Everglades National Park, a world-renowned wetland in southern Florida, once abounded with rabbits, raccoons, muskrats, and other small mammals. But roughly 15 years ago, these species started to become scarce. About the same time, biologists noticed a boom in the population of a predator that had invaded the 64,238-hectare park: the Burmese python. Now, an experiment adds to the evidence that the pythons, which grow up to 5 meters long, are to blame for the collapse of the mammals' populations.
“There’s no question that this is an environmental disaster,” says J. D. Willson of the University of Arkansas, Fayetteville, who was not involved in the study.
The pythons are native to Southeast Asia and may have been released into the Everglades decades ago by people who kept them as exotic pets. From studying the stomach contents of dead snakes, biologists knew that they eat a variety of animals—even large alligators. But mammals account for 75% of their diet. The snakes' voracious appetite has apparently had a big impact. Between 2003 and 2011, sightings of raccoons and opossums in the Everglades dropped by 99%, according to paper published in 2012 in the Proceedings of the National Academy of Sciences (PNAS). In addition, these and other mammal species were more common in parts of the Everglades where pythons had arrived only recently.
Some scientists were skeptical that a single species of predator could have such a dramatic impact, especially in an ecosystem that already contained other kinds of snakes. So two of the authors of the PNAS paper teamed up with Robert McCleery, a conservation biologist at the University of Florida in Gainesville, to undertake a field experiment. The team members spent 6 months capturing 95 adult marsh rabbits, which are native to the southeastern United States and were once quite common in the park. “It was a lot of sweat,” McCleery says.
After outfitting the rabbits with collars that carried radio transmitters, the researchers released some of them into two parts of Everglades National Park. For comparison, other rabbits were let go inside a wildlife refuge about 100 kilometers northeast of the park, where pythons have not been reported. The collars had sensors that signal if a rabbit has not moved recently and might be dead. Once researchers located a rabbit’s carcass, they could look for bite marks and other evidence that reveals whether it had been killed by a bird or a mammal. Predation by pythons or alligators was easy to identify—they would still have the entire rabbit and its transmitter in their stomachs.
After a few months, the rabbits were reproducing at both sites, but they also fell victim to various predators. In the snake-free refuge, coyotes and bobcats killed most of the rabbits, as the scientists expected. In Everglades National Park, pythons were responsible for 77% of rabbit deaths, with only one killed by a mammal, the researchers report online this week in the Proceedings of the Royal Society B: Biological Sciences.
The difference between the two sites grew alarming. The risk of predation in the Everglades exceeded that in the refuge, likely because pythons increased their activity as the weather warmed up, wiping out all the rabbits. “This is very strong, convincing evidence that pythons are the cause of the mammal declines,” says herpetologist and conservation ecologist Michael Dorcas of Davidson College in North Carolina, who was not involved in the study.
It’s not clear why pythons have a larger impact than coyotes or bobcats. The snakes might be more abundant, or the rabbits may lack defensive strategies for dealing with them, such as staying away from the water’s edge where the pythons lurk. Whatever the reason for the mammals' vulnerability, the pythons pose an “enormous threat to ecological functioning of the entire Everglades system,” McCleery and his co-authors conclude. Willson agrees, noting that small mammals likely influence key aspects of the Everglades environment, such as the flow of nutrients and patterns of plant growth.
At the moment, there are no solutions to the python problem. The snakes are elusive, so hunting has proven ineffective. Researchers don’t know how to measure the size of the population, or how they might effectively lure them into traps. In the meantime, one hope is that the small mammals—some of which are quite adaptable—will learn how to survive their new predators.