Severe Mammal Declines Coincide with Proliferation of Invasive Burmese Pythons in Everglades National Park

Proceedings of the National Academy of Sciences (PNAS)Proceedings of the National Academy of Sciences (PNAS), Published Online 30 January 2012

Michael E. Dorcas, John D. Willson, Robert N. Reed, Ray W. Snow, Michael R. Rochford, Melissa A. Miller, Walter E. Meshaka, Jr., Paul T. Andreadis, Frank J. Mazzotti, Christina M. Romagosa, and Kristen M. Hart

“Invasive species represent a significant threat to global biodiversity and a substantial economic burden. Burmese pythons, giant constricting snakes native to Asia, now are found throughout much of southern Florida, including all of Everglades National Park (ENP). Pythons have increased dramatically in both abundance and geographic range since 2000 and consume a wide variety of mammals and birds. Here we report severe apparent declines in mammal populations that coincide temporally and spatially with the proliferation of pythons in ENP. Before 2000, mammals were encountered frequently during nocturnal road surveys within ENP. In contrast, road surveys totaling 56,971 km from 2003–2011 documented a 99.3% decrease in the frequency of raccoon observations, decreases of 98.9% and 87.5% for opossum and bobcat observations, respectively, and failed to detect rabbits.

Map of South Florida illustrating sampling locations in relation to python distribution.

Map of South Florida illustrating sampling locations in relation to python distribution. Road surveys for mammals were conducted in the 1990s and 2000s along the Main Park Road (MPR) in Everglades National Park (ENP). Areas recently invaded by pythons and surveyed for mammals in 2009–2011 include Big Cypress National Preserve (BCNP), Collier-Seminole State Park (CSSP), Chekika (CHK), and Key Largo. Immokalee and Corbett Wildlife Management Area (CWMA; north of the map) are two sampled sites where pythons have not yet become established. The purple region represents the area of ENP where pythons were found in the 1990s and where reproduction was first reported (16). Red triangles represent localities of pythons found during 2008–2009.

“Road surveys also revealed that these species are more common in areas where pythons have been discovered only recently and are most abundant outside the python’s current introduced range. These findings suggest that predation by pythons has resulted in dramatic declines in mammals within ENP and that introduced apex predators, such as giant constrictors, can exert significant top-down pressure on prey populations. Severe declines in easily observed and/or common mammals, such as raccoons and bobcats, bode poorly for species of conservation concern, which often are more difficult to sample and occur at lower densities.”