Hidden killers invade Florida everglades
Since it was established in 1947, Everglades National Park has been a research haven for wildlife biologists, who are drawn to the diverse array of animals who call the 1-and-a-half-million-acre park home. The Everglades is one of the largest remaining wilderness areas in the eastern United States, and because of its subtropical ecosystem, scientists who study reptiles and amphibians visit frequently.
In the early 2000s, a team of herpetologists went to the Everglades to conduct research on the rise of the Burmese python as an invasive species in south Florida. One night, after driving the park’s main road looking for snakes, one of the team’s lead researchers compared observations with J.D. Willson, who was then a doctoral student at the University of Georgia and is now a University of Arkansas biologist.
“I remember commenting to J.D., ‘We haven’t seen a raccoon down here, dead or alive,’” recalls Mike Dorcas, a professor of biology at Davidson College, who was Willson’s undergraduate mentor. “We realized we hadn’t seen opossums, either. So consequently we developed the idea for a study to look at changes in relative abundance of mammals associated with the python invasion.”
From 2003 to 2011, the team led by Dorcas and Willson drove roads at night looking for mammals and compiled observations from other biologists. The scientists amassed more than 57,000 kilometers of surveys on the main road in Everglades National Park, recording observations of mammals that had historically been abundant in the park, including the two most commonly found during the 1990s, raccoons and opossums.
What they found shocked them. In all those miles, they accounted for only nine raccoons and five opossums, declines of more than 98 percent compared to pre-python surveys. They saw only four bobcats, and they didn’t spot a single fox or a rabbit.
“It was an unbelievable reduction,” Willson said. “It is now kind of eerie to drive roads at night in the southern parts of the Everglades. You go hundreds of miles and seldom see a single mammal. We have reports of park visitors who went to go fishing in the 1990s and they would frequently see rabbits scampering across the road.
“As far as we can tell, nobody has seen a wild marsh rabbit along the 40-mile main road in Everglades National Park in almost 10 years.”
The team — which included scientists from the National Park Service, the U.S. Geological Survey, the University of Florida and Auburn University — came to a single conclusion.
“These findings suggest that predation by pythons has resulted in dramatic declines in mammals within Everglades National Park and that introduced apex predators, such as giant constrictors, can exert significant top-down pressure on prey populations,” they wrote in a paper that appeared in the Proceedings of the National Academy of Sciences in 2012.
Burmese pythons are at the top of the food chain in south Florida.
“There’s not much there that they can’t eat,” Willson said. “The largest pythons are capable of eating nearly every native bird or mammal in the southeastern United States — including full-grown whitetail deer and even alligators. We were seeing enough pythons in the Everglades that we expected to see some effects, but we had no idea it would be that dramatic.”
Among the various threats to wildlife and native ecosystems across the United States, invasive species are not getting enough scientific attention, according to Willson. The Burmese python — one of the largest snakes in the world and native throughout much of southern and southeastern Asia — had been one of the most common animals in the pet trade for three decades. Burmese pythons started popping up with increasing frequency in the wild in Everglades National Park starting in the early 2000s.
“It started out as one or two a year, so people assumed that these were recently released animals or those who had escaped from their owners,” Willson said. “But fairly quickly it became apparent that this was an established and reproducing population starting to expand in the region. The first animals were found in some of the most remote areas of Everglades National Park in the mangrove sections near Flamingo. Now, there are records of them showing up well north of Interstate 75 — Alligator Alley. In the last 10 years they have spread over a huge area.”
Realizing this as a rich research opportunity, Dorcas, Willson and their research colleagues began making regular trips to Florida. They found out what the pythons were eating — and soon became concerned about how they could affect species that are endemic to Florida or are federally or locally protected.
For example, consider the Key Largo woodrat, which is only found in a small area on Key Largo in the upper Florida Keys archipelago. The U.S. Fish and Wildlife Service lists the rodent as an endangered species, and estimates suggest that there are only about 200 of the rodents left on the island.
The first python that was captured and euthanized on the island had two Key Largo woodrats in its stomach — perhaps 1 percent of the species’ known population.
The National Park Service and the University of Florida have amassed a large prey list based on the examination of the stomach contents of captured and euthanized pythons.
“They seem to be eating alligators with some regularity,” Willson said. “We are also worried about the beautiful herons and egrets that draw tourists to the Everglades and are frequent prey for pythons.”
Before pythons were common there, the Everglades had a standard medium-sized mammal community. There were high numbers of raccoons and opossums and smaller numbers of carnivores such as bobcats and foxes.
Not any longer.
“The mammal study was important in providing very convincing evidence of the apparent effects that pythons are having on native fauna,” Dorcas said. “Our research has made people aware that these pythons can have major impacts on the prey species that they are feeding on. This is probably going to have major cascading effects on the ecosystem, as well. It’s hard to imagine taking out most of the mammals and not seeing the impacts on plants and other species.”
Nobody has been able to come up with a solid estimate of how many Burmese pythons are slithering around south Florida — rough guesses range from several thousand to hundreds of thousands. Their date of origin in the Everglades hasn’t been determined with any accuracy, either.
Research by Willson’s group disproves the theory that Hurricane Andrew caused the problem. For years, a popular notion in the media has been that the 1992 hurricane destroyed a warehouse or breeding operations that released the snakes into the wild.
“In order to get to what we would consider a plausible population size — more than 10,000 snakes out in the wild — you would have to start with several hundred ‘founders’ to start that population and get that population going quickly enough to get to where we are now,” he said. “The area where pythons were first found and where reproduction was first confirmed is more than 30 miles away from the areas in western Miami that have reptile dealers.
“It makes much more sense that the problem started in the southern part of the Everglades and dates back well before Hurricane Andrew,” he said. “Our research, based on population models and examination for the spread of the area of invasion, suggests that the most plausible explanation is that a few snakes were released either intentionally or unintentionally to the southern part of the Everglades in the 1980s or possibly even the 1970s.
“If we don’t know how many are out there we have no ability to judge how well our control efforts are working, to know how much effort it would take to eliminate them even from a small area,” he said. “They are so adept at hiding that there are very few circumstances where we know how many there are with any degree of certainty.”
Willson’s research team is currently working on new methods for estimating python density.
So, will a Burmese python be coming to your neighborhood anytime soon?
Again, there is no good answer.
The technique that has been used most frequently to assess how far these snakes could spread is climate matching — a method that attempts to match the climate of areas where Burmese pythons are known to be found in their native range to similar conditions in areas at risk of invasion.
Most people think of Burmese pythons as tropical animals
that won’t be able to live in colder climates. That’s not exactly true. The species does occur in tropical areas but it is also found in very arid areas in Pakistan and northern India and farther north into the foothills of the Himalayas, where it freezes in the winter. Some climate models predict suitable climate as far north as central Arkansas.
Four years ago, Willson helped lead a cold-tolerance test for pythons while working as a post-doctoral fellow at the Savannah River Ecology Lab in South Carolina. The group translocated 10 adult male pythons from the Everglades to a snake-proof enclosure at the lab that featured a large pond and a variety of habitats.
The researchers kept the snakes outside for almost a full year to monitor their behavior, physiology and body temperatures, but ostensibly it was an experiment to see if they would survive the South Carolina winter. It was one of the coldest winters on record in South Carolina and all of the snakes eventually died — but not before showing some adaptive behavior and resilience to the frigid temperatures.
“In the fall, the pythons stayed underground or in the water at night and they basked in the sun during the day,” Willson said. “One snake made it through six or seven freeze events before he eventually died during an intensive cold spell in January. These results suggest that they may not be able to survive that far north, especially during extreme winters, but we definitely saw some behaviors that were appropriate for cold weather.”
That means that the python’s territory will probably extend beyond south Florida.
“This is probably not just a south Florida problem,” Willson said. “It is probably at least a peninsular Florida problem and likely a Gulf Coast problem. The jury is still very much out on the central and Southern United States.”
J.D. Willson is an assistant professor of biological sciences at the University of Arkansas. His lab focuses on understanding factors that drive population and community dynamics of reptiles and amphibians. Although much of the lab’s work is applied, Willson and colleagues also shed light on basic questions in ecology. They are especially interested in understanding “unique aspects of reptile and amphibian population biology that differentiate them from other vertebrates.”
Willson and Mike Dorcas are the co-authors of Invasive Pythons in the United States: Ecology of an Introduced Predator, published in 2011 by the University of Georgia Press.