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Along the western edge of Alaska’s Aleutian Archipelago came a group of islands that were accidentally populated by rodents in order to earn the nefarious label of the “Rat Islands”. The non-native invaders were accidentally introduced to these islands and others across the Aleutian chain through shipwrecks dating back to the 18th century. The resilient rodents, known to be some of the most harmful invasive animals, have adapted and thrived and adapted to the new environment Eventually the island’s ecosystems were overwhelmed, the natural ecological order disrupted and native species displaced.
A coordinated conservation effort that has removed the rats from one of the islands formerly known as Rat Island has become a new example of how ecosystems can fully recover to their natural state in just over a decade. The ecological boom on the newly named Hawadax Island (a return to the original Aleutian name meaning “the island over there with two hills”) extended from land to the island’s interconnected marine community. The results of a study published in Scientific Reports led by a researcher from the University of California at San Diego have documented the remarkable recovery.
“We were surprised that the recovery was moving so quickly – we thought it might take longer,” said Carolyn Kurle, associate professor in the UC San Diego Division of Biological Sciences Section of Ecology, Behavior and Evolution and lead author of the new study , which involves researchers from UC Santa Cruz, Island Conservation, the US Fish and Wildlife Service, and The Nature Conservancy.
Kurle has participated in research expeditions to more than 35 of the islands of the Aleutian chain. She and her colleagues conducted surveys in Hawadax in 2008 when the invasive rodents were dominating the island ecosystem. As the new direct predator for native island species, the rats triggered a cascade of disruptions to the island’s food chain. They hunted for eggs and chicks from shorebirds, which nearly wiped out the island’s breeding shorebird population. Without birds, herbivorous marine invertebrates such as snails and limpets ate, the island’s tidal herbivores flourished, significantly reducing the abundance of seaweed.
To reverse these effects, a coordinated conservation strategy to save the native species on Hawadax removed the rats in 2008. The effort represented a rare case where researchers could compare ecosystem data from surveys during rat dominance with a recovering ecosystem five years later and a fully restored system after 11 years.
“You don’t often have the opportunity to go back to a remote location and collect data afterwards,” said Kurle. “Sometimes it is difficult to say that a conservation measure has had any effect, but in this particular case we did an expensive and difficult conservation measure and actually showed it to work. But we didn’t expect it to be that fast. “
After the rats were removed in Hawadax, the seabirds returned and began to eat the marine invertebrates, which has allowed the kelp community to relax and rejuvenate.
“Invasive rats are almost always direct predators of native animals when they are introduced onto islands,” said Kurle. “When the birds returned, it created a completely different structure in the marine community on this island. It now has a structure more similar to what we see on islands where rat invaders have never been seen before.”
The researchers say more studies focused on understanding and measuring the direct and indirect effects of invaders, and how interconnected communities respond after those effects are removed, are needed in order to understand the full conservation achievements related to eradication invasive species, especially on islands.
“This study both confirms the profound effects of introduced species such as rats on sensitive island ecosystems while demonstrating the remarkable conservation benefits of their removal,” said Donald Croll, study co-author and professor at UC Santa’s Institute of Ecology and Evolutionary Biology Cruz.
Source: UC-San Diego