Every child learns this sad and basic truth about nature: The snake eats the rabbit.

But in the southernmost part of the Florida Everglades, things have taken a really wild turn. Pythons and anacondas are eating everything. The most common animals in Everglades National Park — rabbits, raccoons, opposums and bobcats — are almost gone, according to a study released Monday.

The snakes are literally fighting with alligators to sit atop the swamp’s food chain. In October, a 16-foot python was found resting after devouring a deer.

“There aren’t many native mammals that pythons can’t choke down,” said Robert N. Reed, a research wildlife biologist at the U.S. Geologial Survey’s Fort Collins Science Center and a co-author of the study, published in the Proceedings of the National Academy of Sciences.

Officials can’t stop invasive pythons and anacondas from marauding in the Everglades, Reed said; they can only hope to contain them. “We’re trying to prevent spread to the Florida Keys and elsewhere north.”

The snakes were released by pet owners into the Everglades, where they started to breed. A female python can lay 100 eggs, though 54 is considered the norm. The study was described as the first to show pythons are causing the decline of native mammals in the Everglades.

When researchers struck out to count animals along a main road that runs to the southernmost tip of the park, more than 99 percent of raccoons were gone, along with nearly the same percentage of opossums and about 88 percent of bobcats. Marsh and cottontail rabbits, as well as foxes, could not be found.

The Obama administration recently banned the import and interstate commerce of Burmese python, two species of African pythons, and the yellow anaconda. But under pressure from the U.S. Association of Reptile Keepers, trade of the world’s longest snake, the reticulated python, and the boa constrictor were allowed to continue.

The reptile trade is a $2 billion business in the United States, according to the Humane Society. About 11 million reptiles were kept as pets in 2005, according to the American Pet Products Manufacturers Association. More reptiles are imported here than anywhere else in the world.

“Pythons are wreaking havoc on one of America’s most beautiful, treasured and naturally bountiful ecosystems,” Marcia McNutt, director of the USGS, said in a statement. “The only hope to halt further python invasion . . . is swift, decisive and deliberate human action.”

But officials do not yet know what can be done to slow the migration of pythons to other areas in Florida, and north to Georgia and Louisiana.

“We need more research into methods to limit the population spread,” said Michael F. Dorcas, one of the authors of the study, Severe Mammal Declines Coincide with Proliferation of Invasive Burmese Pythons in Everglades National Park.

Researchers collected data through repeated night road surveys, traveling 39,000 miles for eight years ending in 2011, counting live animals and road kill. They compared the data with findings of similar surveys conducted in 1996 and 1997, according to a statement by the USGS.

Andrew Wyatt, president of the Reptile Keepers, which advocates on behalf of snake importers, dismissed the study.

“They play fast and loose with facts and make big jumps to conclusions,” Wyatt said. The authors contradict prior studies showing that mercury in the water has played a role in the deaths of small mammals, he said.

Wyatt also said pythons can only survive in southernmost Florida and that they would perish in extreme cold.

Dorcas, who participated in several studies of pythons and cold weather, said it’s not simple. Hundreds of adult pythons and hatchlings were captured and removed from the park months after a cold snap, he said.

Dorcas was also part of a study that removed 10 snakes from the Everglades to winter in South Carolina, where each died of exposure. But researchers noted that the snakes were far more tolerant of cold weather than they had thought.

The U.S. Fish and Wildlife Service predicted that a new generations of Burmese pythons on the edge of their non-native range can adapt and “expand to colder climates.”