Severe Declines in Everglades Mammals Linked to Pythons
Precipitous declines in formerly common mammals in Everglades National Park, southern Florida, have been linked to the presence of invasive Burmese Pythons (Python molurus bivittatus), according to a U.S. Geological Survey (USGS) coauthored study published in the Proceedings of the National Academy of Sciences (http://dx.doi.org/10.1073/pnas.1115226109).
The study, the first to document the ecological impacts of this invasive species, strongly supports that animal communities in this 1.5-million-acre park have been markedly altered by the proliferation of pythons, particularly since 2000. Mid-sized mammals are the most drastically affected.
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 USGS 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 through repeated systematic nighttime road surveys within the park, counting both live and road-killed animals. Over the period of the study, from 2003 to 2011, researchers traveled a total of nearly 39,000 miles. They 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 scientists who authored the paper 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 Michael Dorcas, lead author of the study, a professor at Davidson College in North Carolina, and coauthor of the book Invasive Pythons in the United States. “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 drastic declines in such a short time is that these prey species are “naïve”—that is, they not used to being preyed on by pythons because such large snakes have not existed in the eastern United States for millions of years. Burmese Pythons more than 16 feet long have been found in the Everglades. In addition, some of the declining species could be victims both of being eaten by pythons and of having to compete with pythons for food.
“It took 30 years for the Brown Treesnake (Boiga irregularis) 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 coauthor 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, showing that even large animals, including top predators, are susceptible to python predation. For example, pythons have been documented consuming full-grown deer and alligators. 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.
“Our research adds to the increasing evidence that predators, whether native or exotic, exert major influence on the structure of animal communities,” said John Willson, a study coauthor, research scientist at Virginia Tech University, and coauthor of the book Invasive Pythons in the United States. “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.”
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 nonnative python and to better understand the impacts on the park. No incidents involving visitor safety and pythons have occurred in the park. Encounters with pythons are very rare; that said, visitors should be vigilant and report all python sightings to park rangers,” Kimball said.
The odds of eradicating an introduced population of reptiles once it has spread across a large area are very low, pointing to the importance of prevention, early detection, and rapid response. And with the Burmese Python now distributed across more than a thousand square miles of southern Florida, including all of Everglades National Park and areas to the north, such as Big Cypress National Preserve, the chances of eliminating the snake completely from the region are low. However, controlling their numbers and preventing their spread are crucial goals for south Florida land managers. For example, a number of Burmese Pythons have been found in the Florida Keys, but there is no confirmation yet that a breeding population exists in the Keys. Given a recent USGS study that showed the python’s apparent ability to disperse across salt water (http://dx.doi.org/10.1016/j.jembe.2011.11.021), island residents and resource managers need to stay vigilant so as to be able to detect and eliminate arriving pythons before they become established.
Exactly how pythons were introduced to Everglades National Park is controversial, but most agree that the founders of the current population were pets that had been released or had escaped. On January 23, 2012, the U.S. Fish and Wildlife Service (FWS) published a rule in the Federal Register that restricts the importation and interstate transportation of four nonnative constrictor snakes (Burmese Python, Northern and Southern African Pythons [Python sebae and Python natalensis], and the Yellow Anaconda [Eunectes notaeus]) that threaten the Everglades and other sensitive ecosystems. These snakes are now listed as injurious species under the Lacey Act. In addition, the FWS will continue to consider listing as injurious five other species of nonnative snakes: the Reticulated Python (Broghammerus reticulatus or Python reticulatus), Boa Constrictor (Boa constrictor), DeSchauensee’s Anaconda (Eunectes deschauenseei), Green Anaconda (Eunectes murinus), and Beni Anaconda (Eunectes beniensis). For more information about the Lacey Act and the listing of the four constrictors as injurious, please visit the FWS News and Resources Web site.
The online USGS Science Feature “The Big Squeeze: Pythons and Mammals in Everglades National Park” contains additional information about pythons in the Everglades and USGS studies of invasive species; to read it, visit http://www.usgs.gov/blogs/features/usgs_top_story/the-big-squeeze-pythons-and-mammals-in-everglades-national-park/.
The recently published report, “Severe mammal declines coincide with proliferation of invasive Burmese Pythons in Everglades National Park,” is posted at http://dx.doi.org/10.1073/pnas.1115226109. Questions and answers about the study are posted at http://www.fort.usgs.gov/FLConstrictors/FAQPrey.asp. An interview with USGS scientist and study coauthor Robert Reed is posted at http://gallery.usgs.gov/audios/439.
in this issue:
Declines in Everglades Mammals Linked to Pythons