Giant snake invasion!

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Giant snake invasion!

Hoping to avert a biological infestation that could eventually stretch across a band of southern states, the U.S. Fish and Wildlife Service has proposed to ban the import and interstate transport of nine giant constrictor snakes. After the comment period expires May 11, the agency will review its options and consider whether to push forward with regulations.

 Gloved hands holding snakes head with dark and light brown markings, over dirt pile with white eggs

“Home sweet home,” coos a female Burmese python at her nest (see the eggs?) in South Florida. Her extended family, originally imported for the U.S. pet trade, is merrily invading a unique and fragile wilderness in Florida’s Everglades.

The proposal was not popular with snake enthusiasts and the pet trade, but these giant constrictors are big, fearsome predators that can even kill alligators and panthers.

In short, these snakes (including the boa constrictor, four pythons and four anacondas) seem more suited to Tarzan movies than Florida travel brochures.

The major existing threat comes from thousands of Burmese pythons living in and around Everglades National Park, where the 2006 discovery of a nest confirmed that the python is breeding. It’s uncertain far north the snake is established, but Art Roybal, a Fish and Wildlife Service biologist in Florida, says they are “most likely” breeding in the Myakka River drainage in Sarasota County, but “Future events will determine whether that likely population can persist, spread, or extend to additional animals.” (Here’s a map of Burmese python sightings in Florida).

Although Burmese pythons seldom attack humans, some have killed their owners in captivity.
Fish and Wildlife is also proposing to ban the import and interstate transport of eight other giant constrictors, including the boa constrictor, which has been established for decades west of Miami, and the northern African python, which may be reproducing in South Florida. The agency hopes to designate the snakes as “injurious wildlife” under the Lacey Act.

Over the last 10 years, more than 1.8 million live constrictor snakes, including 12 species, were imported into the United States. Most, however, are ball pythons, which are not included in the Lacey-act proposal.

The snake problem arises almost entirely from pet owners, says the wildlife agency. Some of the invaders (or their ancestors) are escaped pets. Others came from pets that grew big and were dumped into the wild by owners who did not know, or did not care, that these animals could start to play house in the hospitable southern clime.

Although snake dealers generally disdain these proposed restrictions, Gordon Rodda, a U.S. Geological Survey invasive snake expert, says “there already are tens of thousands of these snakes in captivity in the United States, and no-one is proposing any restrictions on that. Private possession of pets is a state responsibility. These proposed rules would affect only the import of new stock and transport across state lines, not the keeping of existing stock or their progeny.”

Head and torso of large snake with dark brown marking, black and light brown lines

The Burmese python can grow up to 20 feet long, and weigh up to 200 pounds. One of the world’s largest snakes, it is firmly established in South Florida and moving north.

The art of the invader

Invasive species, whether plants, mammals, insects or reptiles, have inherent advantages over natives: they often lack diseases or predators, and native wildlife and vegetation cannot counter their competitive tactics. “We are talking about non-native predators that our native species have not evolved to cope with,” says Roybal. “They get so large, up to 200 pounds and up to 20 to 23 feet long. They are a sit-and-wait predator. The native species are not used to animals of that sort, and haven’t developed behaviors to avoid them.”

The large constrictors are especially dangerous to threatened and endangered species, says Roybal. “The thing that really opened our eyes was the consumption of three key largo woodrats [by the python] in the Florida keys. This is a very imperiled species, there probably are only 200 to 300 left in the wild. If a large constrictor got established in the keys, it could mean severe impacts and possibly extinction.”

The Burmese python could occupy all of Florida, experts say, and even spread far beyond, according to a U.S. Geological Survey study of the snake’s climate requirements.

Maps: USGS.
Climate in green areas matches climate in the python’s native range. Mouse over to see the python’s range in 2100, after the climate warms.
Large coiled snake with black diamond shaped markings, mostly light brown with some yellow

Photo: USGS
The reticulated python may be among the former pet snakes now breeding in Puerto Rico.

Roybal adds that the boa constrictor, and possibly the reticulated and Burmese pythons, recently started breeding in the forests of Puerto Rico, where officials has already removed more than 100 boas. “This is not just a Florida problem,” he says.

A small bird with dark feathers and a light yellow chest, perched on a person's hand

Photo: Guam DOA
The last wild Guam flycatcher was seen in 1985; the bird is extinct due to the brown tree snake.

Unimpressed with the threat of invasive snakes? Then check out Guam, where the brown tree snake has eliminated 10 native birds since arriving in about 1950. As many as 13,000 brown tree snakes occupy a square mile on the island.

After the brown tree snake invaded the Pacific island of Guam, 10 of 12 native birds disappeared.

If the brown tree snake can be eradicated, it would be logical to restore native birds from other islands, but Rodda says most of Guam’s suitable habitat is owned by the U.S. military, which is unreceptive to the idea.

This venomous snake does not shy away from housing, so it also poses a safety threat.

Brown tree snakes have since hitchhiked from Guam to several Pacific islands, yet there are finally signs of progress in controlling the repellent reptile, says Gordon Rodda, brown tree snake guru for the U.S. Geological Survey. To halt further invasions, Rodda says, “The first and most intense effort was getting them away from the port and airport; that’s been very successful.”

The next step, Rodda adds, is to “expand the defensive perimeter and clear the whole island” with poison bait. Because Guam has no native snakes, “We can put out bait and only the brown tree snake will eat it. In Florida, they have a large number of native snakes and … you can’t go and poison them willy nilly.”

Pythons are no pansies, says an invasive species expert: “If the alligator was sufficient to control the Burmese python, it would not have spread.”

When it comes to invasive species, and especially snakes, experts agree that prevention trumps eradication. The Burmese python is extremely difficult to find, as it “lives in the middle of the Everglades, the largest wilderness east of the Mississippi River, and in national wildlife refuges,” says Frank Mazzotti, an associate professor wildlife ecology at the University of Florida.

Man standing and smiling in forest, snake twisted around arms, open cage on ground next to him

Photo by R. Rozar, USGS
A biologist removes a trapped Burmese python on Key Largo. How many evaded the traps?

“People don’t really appreciate how hard it is to catch a Burmese python,” says Mazzotti, an expert on snakes in South Florida. In studies of other snakes, he adds, “when people knew the snake population and had well-developed methods for finding and catching them, if they catch 10 percent of them, they feel extremely lucky. It’s not unusual to catch more than 50 percent of a mammal population.”

In an effort to contain the invasion in Florida, federal agents have already trapped a Burmese python on Key Largo, the first major link in the chain of fragile islands called the Florida Keys.

Snakes in general are camouflage experts, says Art Roybal of the Fish and Wildlife Service. “We are told by scientists that for every one snake that you do see, there are 1,000 that you don’t see. … Large constrictor snakes are notoriously cryptic and often hidden and immobile and are therefore difficult to detect.”

“You have to understand the difficulty of finding an animal that looks like a vine that is spread over 10,000 square kilometers of woody swamps,” adds Gordon Rodda of the U.S. Geological Survey. “There is no track record of success in controlling invasive reptiles and snakes. There are reasons to be pessimistic that the Florida population [of Burmese pythons] will ever be controlled.”


 Looking up at freeway billboard, brown python and message, website to not let pet snakes loose

To slow the invasions, agencies in South Florida are imploring pet owners to find a responsible way to get rid of their pets. Florida now stages periodic “amnesty days” where owners can safely dispose of unwanted snakes.

Along with many other biologists, Rodda, who heads the USGS response to the brown tree snake on Guam, says, “Prevention is way better than fighting the crisis once it’s occurred. Once the snake is established, your chances of eradication are virtually gone.”

Meager options for control and eradication

When the USGS looked at predators and diseases for controlling snakes, it saw no good options. Predators are futile against snakes that can fight back against the big teeth: Burmese python can eat leopards and panthers, and Mazzotti says a captured 16-foot Burmese python threw up a 6-foot, 30-pound alligator. “‘Wow!’ doesn’t even do it justice. It was amazing!”

In these struggles, “Probably more often than not, the alligator win, but that doesn’t mean their numbers will be significantly be depressed by alligators,” says Rodda. The death-tangle between a giant snake and a big toothy gator may be eye candy for TV, “but if a small alligator gets pulled from the water at night and swallowed, you are never going to see that. If the alligator was sufficient to control the Burmese python, the snake would not have spread.”


ark grey alligator and dark brown snake fight in grass and water, snake clutched in gator's mouth

Photo: USGS
An American alligator wrestles for keeps with a Burmese python in Everglades National Park. Although the gator seems to have the upper hand (upper fang?), pythons are powerful enough to eat gators.

When experts examined 57 potential predators to control the brown tree snake on Guam, none were deemed acceptable. Indeed, introduced predators are not much good for controlling invasive animals, according to the USGS:

“The record of successes is small, and the number of catastrophic failures (non-target species decimated or even driven extinct by the predator) is large. As a result, it is against U.S. Fish and Wildlife Service policy to introduce predator-based biocontrol agents for vertebrates (there is also a question as to which higher order predator would qualify for preying on giant constrictors!), and we will not entertain that notion further.”

python on table

Photo: USGS
Researchers implant a radio transmitter in a 16-foot, 155-pound female Burmese python at Everglades National Park. Radio-tracking shows the snake’s movement, helping guide control efforts.

Could bugs or bounties work?

Pathogens – viruses, bacteria or fungi – could theoretically be used if they were specific enough to target the invasive snakes, but Rodda says that the knowledge of snake diseases “is all about snakes in captivity. Nothing is known about transmission of disease in the wild.”

Considerable time and money would be needed to research pathogen control against the giant constrictors, according to the USGS.

The vertebrate immune system can adapt to many new pathogens, and viruses sometimes evolve to become less lethal. When Australian authorities tried to eliminate a massive outbreak of introduced rabbits with a virus, the virus worked at first, and then the rabbits returned to running rampant.

Bounties were used to eradicate wolves from most of the lower 48 states, but bounties can boomerang if hunters breed the target animals in captivity or distribute them to new habitat. On the Caribbean island of St. Lucia, for example, bounty hunters began breeding the fer-de-lance, a deadly invasive snake from Africa, that was targeted for elimination. “Once you start assigning a value to invasive animals, there is an impetus to some people to keep them out there,” says Robert Reed, a USGS invasive species scientist and herpetologist, “And there is no funding that would allow us to hire people to catch snakes.”

Burmese Python trap in Key Largo

Photo by R. Rozar, USGS
This trap for Burmese pythons, in use at Key Largo, is about 2 meters long and has entrance funnels at each end; the live rat that serves as bait is protected by a smaller cage from the snake.

The trapping option

Several projects have tested whether Burmese pythons in Florida can be trapped without harming native snakes and wildlife. Traps can either attract snakes with bait, or be located along a fence that intercepts the snakes and ushers them into a trap.

Reed says both types of traps have caught Burmese pythons in South Florida, but trapping experiments are at an early stage, and the ideal trap probably depends on the context. For example, some former farmland on Florida’s mainland has so many rats that “attractants may not work well because the snakes don’t have to hunt around for food.”

Although it’s too early to know how much trapping will cost, they are not a magic bullet. “Traps alone will not be sufficient to eradicate the Burmese python from the Everglades,” Reed says. “It might be a great option if we are trying to locally control snakes around an area with high ecological value, such as a wood stork rookery,” where reducing the snakes without eliminating them might allow the birds to survive.

A changing ecosystem?

As Burmese pythons spread through South Florida, it’s too early to say for sure how they are changing the Everglades, an ecosystem that has already sustained massive invasions from other animals and plants, not to mention numerous disturbances to natural water flow.

Some rare species have already been found inside snakes, but biologists are also noticing that some common species are becoming scarce. “In the 1970s, ’80s and ’90s, when I used drive the main [Everglades National] park road, I would see marsh rabbits maybe every 50 meters,” says Mazzotti. “Now I don’t see them anymore, zero; and we are finding fewer in the stomachs of pythons. We find lots of marsh rabbits in areas without pythons. That’s a correlation that does not imply cause and effect, but these are the kind of impacts you would expect.”

As the spotlight turns to the nine giant constrictors, Reed says invasive reptiles are always tough to control. A recent scientific review, he notes, “saw no evidence that an introduced reptile population had every been intentionally eradicated. These are cryptic animals and by the time you notice them, the odds are pretty decent that they are already established.”

– David Tenenbaum


Terry Devitt, editor; Steve Furay, project assistant; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive


  1. Proposal to ban import and transport of giant constrictors.
  2. Snake invasions around the world.
  3. Snakes of the Everglades agricultural area.
  4. Florida’s non-native species.
  5. Threats to Florida’s biodiversity.
  6. Risks of giant invasive snakes in the U.S.
  7. Burmese Pythons in South Florida: scientific support for invasive species management.
  8. Giant Constrictors: biological and management profiles for nine large species.
  9. Python snakes could spread to one third of U.S.
  10. Burmese python distribution map.
  11. National Reptile Improvement Plan
  12. Global Invasive Species Program
  13. Database of international invasive species.
  14. Brown tree snake invades Guam.