Nine giant invasive snake species threaten U.S. ecosystems, study finds

Five giant non-native snake species would pose high risks to the health of ecosystems in the United States should they become established in the country, the U.S. Geological Survey (USGS) said today.

A 300-page report prepared by the agency details the risks of nine non-native boa, anaconda and python species that are invasive or potentially invasive in the U.S.

Burmese-python-picture 11.jpg

Burmese python (Python molurus), a Giant Constrictor Snake

Photo courtesy of Roy Wood, National Park Service

“Because all nine species share characteristics associated with greater risks, none was found to be a low ecological risk,” USGS said in a statement released with the report.

Two of the giant snake species are documented as reproducing in the wild in South Florida, with population estimates for Burmese pythons in the tens of thousands, the agency noted.


A Burmese python peeks over the head of an alligator that holds the python’s body in its mouth in Everglades National Park.

Photo courtesy of Lori Oberhofer, National Park Service

Small risk to people

Based on the biology and known natural history of the giant constrictors, individuals of some species may also pose a small risk to people, although most snakes would not be large enough to consider a person as suitable prey, USGS added.

“Mature individuals of the largest species—Burmese, reticulated, and northern and southern African pythons—have been documented as attacking and killing people in the wild in their native range, though such unprovoked attacks appear to be quite rare,” the report authors wrote. The snake most associated with unprovoked human fatalities in the wild is the reticulated python.

“The situation with human risk is similar to that experienced with alligators: attacks in the wild are improbable but possible.”

“The situation with human risk is similar to that experienced with alligators: attacks in the wild are improbable but possible.”

“This report clearly reveals that these giant snakes threaten to destabilize some of our most precious ecosystems and parks, primarily through predation on vulnerable native species,” said Robert Reed, a coauthor of the report and a USGS invasive species scientist and herpetologist.


Boa Constrictor (Boa constrictor) at a county park in southeastern Miami.

Photo courtesy of Mike Rochford, University of Florida.

USGS sorted the giant snakes into high-risk and medium-risk categories.

“High-risk species are Burmese pythons, northern and southern African pythons, boa constrictors, and yellow anacondas. High-risk species, if established in this country, put larger portions of the U.S. mainland at risk, constitute a greater ecological threat, or are more common in trade and commerce.

“Medium-risk species were reticulated python, DeSchauensee’s anaconda, green anaconda, and Beni anaconda. These species constitute lesser threats in these areas, but still are potentially serious threats.

“Because all nine species share characteristics associated with greater risks, none was found to be low-risk.”


A female Burmese python on her nest with eggs. Photo by Jemeema Carrigan, University of Florida. Courtesy of Skip Snow, National Park Service

The USGS scientists who authored the report emphasized that native U.S. birds, mammals, and reptiles in areas of potential invasion have never had to deal with huge predatory snakes before, the agency said in its statement.

“Individuals of the largest three species reach lengths of more than 20 feet and upwards of 200 pounds. The reticulated python is the world’s longest snake, and the green anaconda is the heaviest snake. Both species have been found in the wild in South Florida, although breeding populations are not yet confirmed for either.

“Breeding populations have been confirmed in South Florida for Burmese pythons and the boa constrictor, and there is strong evidence that the northern African python may have a breeding population in the wild as well.”


Skip Snow (National Park Service) measures the length of a captured Burmese python at the South Florida Research Center, Everglades National Park.

Photo courtesy of Lori Oberhofer, NPS

“Compounding their risk to native species and ecosystems is that these snakes mature early, produce large numbers of offspring, travel long distances, and have broad diets that allow them to eat most native birds and mammals,” said Gordon Rodda, a USGS scientist at the Fort Collins Science Center and the other coauthor of the report.

“Boa constrictors and northern African pythons…already live wild in the Miami metropolitan area.”

In addition, he said, most of these snakes can inhabit a variety of habitats and are quite tolerant of urban or suburban areas. Boa constrictors and northern African pythons, for example, already live wild in the Miami metropolitan area.


This map from the USGS report suggests how much of the United States has a climate suitable (green area) for the establishment of the Burmese python. 

Eradication is difficult

The report notes that there are no control tools yet that seem adequate for eradicating an established population of giant snakes once they have spread over a large area. “Making the task of eradication more difficult is that in the wild these snakes are extremely difficult to find since their camouflaged coloration enables them to blend in well with their surroundings,” USGS added.


Yellow anaconda (Eunectes notaeus) specimen captured at Big Cypress National Preserve. Photo courtesy of Skip Snow, National Park Service.

The lesson of Guam

“We have a cautionary tale with the American island of Guam and the brown treesnake,” said Reed. “Within 40 years of its arrival, this invasive snake has decimated the island’s native wildlife–10 of Guam’s 12 native forest birds, one of its two bat species, and about half of its native lizards are gone. The python introduction to Florida is so recent that the tally of ecological damage cannot yet be made.”

The researchers used the best available science to forecast areas of the country most at risk of invasion by these giant snakes, USGS said.


The USGS map shows where in the U.S. the climate is suitable for establishment of the southern African python.

“Based on climate alone, many of the species would be limited to the warmest areas of the United States, including parts of Florida, extreme south Texas, Hawaii, and America’s tropical islands, such as Puerto Rico, Guam, and other Pacific islands,” the agency said.

invasive-snakes-report-cover.jpg“For a few species, however, larger areas of the continental United States appear to exhibit suitable climatic conditions. For example, much of the southern U.S. climatic conditions are similar to those experienced by the Burmese python in its native range.

However, many factors other than climate alone can influence whether a species can establish a population in a particular location,” the report notes.

The Fish and Wildlife Service and the National Park Service will use the report to assist in further development of management actions concerning the snakes when and where these species appear in the wild.

In addition, the risk assessment will provide current, science-based information for management authorities to evaluate prospective regulations that might prevent further colonization of the U.S. by these snakes, USGS said.

Scientific names of the invasive giant snakes

Indian or Burmese python (Python molurus)

Northern African python (Python sebae)

Southern African python (Python natalensis)

Reticulated python (Python [or Broghammerus] reticulatus)

Boa constrictor (Boa constrictor)

Green anaconda (Eunectes murinus)

Yellow anaconda (Eunectes notaeus)

Beni or Bolivian anaconda (Eunectes beniensis)

De Schauensee’s anaconda (Eunectes deschauenseei)

Giant Constrictor Risk Assessment:

Frequently Asked Questions (USGS)

Forty years in U.S., UK, and South African media gives David Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. He has 120,000 followers on social media. David Braun edits the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directs the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. Follow David on Facebook  Twitter  LinkedIn
  • dangles

    That report is FRAUGHT with errors, misconceptions and downright lies. The authors have taken excessive liberties and have estimated the potential span of these large constrictors WAY WAY WAY outside the realistic range.
    Boa constrictors have been household pets for DECADES and have only established insignificant breeding populations in one or two tiny spots in Florida. This is also true of the Burms.
    Why is it that the Boa lives (natively) within a couple hundred miles of the Arizona/Mexico border, BUT HAVE NOT IN THOUSANDS OF YEARS expanded ANY farther north? And we’re supposed to believe now that a couple irresponsible snake owners have released their snakes that they are all of a sudden a “high risk”?!
    And why haven’t the Burms expanded ANY farther north than the Everglades? Maybe it’s because THEY CAN’T. This report only used TWO factors to determine “likely” suitable habitat. There are too many other factors to consider when determining this. Another (more complete) study used 19 factors, and found they are unlikely to spread any farther north than southern Florida! Why haven’t you done an article on THAT? Maybe because it doesn’t SCARE people like this one does…
    This is INCREDIBLY irresponsible and will put THOUSANDS of people out of jobs. AND IT’S ALL LIES.

  • dangles

    and of course NOBODY will question this because “it’s national geographic” or “it’s the USGS”
    “they don’t have agendas.”

  • Dirty Boas

    Here’s the report Dangles is talking about.
    Pyron et al. Claims of Potential Expansion Throughout the U.S. by Invasive Python Species Are Contradicted by Ecological Niche Models. PLoS ONE, 2008; 3 (8): e2931 DOI: 10.1371/journal.pone.0002931
    It was written by two CUNY (City University New York) graduate students and Dr. Frank Burbrink. The study uses 19 variables to map potential US habitat for the Burmese Python. USGS survey only uses two.
    “By using more complete climate data, in this case 19 variables measuring climatic extremes, averages and seasonal variation, we can make more accurate predictions of species distributions,” said Alex Pyron. “Combining this climatic data with localities for the Burmese python allows us to create powerful models for predicting suitable habitat for the snakes.”

  • MLynch

    Why wasn’t this an issue 20 years ago? Or even 10 years ago? The answer: because it never was an issue. The reptiles that the USGS seeks to eliminate from the pet trade have been residing in households across the United States for decades and have never established a foot hold in any of the projected ecosystems, with the exception of only a couple of very small pockets. If the scenario put forth by the USGS were at all accurate residents of Washington DC; Richmond, VA; Bethesda, MD, Lawton, OK; Phoenix, AZ; and San Bernardino, CA would already be facing an ecological disaster as a result of these “invasive species.” …yet, oddly they are not.
    This report is based more on media spurred paranoia, ignorance, and prejudice than actual scientific fact. …And at what time did National Geographic stop caring about research? Had NatGeo actually looked at the situation in the Florida Everglades, studied it, as well as studied the 9 snakes the USGS is targeting the USGS report would have been found to be severely flawed.
    And something the USGS report fails to address is how eliminating these snakes from the pet trade will have any benefit to the situation in the Everglades. Why? Because it won’t. The problems faced in the Everglades will remain. The USGS has also stated that these snakes are moving north at a rate of about 1.5 miles a day. Really? Well if that’s true then eliminating these snakes from the pet trade will do nothing to stop this northerly migration. However, I’d like to point out that if these snakes were moving north at such a rate we would have seen them well north of the Everglades long ago… and yet we haven’t. So how is banning the 9 large constrictors an environmental issue? It’s not because it will have no impact, what so ever, on any environmental issue we may be facing in southern Florida.
    I’d also like to point out how much the USGS report resembles the so-called “studies” of Africanized Honey Bees back in the 1970’s. They were also media fueled paranoia. We were told that these bees posed a serious risk to the environment and personal well being given their aggressive nature. We were also told that they were heading north and making headway every year and that by the early 1980’s they’d be as far north as New York City… I live in Northern New Jersey, and I’m still waiting for the “Killer Bees” to arrive. Could it be that these “reports” were false and filled with little clinical study?
    If reports are going to be made about the issue of non-native species thriving in the Florida Everglades then let’s focus on facts and actual studies of the animals present in the Everglades, and how to address the issues. Instead of making sweeping theories based on paranoia and ignorance.
    – Mike Lynch

  • Daniel

    PRESS RELEASE November 24, 2009, 5 AM EST
    Scientists Characterize Justification for Congressional Python Ban as “Unscientific”
    November 24, 2009, Wilmington, NC- In a letter to the U.S. House Committee on the Judiciary an independent group of scientists today characterized a United States Geological Survey (USGS) report being touted as the justification for a ban on import and trade in pythons as “unscientific”.
    The independent group of scientists and herpetologists, including professors from the University of Florida, Arizona State, and Texas A&M among others penned members of Congress in response to comments made by the U.S. Fish & Wildlife Service (USFWS) during a November 6th hearing on H.R. 2811, a bill that could determine the fate of much of the reptile trade in the United States. During that hearing USFWS Deputy Director Dan Ashe characterized the USGS report as “peer-reviewed science”, a claim that struck a nerve within the scientific community.
    “It is a misrepresentation to call the USGS document ‘scientific’” stated the scientists. “As written, this [USGS] document is not suitable as the basis for legislative or regulatory policies, as its content is not based on best science practices, it has not undergone external peer-review, and it diverts attention away from the primary concern. We encourage the USFWS and USGS to submit this document to an independent body for proper and legitimate peer review. Additionally, we encourage the Committee to review this document, not as an authoritative scientific publication, but rather as a report currently drafted to support a predetermined policy”.
    H.R. 2811, Introduced by U.S. Representative Kendrick Meek (D-FL), who recently announced his candidacy for the U.S. Senate, could add all pythons, and even boas, to the Injurious Wildlife list of the Lacey Act; a designation reserved for only the most dangerous alien invaders to our natural ecosystem. Such a move would prevent all import, export, and interstate transport of pythons in the U.S. The scientific justification for such a move hinges on a recently published report of the United States Geological Survey (USGS) entitled ‘Risk Assessment of Nine Large Constricting Snakes’, which attempts to paints a picture of large constrictor snakes as an immediate threat to eco-systems over much of the U.S.
    Source: United States Association of Reptile Keepers (USARK)
    Contact: Andrew Wyatt president@usark.org
    Letter To Congress:
    24 November 2009
    U.S. House of Representatives Committee on the Judiciary
    The Subcommittee on Crime, Terrorism & Homeland Security
    2138 Rayburn House Office Building
    Washington, DC 20515
    Dear Chairman Bobby Scott and Ranking Member Louie Gohmert:
    We write in regard to the recent Congressional hearing on HR 2811. As scientists who have worked with reptiles including those cited in HR2811, we express our reservations regarding the document recently released by USGS as an “Open-Report”, titled Giant Constrictors: Biological and Management Profiles and an Establishment Risk Assessment for Nine Large Species of Pythons, Anacondas, and the Boa Constrictor.
    Simply put, this report is not a bona-fide “scientific” paper that has gone through external peer review. Part of this report is fact-driven, described by the authors as “traditional library scholarship.” By the authors’ admissions, there are surprisingly little data available regarding the natural history of these species. In their attempt to compile as much information as possible, the authors draw from a wide variety of references, ranging from articles published in peer-reviewed professional journals to far less authoritative hobbyist sources, including popular magazines, the internet, pet industry publications, and even various media sources. While such an approach is inclusive, it tends to include information that is unsubstantiated and, in some cases, contradicts sound existing data.
    As scientists whose careers are focused around publishing in peer-reviewed journals and providing expert reviews of papers submitted to these journals, we feel it is a misrepresentation to call the USGS document “scientific”. In fact, much of this report is based on an unproven risk assessment model that produces results that contradict the findings presented in a recently published scientific paper that used a more complex and superior model (see: Pyron R.A., F.T. Burbrink, and T.J. Guiher. 2008. Claims of Potential Expansion throughout the U.S. by Invasive Python Species Are Contradicted by Ecological Niche Models, PLoS One 3: e2931. doi:10.1371/journal.pone.0002931). Unfortunately, the authors of the USGS document limit their reference to this scientific work to an unsubstantiated criticism. To the contrary, this alternate model is validated by its relatively accurate prediction of the natural distribution of the species in question (something the USGS model does not even attempt). Furthermore, despite its conclusion of a limited potential distribution of Burmese pythons in the United States, the model presented by Pyron et al. accurately predicts the presence of Burmese pythons in the Everglades.
    The USGS model likely provides a gross overestimate of potential habitat for these snake species. People throughout the United States keep pythons as pets, yet the only known breeding populations in the United States are in the Everglades. Such a wide distribution of potential sources of invasion, but only a localized invasive event, suggests that factors beyond those used in the USGS model are critical to limiting the suitability of habitat for pythons. The authors even state that climate is only one factor of several that affect the distribution of an animal, yet they develop a model that only uses overly simplistic climatic data (e.g., the climatic data did not take seasonality into consideration).
    We are further concerned by the pervasive bias throughout this report. There is an obvious effort to emphasize the size, fecundity and dangers posed by each species; no chance is missed to speculate on negative scenarios. The report appears designed to promote the tenuous concept that invasive giant snakes are a national threat. However, throughout the report there is a preponderance of grammatical qualifiers that serve to weaken many, if not most, statements that are made.
    We fully recognize the serious concerns associated with the presence of persistent python populations in southern Florida. As top predators, these animals can and will have a dramatic impact on the community of wildlife that lives in the Everglades. Inaccurately extending this threat to a much large geographic area is not only inappropriate, but likely takes needed focus away from the real problem in the Everglades.
    In conclusion, as written, this document is not suitable as the basis for legislative or regulatory policies, as its content is not based on best science practices, it has not gone through external peer-review, and it diverts attention away from the primary concern. We encourage the USFWS and USGS to submit this document to an independent body for proper and legitimate peer review. Additionally, we encourage the Committee to review this document, not as an authoritative scientific publication, but rather as a report currently drafted to support a predetermined policy.
    Elliott Jacobson, MS, DVM, PhD, Dipl. ACZM
    Professor of Zoological Medicine
    University of Florida
    Dale DeNardo, DVM, PhD
    Associate Professor School of Life Sciences
    Arizona State University
    Paul M. Gibbons, DVM, MS, Dipl. ABVP (Avian)
    President-Elect, Association of Reptilian and Amphibian Veterinarians
    Interim Regent, Reptiles & Amphibians, American Board of Veterinary Practitioners
    Director, Exotic Species Specialty Service
    Animal Emergency Center and Specialty Services
    Chris Griffin, DVM, Dipl. ABVP (Avian)
    President, Association of Reptilian and Amphibian Veterinarians
    Owner and Medical Director
    Griffin Avian and Exotic Veterinary Hospital
    Brady Barr, PhD
    Resident Herpetologist
    National Geographic Society
    Endangered Species Coalition of the Council of State Governments
    Crocodilian Specialist Group
    Warren Booth, PhD
    Invasive Species Biologist
    Research Associate
    North Carolina State University
    Director of Science
    United States Association of Reptile Keepers
    Ray E. Ashton, Jr.
    Ashton Biodiversity Research & Preservation Institute
    Robert Herrington, PhD
    Professor of Biology
    Georgia Southwestern State University
    Douglas L. Hotle
    Curator of Herpetology/Conservation/Research
    Natural Toxins Research Center
    Texas A&M University
    Francis L. Rose (Retired) , B.S., M.S. (Zoology), PhD (Zoology)
    Professor Emeritus
    Texas State University
    Edward J. Wozniak DVM, PhD
    Regional Veterinarian
    Zoonosis Control Division
    Texas Department of State Health Services
    CC: Secretary Kenneth Salazar, US Dept of the Interior; Director Marcia McNutt, US Geological Survey; Director Sam Hamilton, US Fish & Wildlife Service

  • michael

    I saw a black snake on my dirt road it was about eight feet long and really fat . I just thought someone would like to know . it’s right on the south Georgia and North Florida border beside interstate 75

  • LisainPhilly


    Sounds to me like YOU are the one with the agenda! All you care about are the thousands of jobs that will be lost if the US inforces legislation that currently bans “unrestricted” trade of nonnative snakes into the US (like pythons, for example).

    If you bothered to read the article at all–and apparently you didn’t–USGS and the NPS have already found nonnative snakes in the Everglades and in other swamp-like areas. They will breed in an environment that is suitable, so, YES, they are a threat to the US ecosystem if their numbers go unchecked.

    But I guess you value the almighty dollar more than the fact that these species are brought into this country “illegally” and the possible negative repercussions to our fragile ecosystem!

    Shame on you!

  • Ariana Smith

    The reason pythons and annacondas pose a threat is not that they have never been in the US it is that there population has increased so much that there eating habits and numbers are decreasing native wildlife. When native wildlife becomes in short supply other animals eat the same thing and unlike pythons some animals have a specific diet. Pythons are not going after large mammals like deer, dogs, pigs and other animals some of which we eat. The Everglades are unique. Marsh land is not evverywhere and certain species live there. Its like saying “Gee the Nornern Snakehead is just a fish, I dont see why its a big deal” When a dominate predatory species enters and ecosystem it changes ho wthe food chain works which affects every species below it.

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