"This is the only place in the United States where this is occurring," said Gary Witmer, a biologist with the US Department of Agriculture's National Wildlife Research Center in Fort Collins, Colorado.

"They don't belong here and they need to be controlled."

A former exotic pet breeder, living in a small house, bred the species and allowed the critters to escape.

Without eradication, wildlife officials fear the rats could eventually make their way onto the Florida mainland where they could quickly destroy fragile ecosystems.

"They could cause a lot of damage," Witmer said.

In Zimbabwe, for example, ravenous Gambian rats are blamed for damaging nut and young pea crops.

Grassy Key is a 1,500-acre (607-hectare) spit of land, lined with subtropical hardwood hammocks and flowering bougainvillea bushes, about 60 miles (95 km) north of Key West at Florida's southern tip. Streets are named after limes, lemons, peaches and avocados.

Like other islands in the Florida Keys, Grassy Key is a contrast of inland rustic wooden cottages just a stone's throw from multimillion-dollar waterfront mansions.

"Florida's become quite the hotbed. Florida and Hawaii are vying for which state has the most invasive species," Witmer said.

That dubious honor is attributed to the region's encroaching development, subtropical climate and free-spirited residents who like to keep exotic species, Witmer said.

"VERY MESSY ANIMALS"

In mid-April, Florida Keys wildlife officials found another invasive species: an 8-foot (2.4-metre) Burmese python. The first wild Burmese snake to be discovered in the archipelago, officials say, was found in a Key Largo state park.

The snake had swallowed two of an estimated 500 remaining and endangered Key Largo wood rats, one outfitted with a radio-tracking collar.

Unlike the wood rats, the Gambian rats "don't have any real friends, that we can tell," said Scott Hardin, the Florida Fish and Wildlife Conservation Commission's exotic species coordinator.

Gambian pouched rats, targeted for eradication during the next six weeks, are not related to the Key Largo wood rat.

The African rats can weigh 6 to 9 pounds (2.7-4 kg), with body shades ranging from brown to gray. They have large ears, black, beady eyes, hamster-like pouched facial cheeks, sharp teeth and distinctive long, stringy and white-marked tails.

This week, wildlife officials began baiting 1,000 traps laid out in a grid with narrow four-inch (10-cm) openings. Peanut butter, almond extract and anise are the lures.

Most of the rats will die quickly in underground burrows after ingesting the bait laced with toxic zinc phosphide.

"They're a big rodent. They're not particularly attractive. I don't understand why anyone would want them as a pet," Witmer said. "They're very messy animals."

The US Centers for Disease Control (CDC) in Atlanta and the Food and Drug Administration have banned importation of Gambian rats since 2003.

That was after an outbreak of monkey-pox, similar to but milder to humans than smallpox, was linked to Gambian rat contact with prairie dogs in the US Midwest.

The CDC hopes to study the carcasses and fecal samples of Gambian rats from the Grassy Keys to learn about internal parasites, but they have shown no signs of monkey-pox.