Predatory pythons shift Everglades ecology

As invasive snakes expand territory, some mammal populations drop by more than 90 percent

Giant snakes are eating their way through the Everglades, leaving a drastically changed ecosystem in their wake, a new study shows.

PREDATORY PYTHONS University of Florida scientists show off a 15-foot Burmese python, weighing more than 160 pounds, that was captured in the Everglades. Its stomach contained a 6-foot gator. Michael R. Rochford, University of Florida

The snakes, many of which measure 10 to 16 feet, are called Burmese pythons. But make no mistake: Virtually all of the roughly 30,000 living in southern Florida were born in the Everglades. Ecologists now report that populations of mammals have begun plummeting throughout the pythons’ expanding range. And the timing of these mammal losses matches the geographic spread of the snakes, which federal officials believe were initially released into the wild by snake fanciers, probably 15 to 30 years ago.

Raccoons, opossums, deer and other mammals, along with birds and gators, have all turned up in the stomachs of captured pythons, testifying to the snakes’ varied appetite, notes ecologist Michael Dorcas of Davidson College in North Carolina. “But until now, there hadn’t been any indication that the snakes were altering the ecosystem,” says Dorcas, who led the study.

The new data “make a persuasive case for cause and effect,” says herpetologist J. Whitfield Gibbons of the Savannah River Ecology Lab in Aiken, S.C., who was not affiliated with the new analysis. “The investigators take a convincing position that introduced predatory pythons are responsible for the decline in numbers of large- and medium-size mammals in the Everglades.”

With much of the roughly 6,000-square-kilometer Everglades National Park virtually inaccessible, the team of 11 university and federal scientists took an indirect approach to surveying the region’s mammal populations. Between 2003 and 2011, researchers cruised roads on 313 nights and compared the number of individuals in each species they saw per 100 kilometers traveled to rates witnessed along the same roadways over 51 nights in the 1990s — before pythons had established local breeding populations.

As in the earlier survey, raccoons and Virginia opossums were the most commonly observed mammals, although sightings of each were down by more than 98 percent in the most recent survey. Counts were 94 percent lower for white-tail deer and bobcat sightings were down 87 percent. And in contrast to the earlier survey, scientists saw no rabbits or foxes. Rabbits had been among the most common mammals witnessed in the 90s.

Roadside sightings of mammal species remained unchanged between the two periods in areas outside the python range. In spots of recent python migration, mammal sightings were also down somewhat from a decade earlier, drops ranging from 20 to 80 percent, Dorcas’ team reports online January 30 in the Proceedings of the National Academy of Sciences.

The new diminished mammal counts in python territory “are pretty similar to what we found,” says Joshua Holbrook of Florida Atlantic University in Davie. A more circumscribed road-sampling survey he coauthored in 2010 in Florida Scientist turned up nine mammals over four nights: seven deer, a possum and an unidentified small mammal. On five nights, he and his colleagues saw none. Meanwhile, beyond the python’s range in the nearby Corbett Wildlife Management Area, he and Thomas Chesnes of Palm Beach Atlantic University sighted 40 mammals over nine nights.

“This study paints a stark picture of the real damage that Burmese pythons are causing to native wildlife and the Florida economy,” says U.S. Interior Secretary Ken Salazar. His agency announced new rules on January 17 that will ban the importation and interstate transport of Burmese pythons, yellow anacondas and two other invasive constrictors sold in the pet trade. All have been found in Everglades National Park.

With so many invasive constrictors already breeding in South Florida, research is now focusing on ways to limit their spread and better understand the prey they threaten. Although Burmese pythons need freshwater to survive, a team of biologists with the U.S. Geological Survey led by Kristen Hart in its Davie, Fla., lab showed that the snakes can apparently derive much of their needed moisture from the tissue of prey animals.

The team’s experiments suggest that python hatchlings can’t survive more than two months with access to only saltwater. But a pair of hatchlings was still alive after 200 days with access to only brackish water. And a yearling snake with access to only saltwater survived 7 months — holding open the prospect that these adept swimmers could, if motivated, enter the marine realm and migrate long distances through seawater, the team reports in the Feb. 10 Journal of Experimental Marine Biology and Ecology. In fact, Burmese pythons have already been found eating endangered wood rats on Key Largo, off the mainland coast.


Back Story – SNAKES IN THE GRASS

Source: Center for Invasive Species and Ecosystem Health/Univ. Of Georgia; Credit: © 2012 Google, © 2012 Terrametrics, SIO, NOAA, U.S. Navy, NGA, GEBCO

In most of North America, a constrictor found slithering around in the wild is bound to be a lost or intentionally released pet. But in Florida, Burmese pythons have started expanding from the southernmost reaches of the Everglades (red dots indicate numbers of sightings). Researchers believe a breeder or dealer may have deliberately released a number of snakes in the remote location.

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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