Precipitous declines in formerly common mammals in Everglades National Park in Florida have been linked to the presence of invasive Burmese pythons, according to a study by Michael Dorcas, an associate professor of biology at Davidson College, and colleagues. The research is published in the Proceedings of the National Academy of Sciences.
The study, the first to document the ecological impacts of this invasive species, strongly supports that animal communities in the 1.5-million-acre park have been markedly altered by the introduction of pythons within 11 years of their establishment as an invasive species. Mid-sized mammals are the most dramatically affected, but some Everglades pythons are as large as 16 feet long, and their prey have included animals as large as deer and alligators.
"The magnitude of these declines underscores the apparent incredible density of pythons in Everglades National Park and justifies the argument for more intensive investigation into their ecological effects, as well as the development of effective control methods," said Dorcas, lead author of the study and author of the 2010 book Invasive Pythons in the United States.
He continued, "Such severe declines in easily seen mammals bode poorly for the many species of conservation concern that are more difficult to sample but that may also be vulnerable to python predation."
The most severe declines, including a nearly complete disappearance of raccoons, rabbits and opossums, have occurred in the remote southernmost regions of the park, where pythons have been established the longest. In this area, populations of raccoons dropped 99.3 percent, opossums 98.9 percent and bobcats 87.5 percent. Marsh and cottontail rabbits, as well as foxes, were not seen at all.
The researchers collected their information via repeated systematic night-time road surveys within the park, counting both live and road-killed animals. Over the period of the study, researchers traveled a total of nearly 39,000 miles from 2003 to 2011 and compared their findings with similar surveys conducted in 1996 and 1997 along the same roadways before pythons were recognized as established in Everglades National Park.
The authors also conducted surveys in ecologically similar areas north of the park where pythons have not yet been discovered. In those areas, mammal abundances were similar to those in the park before pythons proliferated. At sites where pythons have only recently been documented, however, mammal populations were reduced, though not to the dramatic extent observed within the park where pythons are well established.
"Pythons are wreaking havoc on one of America's most beautiful, treasured and naturally bountiful ecosystems," said U.S. Geological Survey director Marcia McNutt. "Right now, the only hope to help halt further python invasion into new areas is swift, decisive and deliberate human action."
The authors suggested that one reason for such dramatic declines in such a short time is that these prey species are "naïve" -- that is, they not used to being preyed upon by pythons since such large snakes have not previously existed in that ecosystem.
"It took 30 years for the brown treesnake to be implicated in the nearly complete disappearance of mammals and birds on Guam; it has apparently taken only 11 years since pythons were recognized as being established in the Everglades for researchers to implicate pythons in the same kind of severe mammal declines," said Robert Reed, a USGS scientist and co-author of the paper. "It is possible that other mammal species, including at-risk ones, have declined as well because of python predation, but at this time, the status of those species is unknown."
Another coauthor of the study was John Willson '02, a research scientist at Virginia Tech University who has worked with Dorcas on several studies, and co-authored the book Invasive Pythons in the United States.
Willson commented, "Our research adds to the increasing evidence that predators, whether native or exotic, exert major influence on the structure of animal communities. The effects of declining mammal populations on the overall Everglades ecosystem, which extends well beyond the national park boundaries, are likely profound, but are probably complex and difficult to predict. Studies examining such effects are sorely needed to more fully understand the impacts pythons are having on one of our most unique and valued national parks."
On January 23 the U.S. Fish and Wildlife Service published a rule in the Federal Register that will ban the importation and interstate transportation of four non-native constrictor snakes that threaten the Everglades, including the Burmese python. These snakes are being listed as injurious species under the Lacey Act.
In addition to Dorcas and Willson, authors of the study are Robert N. Reed, USGS; Ray W. Snow, NPS; Michael R. Rochford, University of Florida; Melissa A. Miller, Auburn University; Walter E. Meshaka, Jr., State Museum of Pennsylvania; Paul T. Andreadis, Denison University; Frank J. Mazzotti, University of Florida; Christina M. Romagosa, Auburn University; and Kristen M. Hart, USGS.
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