Scientists and researchers have spent years trying to eradicate rats from islands in Haida Gwaii, B.C., and for a time they were successful, but the unwelcome rodents have returned.

Four years ago, a team of scientists with Parks Canada declared two islands in Gwaii Haanas National Park Reserve free of black rats.

But in September, they were met with an unwelcome surprise. A monitoring camera set up by wildlife officials revealed the reinvasion.

"Unfortunately this time around they showed evidence that Norway rats had invaded the islands," said Parks Canada conservation manager Tyler Peet. "That's a different species from the ones that had been eradicated."

Rats eat at-risk species of bird

Local conservationists fought to rid the islands of rats because they eat the eggs and chicks of ancient murrelets, also known as night birds — an at-risk species that is culturally significant to the Haida Nation.

Rats were completely eradicated on two Gwaii Haanas National Park Reserve islands, from 2013 until this September. (Andrew Wright/Parks Canada)

Half the world's population of ancient murrelets lives in the Haida Gwaii archipelago.

Efforts to eradicate the rats began in 2009 when four islands in the park were targeted for their small size and distance from other islands in the archipelago.

Rats are prodigious swimmers, which is part of what has eased their spread in the sensitive ecological area.

Rats killed with poison pellets

The rats were killed by air-dropping poison pellets across three of the four islands. The one island where the poison was spread by hand was reinfested. Two islands were declared rat-free in 2013.

Peet says the resurgence of rats means they either swam to the islands or got a ride on a boat — and that underlines the need for what he calls a "biosecurity plan."

"All it means is thinking of ways to control living organisms from getting to places you don't them want to be — and in order to be effective, it has to be a community-wide effort."

Conservation officials are now sending tissue samples to a genetics lab in the hope the results will shed light on how the invasive rodents got to the islands.