A big African rat's

swell U.S. prospects

may mean big trouble

By Alex Danoff

The invasive and potentially threatening Gambian Rat could survive in the entire eastern half of the United States, as well as the coastlines of Texas and Mexico and parts of the Pacific Northwest, according to a recent study in the Journal of Mammalogy.

The Gambian Rat (Cricetomys gambianus), also known as the African Pouched Rat, has found a home in the Florida Keys and is breeding.

It is the biggest rat in the world and can weigh up to six pounds. Native to sub-Saharan Africa, the rat is a known carrier of human disease including monkeypox, a rare viral disease which can cause symptoms similar to smallpox.

In 2003 the US government issued an order to prevent further importation of the rats because of fears that they were a carrier of the monkeypox virus.

Too late: before the ban, the Gambian Rat had become popular in the exotic pet industry. They got to Grassy Key through a pet dealer, who had brought them from Africa to Florida to sell them, said Darin Carroll, an evolutionary biologist with the Centers for Disease Control and Prevention in Atlanta.

"Basically, if you had some before, you can hold onto them until they die, but you can't do anything with them," Carroll said. "That kind of came out of the outbreak investigation. They were pretty popular in the exotic pet trade and that's why the guy sent them here.

"The population right now is relatively small and it's relatively isolated on one island," Carroll said.

That island is Grassy Key, a small land mass in the Florida Keys roughly 25 miles south of the tip of mainland Florida. It is somewhat isolated, separated from the other islands by causeways and mangroves.

In the summer of 2004, Neil Perry, then a graduate student at Texas A&M University, received word from a biologist on Grassy Key who had caught one of the Gambian Rats.

"He had a woman complaining about these giant rats," Perry said. "He trapped one and had no idea what it was. I've had a lot of background in small mammals and identified it as the Gambian Rat.

"Pretty much right off the bat anyone in ecology understands the potential consequences of having an animal that big, breeding and potentially spreading into an area that's that very sensitive, in particular the threat to some of the other endangered small mammals down there."

In August, 2004, Perry and a group of researchers went to Grassy Key and collected five of the rats. They ran preliminary tests then sent them to the CDC for disease testing. All five tested negatively for monkeypox.

The primary objective of the study was to see if the rats were breeding, and researchers were able to confirm that they were, Perry said. He said they were also curious about the animal's nesting habits on Grassy Key.

"They're burrowers in their native country and most of the land in the Keys is hard rock limestone, he said. "It turns out that they found some loose soil in some places and burrowed."

The US government initially became interested in the pouched rats because they were on the list of species that were involved in the 2003 monkeypox outbreak, in which the disease was reported by several people in the United States, Carroll said.

"All signs indicate that that particular population in Florida is not infected, but just the fact that they're there is bad in a lot of ways," Perry said. "From the public health angle, it doesn't look like there's anything wrong with them, but from an ecological angle it's never good to have some introduced species establish itself in a new place because you never know what's going to come out of that and how it's going to interact with all of the local things."

Perry said it is extremely difficult to anticipate what kind of effect the rodents would have on the local environment if they were to move onto the mainland.

"Ecologically speaking, we really can't fathom what kind of impact they would have, but rodents probably have the biggest sway," he said.

Compared to most other animals, rodents typically consume more resources and have a greater impact on an area, he said.

Scott Hardin, invasive species coordinator for the Florida Fish and Wildlife Conservation Commission, said there are several reasons why the Gambian Rat could be a threat to mainland Florida.

"Among other things, it has the potential to cause some economic damage to a lot of the fruit and vegetable and ornamental crops that are grown in South Florida," he said.

The rats could also have an impact on the other rodent species of Southern Florida because of competition for resources, he said.

"We sort of have a practice, even if we can't quantify what [an animal's] risk might be, we will try to eliminate an introduced species if it's practical to do so, and not even take the chance on it being good, bad or somewhere in between," Hardin said.

To demonstrate the unpredictability of invasive species, Carroll pointed to the case of kudzu, a viney plant from Asia that has thrived in the southeastern United States.

"The thing is you just really don't know," Carroll said. "A lot of people call kudzu 'The vine that ate the South.' That was one of those species that was imported here as an ornamental plant and no one really gave it a second thought.

"But once it got here it went crazy and now it's strangling trees and causing all sorts of problems. That's one of a long list of species that are problematic."

The best way to avoid running into a similar problem is to cut the invasive species off before it has a chance to affect the local population, Carroll said.

"The bottom line is that no one really knows what kind of problems they would cause, but the hope is to not have to find out," he said. "Because usually by the time you do find out, then itís too late to do anything about it, especially not at some really great expense."

Hardin said that the commission's main concern is that the rats will find their way up to the mainland by way of garbage trucks that go between the keys and Southern Florida. There is no place to stockpile trash on the keys, so they are forced to transport it to the mainland on a regular basis.

"We're very much concerned that these rats might hitchhike and end up where all that agriculture is,"  Hardin said. "And based on their distribution in Africa, which is anywhere south of the Sahara, almost all the way to South Africa, they wouldnít be confined to Florida. They could easily move throughout the Southeast."

Such a possibility was supported by researchers who conducted the study on the Gambian Rat that appeared in the June, 2006, issue of the Journal of Mammalogy.

Using points in Africa where the rats have been able to successfully live, the researchers overlaid this data onto a map of the United States. They then used this as a hypothesis as to where the animal could find an acceptable habitat.

"They're very adaptable animals," Perry said. "You think about an animal that is native to the savannah of Africa and then it can adapt to and establish in a place like the Florida Keys, which is a tropical environment- it's impressive."

The Florida Fish and Wildlife Conservation Commission has led the efforts to prevent the Gambian Rat's spread from happening.

It has partnered with the US Department of Agriculture as well as the Animal and Plant Health Inspection Service in an effort to eradicate the rats from Grassy Key. APHIS has a wildlife services group that specializes in removing animals that threaten agriculture and present problems for other species.

"Very rarely does an introduced species give you any chance to eradicate it," Hardin said. "Normally once it's out, it's out and there is nothing you can do. This happens to be confined by some unusual circumstances, so we have one of those weird opportunities to see if we can really eradicate them."

Although no concrete measures have been taken yet to eliminate the rats from the island, the partnering organizations have begun to develop a plan to remove them that would likely begin in January,2007, and go through April, Hardin said.

Photos and graphics courtesy of Neil Perry.