John J.D. Willson, Virginia Tech: Severe Declines in Everglades Mammals Linked with Invasive Burmese Pythons

January 31, 2012

Collaborative research, led by Michael Dorcas of Davidson College and John “J.D.” Willson of Virginia Tech’s College of Natural Resources and Environment, has linked precipitous declines in formerly common mammals in Everglades National Park to the presence of invasive Burmese pythons.

Burmese python (Python molurus). Photo courtesy of Roy Wood, National Park Service.

The study, published on Jan 30, 2012, in the Proceedings of the National Academy of Sciences, is the first to document the ecological impacts of this invasive species and 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.

“Our research adds to the increasing evidence that predators, whether native or exotic, exert major influence on the structure of animal communities,” said Willson. “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.”

Willson is a post-doctoral researcher in the Wildlife Ecotoxicology and Physiological Ecology Program in the Department of Fish and Wildlife Conservation at Virginia Tech and is a co-author of the book “Invasive Pythons in the United States.”

“Dr. Willson’s recent work on pythons provides significant insights into the important roles that reptiles can play in community structure and ecosystem processes,” said Associate Professor Bill Hopkins, who directs the ecotoxicology program. “Understanding how introduced predators like pythons influence community structure will ultimately prove critical to conserving important ecological systems like the Everglades.”

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.

“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 halt further python invasion into new areas is swift, decisive, and deliberate human action.”

The researchers collected their information via repeated systematic nighttime road surveys within Everglades National Park, counting both live and road-killed animals. Researchers traveled a total of nearly 39,000 miles from 2003 to 2011 and compared their findings with similar surveys conducted along the same roadways in 1996 and 1997 before pythons were recognized as established in the park.

The study’s authors noted that the timing and geographic patterns of the documented mammal declines are consistent with the timing and geographic spread of pythons.

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.

“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 lead author Michael Dorcas, a professor in the Department of Biology at Davidson College in North Carolina, who co-authored “Invasive Pythons in the United States” with Willson. “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 mammals that have declined most significantly have been regularly found in the stomachs of Burmese pythons removed from Everglades National Park and elsewhere in Florida. The authors noted that raccoons and opossums often forage for food near the water’s edge, a habitat frequented by pythons in search of prey.

The authors suggested that one reason for such dramatic declines in such a short time is that these prey species are “naive” since such large snakes have not existed in the eastern United States for millions of years. Burmese pythons over 16 feet long have been found in the Everglades. In addition, some of the declining species could be both victims of being eaten by pythons and of having to compete with pythons for food.

“It took 30 years for the brown tree snake 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 U.S. Geological Survey scientist and a 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.”

The scientists noted that in their native range in Asia, pythons have been documented to consume leopards. Consequently, even large animals, including top predators, are susceptible to python predation. For example, pythons in the Everglades have been documented consuming alligators and full-grown deer. Likewise, the authors state that birds, including highly secretive birds such as rails, make up about a fourth of the diet of Everglades pythons, and declines in these species could be occurring without managers realizing it.

The authors found little support for alternative explanations for the mammal declines, such as disease or changes in habitat structure or water management regimes.

“This severe decline in mammals is of significant concern to the overall health of the park’s large and complex ecosystem,” said Everglades National Park Superintendent Dan Kimball. “We will continue to enhance our efforts to control and manage the non-native python and to better understand the impacts on the park.”

“No incidents involving visitor safety and pythons have occurred in the park,” Kimball continued. “Encounters with pythons are very rare; that said, visitors should be vigilant and report all python sightings to park rangers.”

The U.S. Fish and Wildlife Service published a rule in the Federal Register on Jan. 23, 2012, that will ban the importation and interstate transportation of four non-native constrictor snakes (Burmese python, yellow anaconda, and northern and southern African pythons) that threaten the Everglades and other sensitive ecosystems. These snakes are being listed as injurious species under the Lacey Act. In addition, the U.S Fish and Wildlife Service will continue to consider listing as injurious five other species of nonnative snakes (reticulated python, boa constrictor, DeSchauensee’s anaconda, green anaconda, and Beni anaconda).

The authors of the research paper, “Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park,” are Michael E. Dorcas, Davidson College; John D. Willson, Virginia Tech; Robert N. Reed, U.S. Geological Survey; Ray W. Snow, National Park Service; 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, U.S. Geological Survey.

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