A Serpentine Threat: Burmese Pythons Linked to Severe Declines in Everglades Mammals

John “J.D.” Willson, a post-doctoral researcher in the Department of Fish and Wildlife Conservation, came to the university to do research on the effects of pollution on native reptiles and amphibians. Now, however, he’s getting national attention for his collaborative study on the environmental threat of an elusive, non-native snake proliferating in the swamps of southern Florida.

The study, published in the Proceedings of the National Academy of Sciences, is the first to document the ecological impacts of the invasive Burmese python and has set off waves of alarm across Florida and neighboring states.

Burmese pythons are the likely cause of a dramatic mammal decline in Florida’s Everglades. Sightings of medium-size mammals are down substantially — as much as 99 percent — in areas now inhabited by pythons. Willson’s study reveals that the huge snakes — probably descended from escaped or released pets — appear to be wiping out raccoons, opossums, bobcats, and other mammals. Scientists fear for the environmental balance of the 1.5-million-acre Everglades National Park and surroundings areas.

“Such severe declines in easily seen mammals don’t bode well for the many species of conservation concern that are more difficult to monitor, particularly wading birds,” said Willson. “Our findings suggest that pythons are changing the Everglades ecosystem, causing other wildlife to disappear, and that will have dramatic effects on the overall ecosystem.”

Tens of thousands of Burmese pythons, which are native to Southeast Asia, are believed to be living in the Everglades. They can grow to 15 feet long or more in the wild and can live more than 25 years. Although alligators may attack young pythons, the large adult snakes are likely at the top of the food chain. While birds make up more than a quarter of their diet, the snakes have been found with alligators, and even deer, in their stomachs. Some of the declining species could be both victims of being eaten by pythons and of having to compete with the pythons for food.

“Pythons are ambush predators,” Willson said. “They lie hidden in water and vegetation waiting for prey. They have an acute sense of smell and will eat a variety of warm-blooded animals as well as alligators.”

Pythons and other constrictor snakes kill their prey by coiling around it and suffocating it. Although Willson says he’s not in serious danger when he searches for pythons in the Everglades, he always hunts with a partner. “I have to be as careful around them as I would be around alligators,” he noted.

At least 1,800 Burmese pythons have been caught in and around Everglades National Park since 2000. For this study, researchers drove 39,000 miles along Everglades-area roads from 2003 to 2011, counting wildlife and comparing the results with surveys conducted along the same routes in 1996 and 1997. The researchers found staggering declines in animal sightings: a drop of 99.3 percent among raccoons, 98.9 percent for opossums, 94.1 percent for white-tailed deer, and 87.5 percent for bobcats. Rabbits and foxes, commonly spotted in earlier years, were totally absent. Along roads where python populations are believed to be smaller, declines were lower but still notable.

Officials must pick their battles with the pythons carefully, Willson says. “There’s no easy solution for controlling them, so we need to concentrate on sensitive areas, such as the heron rookeries and the Florida Keys. We’re refining our collection methods, but we don’t know how many pythons there are and there is no way to eradicate them completely at this point.”

“Non-native invasive species have become a problem worldwide,” Willson continued. “Animals are getting introduced, whether it’s intentional or accidental. Although there are at least two other species of large constrictors taking up residence in South Florida, it’s the Burmese python that is causing the biggest problems.”