‘Rat Island’ rid of invading rodents

Shorebirds have returned to the volcanic islands offshore of Southwest Alaska

A Norway rat caged on Hawadax Island. Photo courtesy of Shauna Reisewitz

Hawadax Island in the Aleutian archipelago, long known as Rat Island after rats fleeing shipwrecks as far back as the 1700s overran the terrain, has regained its ecosystem, and shorebirds have returned to a restored pre-invasion environment.

“We were surprised that the level of recovery unfolded so quickly; we thought it could be longer,” said Carolyn Kurle, lead author of a new study on the “Island of Rats” recovery, published in Scientific Reports and online in early March at www.eurekalert.org, the website of the American Association for the Advancement of Science.

Kurle, who has taken part in research expeditions to over 35 of the islands in the Aleutian chain, is an associate professor at the University of California San Diego Division of Biological Sciences Section of Ecology, Behavior and Evolution.

Others engaged in the study included researchers from UC Santa Cruz, Island Conservation, the U.S. Fish and Wildlife Service and The Nature Conservancy.

The team did surveys at Hawadax in 2008 when the invasive rodents dominated the island’s ecosystem. The Aleut word Hawadax means “the island over there with two knolls.”

When rats first came to the island, they disrupted the food chain, preying on shorebird eggs and chicks, and nearly wiping out the island’s breeding shorebird population. Without birds to consume herbivorous seashore invertebrates such as snails and limpets, researchers said, the island’s intertidal plant-eaters flourished, driving down the abundance of marine kelp.

Researchers said that to reverse these effects a coordinated conservation strategy was undertaken, presenting a rare case in which researchers were able to compare ecosystem data from surveys during rat dominance with a recovering ecosystem five years later and a fully recovered system after 11 years.

“You don’t often get the opportunity to return to a remote location and collect data after the fact,” Kurle said. “Sometimes it’s hard to say that a conservation action had any sort of impact, but in this particular case we took a conservation action that was expensive and difficult, and we actually demonstrated that it worked. But we didn’t expect it to be so fast.”

These rats are almost always direct predators of native animals when they become introduced on islands, Kurle said. “So when the birds returned it led to an entirely different structure in the marine community on this island. It now has a structure that more closely resembles what we observe on islands that have never had rat invaders.”

More studies are now needed, researchers said, to understand and measure both the direct and indirect impacts of these invaders, and how interconnected communities respond following their removal.

This study both confirms the profound impact of introduced species like rats across sensitive island ecosystems and at the same time demonstrates the remarkable conservation benefits of their removal said Donald Croll, co-author of the study and a professor in the UC Santa Cruz department of Ecology and Evolutionary Biology.