Since the giant constrictors took hold in Florida in 2000, many previously common mammals have plummeted in number—and some, such as cottontail rabbits, may be totally gone from some areas.
Scientists already knew from dissecting the snakes that they prey on a wide range of species within Everglades National Park. (See a picture of a Burmese python that exploded eating an American alligator in the Everglades.)
Also popular as pets, Burmese pythons are one of nine species of constrictor snakes, numbering about a million individuals, that have been imported into the United States over the past three decades, according to the U.S. Fish and Wildlife Service. Many of these animals, which can grow to lengths of 20 feet (6 meters) have either escaped or been dumped into the wild.
But this is "the first study to show that pythons are having impacts on prey populations—and unfortunately those impacts appear to be pretty dramatic," said study leader Michael Dorcas, a herpetologist at Davidson College in North Carolina.
"We started the study after we realized, Man, we're not seeing a lot of these animals around anymore," Dorcas said.
But "when we did the calculations, we were pretty astonished."
Burmese Pythons Causing "Severe Declines?"
For the study, Dorcas and colleagues conducted nighttime surveys of live and dead animals on roads between 2003 and 2011. Such numbers provide estimates of how many animals of a certain species are present in a given area.
The scientists compared these data with similar surveys conducted in 1996 and 1997.
But the 2003 to 2011 surveys—which covered a total of nearly 35,400 miles (57,000) kilometers of road—revealed "severe declines" in mammal sightings, according to the study, published this week in the journal Proceedings of the National Academy of Sciences.
Raccoon observations dropped by 99.3 percent, opossum by 98.9 percent, and bobcat by 87.5 percent. The scientists saw no rabbits or foxes at all during their surveys.
(Read about giant Burmese python meals that went bust.)
Also worrisome is what could be happening to species that were already rare—and thus more difficult to research, Dorcas noted.
For instance, it's unknown whether the snakes are putting the squeeze on the Florida panther, a subspecies of cougar deemed endangered by the U.S. Fish and Wildlife Service.
But "it's not unreasonable to assume that a really large python could consume a Florida panther," he said—the snakes are known to eat leopards in Southeast Asia.
Impact of Everglades Mammal Decline Unknown
It's difficult to predict how the decline in mammal populations will affect the Everglades, Dorcas said. (See Everglades pictures.)
But some species may even benefit from the python's big appetite, he said. For example, turtle numbers are often kept down by raccoons, which eat the reptiles' eggs. Without as many raccoons, "we may be knee-deep in turtles in 20 years," he quipped.
Whit Gibbons is a professor emeritus of ecology and head of outreach for the Savannah River Ecology Lab at the University of Georgia.
"My bet is that some of the mammals that have been affected will partially recover by managing to adapt or adjust," said Gibbons, who wasn't involved with the study.
"It's unlikely," he added, "that raccoons are going to go extinct in Florida."
But as long as pythons are there, the mammals won't bounce back to their former levels, he said.
Meanwhile, some groups are mounting efforts to stem the spread of the Burmese python. The Nature Conservancy's "Python Patrol," for example, works to prevent the reptile from moving into the Florida Keys.
And on January 17 the U.S. Department of the Interior announced a new law banning importation and interstate transport of four species of invasive snakes, including the Burmese python.
"We have taken strong action to battle the spread of the Burmese python and other nonnative species that threaten the Everglades and other areas across the United States," Secretary of the Interior Ken Salazar said in a statement.
"There's no single solution to this conservation challenge, but banning the importation and interstate transport of these invasive snakes is a critical step."
Pythons' Invasion an Opportunity?
The University of Georgia's Gibbons sees the snakes' invasion as a chance for scientists to track what happens to the Everglades.
Though the ecosystem "may not collapse, it will likely change," he said. "That change would be very worthwhile to monitor from a scientific standpoint.
"Maybe next time we could prevent changes we don't want to happen."