A python invasion has exploded out of the Everglades
An ambitious new paper produced by the U.S. Geological Survey found that the python population has exploded in only 20 years from a few snakes at the southern tip of Everglades National Park to an invasion that envelops the southern third of Florida.
Burmese pythons are too good at what they do — they’re nearly undetectable to both humans and their prey, they barely need to move and when they do they’re deadly. On top of that, they have lots of babies.
As a result, according to an ambitious new paper produced by the U.S. Geological Survey, their population has exploded in only 20 years from a few snakes at the southern tip of Everglades National Park to an invasion that envelops the southern third of Florida.
The reptile’s “invasion front” has recently rolled through Broward and Palm Beach counties and is moving up the state. The current front encompasses the southern end of Lake Okeechobee and is pushing westward north of Fort Myers.
The study, which meticulously synthesizes several decades’ worth of findings from more than 250 research initiatives, assesses where we stand in the python invasion and how we might slow it.
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The success of these snakes, which are native to Southeast Asia, and came here via the exotic pet trade, has been a cataclysmic failure for South Florida ecosystems and “represent one of the most intractable invasive-species management issues across the globe,” said the paper.
To put it simply, the snakes are very much on the move, butting up against civilization and heading north — how far it will go depends on several factors, including climate change.
History of an invasion
In the 1970s, Burmese pythons, which are admittedly beautiful, dappled in a rich pattern of mahogany, coffee and taupe, became all the rage in the exotic pet trade. Snakes from Thailand and Myanmar began showing up more and more in the States, including South Florida.
By the end of the decade, there was evidence that some of the snakes were living in Everglades National Park. In 1979, a python measuring more than 12 feet was run over on Tamiami Trail, and there was a spate of unconfirmed sightings in the southwest section of the park through the 1980s.
It wasn’t until 1995, though, that biologists officially documented and collected two snakes — a 7-foot adult and, tellingly, a hatchling — near West Lake at the southern tip of the peninsula.
In the following years, park staff began finding the invasive snakes farther and farther north in the park. “Together, these observations suggest that multiple generations of Burmese pythons were present in ENP by 2000 or earlier and that the population occupied a large geographic area,” writes the USGS’s Jackie Guzy, author of the report.
That year, wildlife officials deemed them to be established and reproducing. Since then the snakes have expanded steadily up the peninsula, with genetic analysis suggesting that a second introduction of snakes with slightly different patterns on their skin occurred to the west, near Naples.
Little did biologists know at the time the massive impact the invaders would have on native animals, and how futile control efforts would be.
The 'invasion front'
One of the most startling aspects of the study is a map depiction of the snake’s “invasion front.” Guzy used occurrence records submitted by both researchers and the public between 1979 and 2021 to create a map that shows the chronology of python removals. She cautions that the leading edge “represents the best professional estimate of the invasion front, which is not exact and will change over time” and could include snakes that are escaped captives, and not part of the wild invasive population.
On the map we can see the tiny nugget of removals at the southern tip of the Everglades from 1995 to 2000. From there the species blossoms rapidly through the wilderness of Everglades National Park and then more slowly, both down into the Keys and laterally into boundary reserve and agricultural areas of Miami-Dade, Broward and Palm Beach counties before finally finding its way into coastal civilization areas starting in about 2013.
The outer band of the invasion front, representing 2019 to 2021, now reaches West Palm Beach, the southern end of Lake Okeechobee, and areas north of Fort Myers.
The snakes clearly thrive in the swamps of the Everglades, but is suburbia suitable? A study that tracked hatchlings showed that fewer survived near urbanized areas. Canals, though, provide not only habitat, but travel routes as they look for territory.
Guzy wrote in an email that “while Burmese pythons may expand into urban areas, or occur in proximity to the urban interface, research thus far indicates they tend to avoid highly urban areas. This may be because urban areas have expansive development and less favorable habitat, which may result in higher rates of detection and removal.”
Detecting the outer band
The study says there are few records along the leading edge of the invasion front, so researchers used environmental DNA to detect the presence of the snakes. Animals release environmental DNA through shed skin, feces, mucous and decomposing flesh, and researchers can detect it by taking water or samples.
The map is dotted with hundreds of spot where pythons were seen or captured, and many occur north of Lake Okeechobee, but Guzy warns that, “Thus far, verified records north of Lake Okeechobee cannot be confidently attributed to the southern Florida population and may represent newly escaped individuals.”
A quick glance at the density of the python occurrences shows that the bulk of them occur along rural roads and canals, where snakes often bask for warmth, and where they butt up against the human-made world. That doesn’t mean there are no snakes out in the middle of the sawgrass, it’s just nearly impossible to find them there.
Just how many invasive pythons now live in Florida is impossible to say. The species is so “cryptic,” the study says, that it’s extremely difficult to ascertain a number. The most committal statement the study gives is that there “may be tens of thousands of pythons across known areas of invasion in southern Florida.”
How are they so successful?
Sight, our dominant sense, is likely the worst way to locate a python. They’re nocturnal, can feed infrequently and don’t need to move. If they don’t move, humans just don’t see them. According to Guzy, the snakes also spend an average of 86.1% of their time resting.
In one study, people walking around looking for the snakes in a semi-natural outdoor enclosure the size of two basketball courts only had a success rate of less than 1%. The snakes were underwater, underground, but sometimes hiding in plain sight.
Their cloaking power makes them super-efficient predators, waiting along animal trails or the water’s edge for prey. As a constrictor, they coil around their prey, tightening the grip every time the animal exhales, eventually suffocating it.
The largest invasive python in Florida measured 18.7 feet long, weighed 213.8 pounds and was a big momma, carrying 122 eggs. They normally lay 11-84 eggs per clutch, but studies suggest an average of 34 in the wilds of Florida.
A brutal toll on native wildlife
When biologists open the invasive snakes up, it’s like rifling through a Florida field guide. All told, they’ve found 76 prey species inside the snakes. That includes lots of birds, such as vultures, crows, ducks, herons, roseate spoonbills and threatened wood storks; small mammals such as the endangered Key Largo woodrat and Key Largo cotton mouse, marsh rabbits, armadillos, possums, raccoons, otters and domestic cats, and larger prey including domestic goats, white-tailed deer, wild hogs and alligators. There has never been a documented human death due to a wild python in Florida.
How much damage have they done? Guzy points out that before 2000, researchers could frequently spot mammals in Everglades National Park. But from 2003 to 2011, the frequency of mammal observations (raccoons, opossums, bobcats, rabbits, gray foxes, and white-tailed deer) declined by 85% to 100%. Outside the python’s range, those species were more common.
The snakes also competed with native predators, like bobcats. One study released marsh rabbits in areas with and without pythons. In python areas, the snakes accounted for 77% of rabbit mortalities within 11 months. At other sites, no rabbits were killed by pythons and mammal predators accounted for 71% of the marsh rabbit deaths. Marsh rabbit declines in southern Florida were caused by pythons, other studies showed.
Cold comfort: How much farther can they go? The study is not ready to commit to predicting how far north the snakes could live, in part because different researchers have come up with vastly different answers.
One study cross-referenced the climate and rainfall of the snake’s range in Asia with that of the U.S. and suggested the snake could reach much of the southern third of the United States. Another analysis foretold all of Florida as eventual python country. Add climate change to the mix and it’s hard to say where the snakes will stop.
Cold tolerance puts the brakes on the snakes, but they often stay warm by sheltering in gopher tortoise and mammal burrows. One study said this strategy could see the snakes surviving in southwest Georgia.
In another study, snakes in north Florida died during a cold snap, even though they had access to a den and a heat source. The wildcard here is the snake’s ability to evolve to tolerate cold — after a severe 2010 cold snap, snake populations dropped, but have recovered. Those cold-hardened snakes are the ones having babies today, the paper said.
How to slow the invasion
“Over the past two decades we have extensively explored methods to capture and remove pythons. But so far, there are no easy solutions,” Guzy said. “The more tools we have at our disposal, the better.”
Those tools currently include the state’s Python Challenge, an annual 10-day event where hunters cruise backroads and canal and slog through swaps to catch and euthanize as many snakes as they can. Last year’s Challenge totaled 231 dead snakes.
Some hunters used trained dogs to sniff the snakes out, and biologists have implanted tracking devices in certain snakes and followed them to breeding aggregations, where they can snag several in a small area.
Another nascent but promising concept is to put tracking collars on prey such as possums and racoons. Snakes are so ubiquitous that the mammals are eventually eaten, usually by a larger snake.
The collar stays in the snake’s digestive tract for some time, and biologists can track the collar to the snake.
This method recently led researchers to two massive female pythons full of egg follicles in Key Largo. The snakes were humanely euthanized.
There is hope that with cheaper collars and drone technology, this prey-tracking method could put a real dent in the snake’s populations, particularly because it leads to larger snakes, which are often female.
The next frontier in python control may be genetic biocontrol, akin to the genetically modified mosquitoes that Florida released in 2021 to prevent females from surviving to adulthood. Someday, biologists might be able to alter the genetics of a population to either become sterile, or almost entirely male, or hamper their survival in some way.
“Genetic biocontrol tools represent exciting possibilities that are actively being explored but which are still a long way from being used,” Guzy said.
In the end, Guzy said, “these snakes are extremely cryptic and secretive, and they are inherently difficult to find. ... Although supremely challenging, python research is active and ongoing, and public support and engagement is an important aspect of these efforts.”