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GAINESVILLE, Fla. – – Nearly 80 percent of radio-tracked marsh rabbits that died in Everglades National Park in a recent study were eaten by Burmese pythons, according to a new publication by University of Florida and U.S. Geological Survey researchers.
A year later, there was no sign of a rabbit population in the study area. The study demonstrates that Burmese pythons are now the dominant predator of marsh rabbits, and likely other mid-sized animals in the park, potentially upsetting the balance of a valuable ecosystem.
The study provides the first empirical evidence that the Burmese python caused reductions in marsh rabbit populations in the park, supporting previous studies that suggested pythons were a significant factor in declines of many other mid-sized mammals since becoming established there a few decades ago.
The estimated tens of thousands of Burmese pythons now populating the greater Everglades present a low risk to people in the park, according to previous research by USGS and NPS.
Scientists know that invasive pythons prey on native Everglades mammals, but they didn’t have experimental evidence that pythons could cause population declines or local extinction of mammals, said Robert McCleery, a UF assistant professor in wildlife ecology and conservation who led the study.
While a 2012 study showed that as pythons were proliferating, mammals were declining, it did not directly link the two phenomena. “This study does just that,” said Bob Reed, a USGS research herpetologist and study co-author.
“Mammals play an important role in the Everglades ecosystem, and so recovery of mammal populations is closely tied with recovering the overall health and functionality of this ecosystem,” McCleery said.
In most Florida wetlands, it’s easy to detect marsh rabbit populations by searching for their scat, but the researchers could not find evidence of rabbits in the parts of Everglades National Park they studied during intensive surveys prior to conducting their experiment.
In 2012, a group of scientists that included researchers from Davidson College, USGS and UF compared data on mammal populations from the 1990s – before pythons became widespread in Everglades National Park ─ to results of population surveys conducted between 2003 and 2011. The 2012 study found that significant mammalian population declines coincided in space and time with the proliferation of invasive pythons in the Everglades.
“Previous studies implicated pythons in mammal declines in the Everglades, but those studies were largely correlative,” said Reed. “This new study moves us from correlation to causation in terms of the impact of invasive pythons on native mammals.”
To conduct the most recent study, researchers found areas outside of the park that supported large and healthy populations of marsh rabbits. They moved 31 marsh rabbits into select areas in the park in two experimental populations. They also put 15 rabbits in the Loxahatchee National Wildlife Refuge, and captured, collared and released another 49 in Fakahatchee Strand State Park, where they knew there would be few, if any, pythons ─ and used those as control sites. All of the rabbits were equipped with radio-collars so that they could be regularly located.
The researchers radio-tracked the rabbits and found that 77 percent of those that died in the Everglades were eaten by Burmese pythons, and that there was no sign of a rabbit population in the areas where they released them in the park one year later. On the other hand, rabbits remained common at the control site after the experiment. Many animals eat marsh rabbits, but outside the park, they’re most often the victims of bobcats and coyotes.
Furthermore, the warmer and wetter the weather, the more rabbits were consumed by pythons in the park. The researchers suggested this may be because higher water levels allow the pythons to easily swim long distances while searching for food and because they feed more frequently when it’s hot.
Scientists chose to study the pythons’ impacts on marsh rabbit populations because the high reproductive rates of rabbits mean that their populations are typically resilient to predators, McCleery said. The conclusion that pythons are capable of eliminating marsh rabbit populations in Everglades National Park led the authors to suggest that the observed declines in other mid-sized mammal species in the park could also be due to predation by pythons.
The study was published online this week in the journal Proceedings of the Royal Academy B. McCleery’s co-authors include UF graduate student Adia Sovie, as well as Robert Reed, Kristen Hart and Margaret Hunter, all research wildlife biologists with the USGS.