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Scientists are reporting that aliens are wiping out the animals in Florida's Everglades. Those aliens are Burmese pythons from Asia. They've been slithering around south Florida for decades. But scientists now say the constrictors are so bad, they're eating their way through the swamps.

As NPR's Christopher Joyce reports, the federal government has decided to do something about it.

CHRISTOPHER JOYCE, BYLINE: Michael Dorcas has been catching snakes since he was a kid in Texas. But a 15-foot Burmese python is a handful, or two.

DR. MICHAEL DORCAS: You typically try to grab them behind the head, get somebody else to grab the back end of them. But often, still they defecate all over you even if they can't bite you. So it's always an unpleasant thing when you catch a wild python.

CORNISH: Dorcas is now a biologist at Davidson College in North Carolina. For the past eight years, he's been driving through the Everglades counting animals, specifically midsized mammals. Dorcas wanted to know how big a bite the pythons are taking out of the mammal population. When he compared the number of mammals now to the 1990s, when pythons were less common, he was shocked.

DORCAS: Once we calculated the percentages, we had no idea they were going to be this dramatic.

JOYCE: How dramatic?

DORCAS: Let's see, 99.3 percent decrease in raccoon observations, decreases of 98.9 percent in possums, 94 percent white-tailed deer, 87.5 percent in bobcats.

JOYCE: Nearly all the raccoons, possums, deer and bobcats gone. Now, counting animals by sight from a car isn't foolproof, but it is an accepted practice in wildlife research. Dorcas reports his findings in the Proceedings of the National Academy of Sciences. He blames pythons because in areas known to be thickest with the snake, the mammals are scarcer. And where there are fewer pythons, there are more mammals. And there's no evidence it's some disease, either.

DORCAS: This is the first study to show actual effects on populations, and that's why it's remarkable in the fact that those effects are extremely severe.

JOYCE: The onset of the python invasion is often blamed on snake owners who release their pets when they get too big for comfort. Lawyer Marshall Meyers represents the pet industry. He says maybe, but it's more complicated than that.

MARSHALL MEYERS: I think it's habitat loss. I think it's - pythons are, obviously, some of it.

JOYCE: Meyers notes that there's less water now in the Everglades, and that could lower animal populations. But he acknowledges that the python invaders and other exotic animals that escape or are released by owners give the pet trade a bad name.

MEYERS: There are species that are not in this country, that we do not want in this country, because if they came in through the pet trade or through the zoos, they can cause a lot of environmental harm, and that's just a big black eye.

JOYCE: The federal Fish and Wildlife Service has been watching the python explosion and is now taking action. This month, they made it illegal to import these snakes or transport them across state lines. That includes three other constrictor species from Africa and South America. Biologist Susan Jewell at the service studies injurious species that invade the U.S. - things like zebra mussels and poisonous lionfish. She says it's possible Florida's pythons could spread if they learn how to survive in colder weather.

SUSAN JEWELL: I think that it's a good heads-up for everybody. This can happen anywhere and most likely will if these snakes get established.

JOYCE: Jewell says the ban allows people who now own these snakes to keep them, and you can still buy and sell them within a state. She says the new rule does not mean you have to give up your snake. Christopher Joyce, NPR News.

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