive rats destroy seabird colonies

Midway Atoll is a small, isolated group of islands in the North Pacific Ocean. It was once one of the most important seabird breeding areas in the world. Before non-native mammals arrived, hundreds of thousands of Bonin’s petrels (Pterodroma hypoleuca) returned to the atoll each year. The birds dug deep burrows in the sand, laid their eggs and raised their young there. Federal wildlife records show that before 1943, more than 500,000 Bonin petrels nested on Sand Island and Eastern Island. This changed during World War II. In 1943, black rats (Rattus rattus) accidentally escaped from cargo and military supply ships arriving at the atoll’s new naval facilities. With few natural mammalian predators, rats spread quickly across the two main islands. The arrival of black rats was disastrous for birds that nested on the ground or underground. Bonin’s petrels dig tunnels in soft sand and lay only one egg each year. They had no natural defenses against predatory mammals. Rats could easily enter underground burrows, where they ate petrel eggs, killed chicks, and attacked adult birds guarding their nests. Over the following decades, constant rat attacks destroyed entire breeding colonies. In the late 1980s, wildlife surveys found fewer than 5,000 breeding pairs of Bonin petrels at Midway. The rats also ate seeds, fruits and insects from native plants. This damaged coastal vegetation and increased soil erosion across the fragile atoll.

The rat eradication campaign.

As the island’s unique bird populations faced severe decline, the U.S. Navy worked with the U.S. Fish and Wildlife Service and the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS-Wildlife Services) to develop a large-scale rat removal program. In 1995, field teams began an island-wide operation using rodenticide baits on Sand Island and Eastern Island. Workers created detailed grid systems to ensure the bait reached potential rat hideouts, including abandoned military buildings, thick vegetation and coastal dunes. The operation had to be carried out with care. Teams needed to kill the entire rat population while reducing the risk to native birds, including resident shorebirds and migratory waterfowl. After two years of detailed monitoring, using tracking tunnels, motion-activated cameras, and physical searches, federal wildlife managers declared Midway Atoll free of black rats in 1996.

A dramatic demographic recovery

Once the black rats were gone, the ecosystem began to recover quickly. Adult Bonin’s petrels could once again use their underground nests without their eggs and chicks being attacked. This greatly improved reproductive success. As the young birds grew, they returned to the places where they were born and created new burrows. The colonies expanded rapidly. In 2008, wildlife surveys by the U.S. Fish and Wildlife Service showed that more than 135,000 pairs of Bonin petrels were breeding at Midway. This represented a more than 27-fold increase from the lowest level recorded by the population at the end of the 20th century. Removing the rats also improved the island’s habitat. Native coastal plants, including ground cover plants and native grasses that had been eaten by rodents, began to recover naturally. The returned vegetation helped keep the sand dunes in place. This provided the petrels with more solid ground to build deeper, safer nesting burrows. Other native birds, including Laysan albatrosses, black-footed albatrosses, and several species of terns, also had better nesting success after rat predation was removed.

New threats and continued protection

Although removing the rats helped restore the Bonin petrel population, Midway Atoll National Wildlife Refuge managers still face new ecological challenges. Conservationists later discovered that invasive house mice (Mus musculus), which had been kept in check by the presence of rats, began to take advantage of the newly available food and habitat. The mice began to attack the adult albatrosses. In response, the US The Fish and Wildlife Service developed additional eradication and pest control plans to protect the refuge’s seabirds. To protect recovery, refuge staff also maintain strict biosecurity measures for research vessels, supply ships and aircraft arriving at Midway. Inspection teams screen incoming cargo and monitor port facilities to prevent rats and other invasive species from returning to the isolated Pacific sanctuary.