How Did Burmese Pythons Get to Florida? The Everglades Invasion

A popular pet that outgrew its home

The Burmese python is native to South and Southeast Asia, from northeastern India to southern China and Indonesia. In the late 20th century it became one of the most popular large snakes in the American reptile trade, admired for its pattern and reputation for calm behavior. Federal records show that more than 90,000 Burmese pythons were imported into the United States between 1996 and 2006 alone, and many more were bred in captivity.

The trouble is that a hatchling measuring about 50 cm (20 in) can grow past 3 m (10 ft) within a few years and may live for more than 20 years. Some owners who could no longer house or handle a large snake released it into the wild. Others escaped from homes, dealers’ premises or breeding facilities. Miami was a major hub of the exotic animal trade, and it sits right next to the Everglades.

The role of Hurricane Andrew

On August 24, 1992, Hurricane Andrew, a Category 5 storm, struck the area south of Miami and destroyed buildings that included reptile breeding and holding facilities. It is often said that hundreds of pythons escaped and seeded the Everglades population.

The evidence suggests a more complicated story. Pythons had already been reported in and around Everglades National Park during the 1980s, years before the hurricane. Genetic studies show that the Florida population is quite uniform, which points to a relatively small group of founding snakes rather than one big release. Most scientists therefore think that released and escaped pets started the invasion, and that the hurricane may have contributed snakes without being the sole cause.

From a few snakes to an established population

For years, pythons found in the Everglades were treated as isolated escapees. Sightings grew steadily through the 1990s, and in 2000 biologists confirmed that the snakes were reproducing in the wild. That is the point at which an introduced species becomes an established one.

Several things helped the pythons succeed:

• A suitable climate. South Florida’s subtropical weather is warm enough most of the year, and the wetlands resemble the marshes and grasslands of the python’s native range.
• Abundant food. Raccoons, opossums, rabbits, rats and wading birds had never faced a predator like a giant constrictor and had no instinct to avoid it.
• Big clutches. A large female can lay 50 or more eggs; one Florida python carried a record 122 developing eggs.
• Few enemies. Once pythons pass about 3 m (10 ft), only large alligators pose a real threat.
• Secrecy. The snakes spend most of their lives hidden, so the population grew largely unnoticed.

Genetic work published in 2018 also showed that some Everglades pythons carry DNA from the closely related Indian python, evidence that the two species interbred at some point, most likely in captivity.

How far have pythons spread?

From the southern Everglades, pythons have spread across much of South Florida, including Big Cypress National Preserve, the western suburbs of the Miami area, and lands toward Lake Okeechobee. They have also turned up on Key Largo. How many live there is uncertain: published estimates range from about 30,000 to more than 300,000.

Cold is the main natural brake. Burmese pythons are tropical animals, and a severe freeze in January 2010 killed many of them in some study areas. Climate models disagree about how far north they could eventually survive. Some predict that only southern Florida and the southern tip of Texas are suitable, while others suggest that much of the coastal Southeast could be. Researchers have also found genetic signs that the snakes that survived the 2010 freeze may be better adapted to cold.

What pythons have done to Everglades wildlife

The most striking evidence comes from road surveys at night in Everglades National Park. A study published in 2012 compared counts from 1996 to 1997, before pythons were common, with counts from 2003 to 2011. Sightings of raccoons fell by 99.3 percent, opossums by 98.9 percent and bobcats by about 87 percent, while marsh rabbits and foxes were not seen at all. The exact figures have been debated, but the declines matched the areas where pythons had been present longest.

An experiment confirmed the link. When marsh rabbits fitted with radio collars were released into python territory, pythons accounted for about three-quarters of the rabbit deaths, and the rabbits soon disappeared. Researchers who examined python stomachs have found more than two dozen bird species, including endangered wood storks, as well as deer, endangered Key Largo woodrats and even American alligators.

Pythons have also brought a parasite, a tongue worm called Raillietiella orientalis, which has spread to native snakes. And by removing mid-sized mammals, they change how the whole food web works. Mosquitoes, for example, now feed more often on rodents, which can affect the spread of some viruses.

Can pythons be stopped?

The python is now considered one of the most serious invasive species in the United States. In 2012, the U.S. Fish and Wildlife Service listed it as injurious under the Lacey Act, which banned imports and movement across state lines. Florida restricts private ownership, and state and federal agencies run long-term research and removal programs. Thousands of pythons have been taken out of the wild, but no one expects the population to be eliminated soon.

The Everglades story is part of a bigger pattern. Florida has more established non-native reptiles and amphibians than almost anywhere in the world, including cane toads and green iguanas. See our guide to snakes in Florida for the native species that share the state with the pythons.

Myths and facts

Myth: all the pythons came from one hurricane. Hurricane Andrew may have contributed, but pythons were already in the Everglades before 1992.

Myth: pythons regularly attack people in Florida. There is no confirmed case of a wild python killing a person in the state. The damage pythons do is to wildlife.

Fact: the species is threatened at home. The Burmese python is listed as Vulnerable in Asia, where its numbers are falling, even as it thrives as an invader in Florida.