In August of 2011, 75 tonnes (82 tons) of rat bait laced with poison was dropped from helicopters on Henderson Island in the South Pacific. Mass mortality of Pacific rats (Rattus exulans) followed immediately afterward. Then, in March of 2012, a single rat was seen, and in the time since, rat numbers have fully recovered. What went wrong? A few dozen didn’t take the bait, according to a case study published in Royal Society Open Science this week.
Remote oceanic islands house a huge diversity of animals that aren’t found anywhere else on the planet. But because the introduction of non-native species – especially rats from the genus Rattus – have driven many native species to extinction, conservation efforts have led to rodent eradication operations on 719 islands. Dropping poisoned bait from helicopters has worked 80 percent of the time. Understanding the reasons behind the failures depends on knowing how close they were to success.
The uninhabited, 43-square-kilometer (16.6-square-mile) Henderson Island is home to four species of land birds found nowhere else and four breeding species of ground-nesting petrels, such as the Henderson petrel (pictured above). Their chicks often fall victim to predation by Pacific rats, which arrived with Polynesian settlers 800 years ago. To eradicate the invasive rats, 75 tonnes (82 tons) of bait laced with the anti-coagulant toxicant brodifacoum was dropped on the island in a $2 million undertaking. And no rats were seen for three months afterward. Seven months after the drop, however, a single rat was spotted, and now somewhere between 50,000 and 100,000 rats thrive on the island.
A team led by Michael Brooke from the University of Cambridge collected rat tissue from before and after the eradication attempt on Henderson. Using snap-trapping, they collected about 2 centimeters (0.78 inches) of the tail, then they extracted DNA from these tail tips. The team also gathered samples from other islands and island groups: Pitcairn, the Gambier archipelago in southeast French Polynesia, and the Cook Islands.
Re-introduction from neighboring islands was not the source of continued rat presence. Nor did the survivors develop brodifacoum resistance, based on experiments with brodifacoum-laced peanut butter.
Instead, a comparison of the genetic diversity of Henderson rats before and after the bait drop revealed that the entire rat population these days descended from about 50 rats that survived. The eradication attempt was, as the team described, "very nearly successful." It’s possible that the survivors either didn’t encounter the poison pellets or they preferred natural food over bait – especially if there was an abundance of available food.