Fugitive Rat Teaches Lesson About Invasive Species Control

Bethany Halford
for National Geographic News
October 19, 2005
It sounds like a plot from a Tom and Jerry cartoon: Intrepid
hunters spend weeks on an island setting complex traps while their
quarry evades them at every turn.

But this seemingly comic cat-and-mouse chase has grim implications for efforts to protect fragile island ecosystems from invasive species.

Conservationists from the University of Auckland wanted to get a better handle on how susceptible a solitary, island-invading rat is to conventional traps. They therefore intentionally unleashed a lone Norway rat upon New Zealand's rat-free Motuhoropapa Island.

The scientists suspected the animal might be more difficult to trap than a rat released in a city. After all, with an island full of food, the rat would tend not to snack on poisoned bait.

But they did not anticipate that this cagey critter would elude them for 18 weeks, eventually swimming across open ocean to a neighboring island.

The rodent's evasive maneuvers indicate that traditional traps might not be effective for catching the first arrival of an invasive species. The findings are reported in tomorrow's issue of the journal Nature.

Cagey, Not Caged

Invasive rodents, such as the Norway rat (Rattus norvegicus), are known to cause major disruptions to isolated ecosystems.

"Invasive animals are the single biggest threat to seabirds and island ecosystems," Bernie Tershy, executive director of the nonprofit organization Island Conservation, said in an e-mail.

"Preventing new introductions and eradicating existing populations of invasive animals is an incredibly effective conservation strategy."

"In parts of the world like New Zealand it's been recognized that [rodent invasions are] a big problem," said Mick N. Clout, a professor of conservation ecology at University of Auckland who co-authored the study with graduate student James C. Russell.

Although rats can be eradicated from islands, they tend to reinvade.

"You want to be able to pick up the first invader, basically, and get rid of it before a population is established," Clout explained.

But early rat invaders have proven very difficult to eliminate.

With permission from local authorities, the team trapped a male Norway rat, took a sample of its DNA, fitted it with a radio collar, and let it loose on the uninhabited island of Motuhoropapa—one of the Noises Islands northeast of Auckland.

The rat, nicknamed Razza, explored Motuhoropapa's 23.5 acres (9.5 hectares) for four weeks before settling into 2.5-acre (1-hectare) area.

For the next four weeks Razza confounded the team by evading conventional trapping techniques, including 70 trapping devices and two trained dogs.

After ten weeks, the researchers lost Razza's radio signal. The rat had vanished, and the conservationists were stumped.

Around this time the owner of Otata—an island 1,300 feet (400 meters) across the water from Motuhoropapa—started noticing distinctive scat on his formerly rat-free island. Using DNA analysis, the researchers confirmed that the droppings were from their missing rodent.

"That was a very big surprise," Clout said. From the presence of rats on offshore islands, scientists had inferred that rats could swim about 1,970 feet (600 meters).

But this, Clout pointed out, is the first confirmed example of an individual Norway rat making a long-distance swim across open water.

While the researchers aren't certain what made the rat seek other shores, Clout says it's possible that Razza was looking for companionship. The experiment had stretched into the Norway rats' mating season.

The team used different types of traps on Otata, but these also failed to snare Razza. After 18 weeks, the conservationists switched to more aggressive measures.

The team set a trap baited with fresh penguin meat in an area where trained dogs had detected concentrated rodent scent. This trap proved irresistible. After vexing pursuers for more than four months, Razza finally met his end in the baited trap.

Rethinking Strategies

Clout acknowledges that it's hard to make broad generalizations based on the behavior of one rat.

But he thinks Razza's remarkable ability to evade conventional traps suggests conservationists may have to rethink the way they go after early rodent invaders.

"We need to be aware of the fact that individual rats may be behaving differently than they are when they are part of a population," Clout said. "We shouldn't be relying on a single method for detecting rats on an island."

Tershy of Island Conservation agrees. "[The report] serves as a warning to managers of rat-free islands that they need to implement rodent introduction prevention plans," he said, "because they can't count on being able to detect or eradicate newly arrived rats."

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