How 50 rats turned into 100,000 after avoiding poison dump

April 21, 2016

A team of researchers from the University of Cambridge and the Royal Society for the Protection of Birds recently conducted a case study on the failed rat eradication project on an island in the South Pacific.

The island's rat problem began approximately 800 years ago when Polynesian sailors introduced Pacific rats to Henderson Island, where they rapidly multiplied. Today, there are no people living on the islands but lots of rats, which are a problem, because they eat the chicks of endangered birds.

In their paper published in Royal Society Open Science, the team reports that the eradication efforts do not fail because of new rat migration onto the island, or because some of the rats were able to withstand the poison that was used. Rather, the team found that the rats simply did not eat the poison to begin with, and then as rats do, began reproducing at a rapid pace.

However, why the rats did not eat the poison puzzled the researchers because of the massive amount — about seventy-five metric tons — that was use. The rat poison was dropped at predetermined locations from a helicopter that flew over the island in 2011. While the rat population was found to significantly decrease after some time, it was not long before it shot back up. Today, the rat population on the island is back to where to where it began, sitting at approximately 100,000 strong.

To rule out other reasons why the population returned, the researchers conducted DNA tests to make sure that the rats that are on the island now are not descendants of rats from somewhere else—they were not, which ruled out migration of new rats as the problem.

Next, the team also ran multiple tests and determined that it was not possible that some of the rats could have survived after eating the rat poison. That left only one possible reason for the failure—some of the rats had not eaten the rat poison at all, and thus were not killed.

But why? After careful analysis, the team found that just prior to the rat poison dump, heavy unexpected rain fell on the island. This increased the growth of fruit and flowers for the rats to eat, and made the rat poison a less attractive option. The researchers think about 50 rats did not eat any poison and survived the eradication efforts.

Then, not surprisingly, the large number of rats today is the result of the rapid pace of reproduction of the remaining rats—one female bears up to six pups every few months. The Royal Society is not ready to give up, however, and will conduct another eradication effort in the future. Next time they said they will try to plan their attack around the weather the best they can.

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