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The cascade effect of invasive species: how one exotic species led to a disease risk in Florida

in the New York Times

Sometime in the 1980s, it became cool to own a pet reptile in Florida. Cooler still was owning one from far away, like from Madagascar, Egypt or Burma. The more exotic, the better. Thousands of cold-blooded creatures moved through Miami’s international airport to their new glass-box homes.

The Burmese python — which can be draped around a neck — was especially popular. A baby python is just 10 inches in length. Much to the surprise of some of their owners, those babies could grow up to 20 times that size.

Maybe it was those overwhelmed owners who let their snakes loose. Maybe it was Hurricane Andrew, which destroyed a reptile breeding facility in 1992 and might have sent its specimens into the wild. One way or another, by the early 2000s, Everglades National Park was teeming with giant non-native pythons, which strangled, and then gobbled whole, almost anything in sight.

Foxes and rabbits, which once commonly roamed the park, appeared to have vanished. Deer, raccoon and opossum populations dwindled by as much as 99 percent. The snakes targeted more than mammals: Once, a 13-foot python ate an entire six-foot alligatorbefore exploding.

In 2015, Bob McCleery, an associate professor of wildlife ecology and conservation at the University of Florida, sent 26 rabbits into the Everglades. Three-quarters of his tracking devices — along with the rabbits — ended up in the stomachs of the snakes.

“The best explanation for the rapid decline of most mammals throughout southern Florida is pythons,” Dr. McCleery said. Most of the mammals, he said, are simply gone.

But not all of them.

One hardy species endured the invasion: the hispid cotton rat. There are a number of theories about why the rat survived. It could be that its population was already dense and abundant, or that its predators — foxes and bobcats — were eliminated. Or it could be that the hispid cotton rat reproduces even more prolifically than rabbits. (A female can breed again within a few hours of giving birth.)

The rat is also one of the only known hosts of a particular strain of the Venezuelan equine encephalitis complex, known as the Everglades virus, which is spread by mosquitoes.

And, with fewer big mammals to feed on, the native Everglades mosquito had fewer choices for its next meal.

According to a new study, the amount of rat blood in the Everglades mosquito’s diet has quadrupled since 1979.

“We were as surprised by the results as anyone else would be,” said Nathan Burkett-Cadena, an assistant professor of entomology at the University of Florida, whose research, published this week in the journal Biology Letters, looks at whether invasive species could increase the risk of disease for humans by setting off such chains of events.

In humans, the Everglades virus can cause fever, headache and in some cases encephalitis, a swelling of the brain.

“I don’t think that anyone could have predicted that this large snake, decimating some native mammals in a relatively wild area, could have some kind of cascading impact for human health,” Dr. Burkett-Cadena said.

While no known human outbreaks have occurred in Florida, Dr. Burkett-Cadena and his team pieced together information from Venezuela, Guatemala and Mexico, where it is thought that viruses in the same family may have spread from rats to people. Together with three of his students, Dr. Burkett-Cadena spent five months collecting “blood-engorged females” from the Everglades and Vero Beach, Fla., to see whether the mosquitoes’ eating habits changed at snake-infested sites. (Only females drink blood, in preparation for egg-laying.)

“We never get to sit down and talk to a mosquito and say, ‘Hey, you feeling okay? Let me feel your forehead — seems like you’ve got Everglades. Do you remember what animal you bit last?’” he said.

Dr. McCleery, who was not involved in his colleague’s study, said the missing mammals would have far-reaching impacts. “As ecologists, we’ve known that this is really bad for some time,” he said. “You can’t completely alter an ecosystem and not expect that there won’t be other implications that will impact humans.”

Kenneth L. Gage, an entomologist and ecologist at the Centers for Disease Control and Prevention, said exotic species could disturb ecosystems in ways that humans could not anticipate.

“We have no idea, really, what we’re doing when we move all these animals and pathogens around the planet,” Dr. Gage said.


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