See the full report at: Giant Constrictors: Biological and Management Profiles and an Establishment Risk Assessment for Nine Large Species of Pythons, Anacondas, and the Boa Constrictor
For U.S. Fish Wildlife Service information on giant invasive snakes, visit:
South Florida Ecological Services Office, Activity Highlights: Invasive Species Issues—The Invasive Burmese Python in South Florida
Q: Why did you do the study that resulted in this report?
USGS research participation was requested by the U.S. Fish and Wildlife Service and National Park Service, because those agencies were faced with a number of complex questions about how they should respond to the presence of giant constrictors in the U.S. and the prospect of additional giant constrictor species in the wild. These agencies particularly wanted USGS to address risks to wildlife, ecosystems, and human safety. This report followed USGS's best scientific practices, including stringent peer-review. For more scientific and statistical details, please see Chapter 2 of the Risk Assessment report.
Q: What will the Risk Assessment report be used for?
The Risk Assessment will be used to assist resource management agencies in developing management actions concerning the snakes when and where these species appear in the wild. It also provides up-to-date and authoritative information for use in evaluating prospective regulations that might prevent further colonization of the U.S. by these snakes.
Q: How risky are the nine giant constrictor species you examined? And which ones pose the most serious threats?
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.
Q: Why were there no snakes in the low-risk category?
The nine large snakes studied in the Risk Assessment were found to share several traits that increase their risk of establishment, increase the damage they might do, or make eradication difficult. Specifically, these snakes:
Had they not possessed these features, they might have constituted a low risk.
Q. Now that the report is done, what is the next step for USGS?
The authors of this work are researchers whose responsibility is to apply the best science to key ecological management problems of our time. Research into invasive reptile species can take many forms. For example, the authors are well aware that many aspects of risk assessment are not as refined as one would like. The authors are therefore studying a variety of ways of improving the science of invasive species risk assessment. They are also studying ways of improving the control of invasive snake populations, such as the Burmese python, boa constrictor, and northern African python in Florida, three species of watersnake in California, and the brown treesnake in Guam. However, this report will also be used by other agencies, such as the U.S. Fish and Wildlife Service, to consider educational and regulatory initiatives that might be taken to keep the U.S. safe from harm by invasive species.
Q: What does “giant” mean and why did you only look at giant snakes?
The selection of giant constrictor species was based on concern over the size of the potential invaders combined with their prevalence in international trade. We include the four largest species of snake, as well as similar and closely related species, and the boa constrictor. The four largest snakes are the true “giants,” with maximum lengths well exceeding 20 feet or 6 meters: northern African python (Python sebae), Indian python (Python molurus, which includes P. m. bivittatus, the Burmese python), reticulated python (Broghammerus or Python reticulatus), and green anaconda (Eunectes murinus). We selected the species not only for their size, but also for their likelihood of establishment. In the known occupied range in Florida, there are records of snakes more than 17 ft long.
The boa is very large, much more commonly owned, and already established in Florida; thus it is a species of special concern. In addition to the four true giants and the boa constrictor, we included four species that are so similar in appearance to one of the giants that they might be confused with them. In some cases, these “look-alike” species are not even distinguished in the international live animal trade.
We also chose to look at very large snakes because these species have no ecological equivalent among native snake species in the United States. Because of this, they pose a special hazard to native wildlife that has never encountered such huge predators before. They may also present a small safety risk to people. Other non-native snake species may also be of concern, but these will have to be addressed in future studies.
Q: Are any of these giant snakes in the wild? How many?
An invasive population of the Burmese python is now distributed across thousands of square miles of south Florida, while boa constrictors are established south of Miami. Additionally, recent evidence strongly suggests a reproducing population of northern African pythons on the western boundaries of Miami. There is currently no evidence of breeding populations of the various anacondas or the reticulated python, although representatives of both groups have been captured or sighted in the wild in Florida and elsewhere. Accurate population estimates for any of these species in Florida are difficult to calculate because, despite their size, the snakes are difficult to find in the wild. However, based on the extent of the Burmese python population, experts estimate that a population of tens of thousands now lives in the wild in Florida.
Q: What do these giant snakes eat?
Giant constrictors are generalist feeders on vertebrates, especially birds and mammals. Some lizards, turtles, and crocodilians are also eaten, and some anacondas regularly eat fish.
Q: Is it normal for giant constrictors to feed at night?
Giant constrictors search for prey using vision, chemical receptors, and heat sensors. They can hunt in the day or in the night.
Q: Will these snakes compete in places where they overlap (e.g., African rock pythons against Burmese pythons)? Or do they have different lifestyles? Could they potentially hybridize?
The northern African python and Burmese python are very closely related to each other (two twigs on the same evolutionary branch). As far as we know, they have the same lifestyle but perhaps slightly different habitat preferences and may have different behaviors that help them regulate their body temperature as well. We do not have enough information to determine how they would interact as species, as they have not previously been found together. There is a possibility that they would hybridize, but the ecological consequences of that possibility would not be predictable with the information presently available.
Q: The report says that quite a few of these snakes pose potentially high risks to wildlife. What does that mean?
The practice of keeping a non-native animal in an area where it might survive in the wild runs the risk that, should the animal escape or be intentionally released, it could survive, reproduce, and establish populations in the wild, as Burmese pythons have in the Florida Everglades. Non-native species can be extremely detrimental to native species, and some non-native species can transmit serious diseases.
Giant constrictors are capable of eating almost every type of land-dwelling vertebrate where they occur, but they are more likely to eat birds and mammals. If a bird or mammal is already rare (perhaps because of habitat loss, or competition or predation from another introduced species), the addition of a novel predator could tip the balance against the native prey species.
For example, the endangered Key Largo woodrat, which occurs naturally only on Key Largo in the Upper Florida Keys, has lost much of its natural habitat to development, and is harmed by competition from introduced black rats, which are now many times more abundant than the native woodrat. When a python reaches Key Largo, it has many black rats to eat, but will take native woodrats when the opportunity arises. It might even prefer to take the relatively defenseless woodrats. Woodrats may be more vulnerable to predation by giant constrictors than non-native black rats because black rats arose in south Asia (in the presence of the pythons), whereas Key Largo woodrats arose on Key Largo, where pythons or large constrictors did not occur. Rats are a primary food for pythons, and if left unchecked, the pythons might become so numerous on Key Largo that the endangered woodrat population would be unable to survive python predation. This same pattern could be replicated for other endangered or at-risk species in the United States.
Q: How could these giant snakes change South Florida’s ecosystems? What about other ecosystems?
The most likely avenue for ecosystem change would be that introduced giant constrictors would change food webs by eliminating or depleting vulnerable native species. If enough species are lost, entire ecosystem processes could be changed. For example, on Guam the introduced brown treesnake has eliminated most native vertebrates (birds, bats, and lizards) that pollinate trees and flowers. Consequently, these native animals are not available to disperse seeds. As a result, some of the native trees have greatly declined in abundance, and may disappear.
Similarly, as fish-eating birds have been lost from Guam, the natural nitrogen transport from aquatic and marine systems to bird rookeries on land has been lost as well. Some Burmese pythons in the Everglades accumulate extremely high levels of mercury in their body tissues, potentially poisoning higher level predators that might eat them, such as alligators and panthers. We do not yet know how a specific system in South Florida would be disrupted by the addition of a novel predator, but from experience with other ecosystems disturbed by introduced snakes we know that serious disruption is a distinct possibility.
Q: Are there other invasive snakes in the country that people should be concerned about? What ones?
Any animal can be problematic when released in places where it is not native. The safest policy is to find an appropriate home for any animal that is no longer wanted. Disposal in the wild can do great environmental harm. For example, the brown treesnake was introduced to the American island of Guam shortly after World War II; it has decimated the native birds, mammals and lizards of Guam, such that only a few small species remain. Fifty years after the snake was introduced, Guam had lost 10 of its 12 native forest birds, most of its bats, and about half of its native lizards. The python introduction to Florida is so recent that the tally of victims cannot yet be made. Similarly, it is too early to determine if the three watersnake species introduced into California (including one species from Florida) will result in any extinctions. Free-ranging snakes representing dozens of species from around the world are discovered in the United States in any given year, usually as a result of escapees or releases from the pet trade, but most of these don’t appear to have established a reproductive population.
Q: USGS generated climate maps that show that other places besides Florida might be at risk. What areas of the U.S. and its territories are most at risk and why?
In terms of the number of giant constrictor species that are climatically matched to a given locality, the areas most at risk are Florida, extreme southern Texas, Hawaii, and America’s tropical islands. However, many other areas with mild winters are potentially vulnerable to colonization by at least one of the giant constrictors. A single giant constrictor species may be sufficient to produce many undesirable effects on the local ecosystems and economies should it become established in an area.
Q: How reliable and accurate are these maps? Does this mean we will get these snakes if we live in, for example, Texas?
The science of invasive species prediction is in its infancy. Many factors besides climate can play a role in determining whether a given species will be successful in a given place. The data tell us that species with wider climatic tolerance are more likely to be successful in a wider variety of American landscapes, but we have too little information to be sure that any given species will or will not survive in a particular place.
Q: Can you tell me how you made these maps?
The basic method was to identify the seasonal range of temperature and rainfall patterns (called the “climate envelope”) that are found in the places where the species is native. We then considered each square kilometer of the United States and asked whether the climate experienced by that American locality was within the climate envelope derived from the species’ native range. If the U.S. area was inside the range of conditions experienced by the snake in its native range, we tentatively registered that square kilometer of the U.S. as suitable for that species of snake. For more scientific and statistical details, please see Chapter 2 of the Risk Assessment report.
Q: Is it possible to eradicate these snakes once they are in an area?
No one has eradicated an invasive snake from any area greater than a few acres through purposeful control measures. Large-scale efforts are just recently underway for these particular giant constrictor species. Scientists familiar with snake-eradication techniques suggest that if eradication is to be successful, it should be conducted when the snake population is still very localized, preferably occurring over no more than a few acres in size. Unfortunately, snake populations are rarely detected until they have spread over a much larger area, such as has occurred with the Burmese python in Florida. Ongoing research may provide new tools that lead to eradication success. In the meantime, agencies such as the National Park Service and others are actively testing and applying control and eradication techniques.
Q: What should at-risk areas of the country do? Is there anything they can do in advance?
Scientists believe that prevention is more effective than a cure in the case of non-native reptiles. Avoiding release or escape of pets is a crucial element for ensuring that non-native species do not occur in the wild and therefore cannot colonize. Everyone can do their part to ensure that non-native species are not released into the wild.
Q. What about the Burmese pythons? Is control working for them?
Since no tools have been refined and implemented for control of Burmese pythons, it is too early to determine whether some of the tools undergoing development and testing may ultimately prove successful.
Q: Which of these snakes is the largest and potentially the most dangerous to people?
None of these snakes pose more than minimal risk to human safety. Human fatalities from non-venomous snakes in the wild are very rare, probably only a few per year worldwide. However, though attacks on people are improbable, they are remotely possible given the large size that some individual snakes can reach. The reticulated python is the world’s longest snake (adult females may exceed 26 feet); the green anaconda is the world’s heaviest snake (upwards of 250 pounds). The snake most associated with unprovoked human fatalities in the wild is the reticulated python, though unprovoked fatalities are known for Burmese and both African pythons as well. Known fatalities caused by these snakes in the wild, though rare, have occurred in the snakes’ native ranges.
Although the green anaconda is the heaviest snake, there are no well-documented unprovoked fatal attacks by green anacondas on humans. Furthermore, it is not a widely held pet, and it prefers tropical bodies of fresh water that are relatively uncommon in the United States. On the other hand, some of the smaller species (e.g., yellow anacondas) that are extremely unlikely to attack humans are more likely to find suitable habitat in the United States; therefore, yellow anacondas constituted a higher ecological risk.
All known fatalities in the United States are from captive snakes, typically while a snake owner is consciously interacting with the animal in some way. The simplest and surest way to reduce the remote risk of human fatalities is to avoid interacting with a giant constrictor. Overall, the risk of attack is miniscule, but because many suburban areas, backyards, and similar areas in Florida include ponds, canals, or other bodies of water where giant snakes would likely feel at home, the situation is similar to that experienced with alligators: attacks are highly improbable but possible in any locality where the animals are present.
Q: This new species, the northern African python (also called the African rock python), sounds particularly dangerous. Is it?
The African pythons are often described as exhibiting unpredictable defensive behavior in captivity, but there’s no information to suggest that free-ranging African pythons are more or less likely to attack humans than are Burmese pythons. The situation is somewhat similar to that occurring in human-alligator interactions. There are rare fatalities recorded, but it is often difficult to determine what precipitated the incidents.
More typical python prey in Florida tends to be rabbits, wading birds, and rodents. We know from press and other accounts that there have been deaths of a few infants, children, adolescents, and adults from pet pythons in this country, but USGS does not track this kind of information. Such attacks are usually defensive and have largely occurred when owners are handling the snakes. A recent exception occurred when a pet python escaped its cage and strangled a 2-year-old girl in the same home. This tragic and unusual incident demonstrates that predatory attacks by Burmese pythons on small people are a possibility, though the rarity of such reports suggests that it is highly unlikely.
Q: I’ve read reports about the recent documenting of the African rock python in Florida and that it is much more aggressive than the Burmese python. What does this mean? Also, can the two snakes interbreed and what would this mean?
Captive northern African pythons (often called African rock pythons) have the reputation of being more prone to biting their owners than Burmese pythons, but this does not necessarily equate to more aggressive behaviors among wild northern African pythons. While there are a few documented attacks on humans in Africa, such attacks are extremely rare as compared to deaths by other wild animals in Africa. In captivity, hybrids between these two species have been observed, but we do not know if the hybrids are fertile or if they are as healthy as their parents. Scientists are concerned about hybridization because it raises the possibility of combinations of traits appearing in the wild that are not now known to exist in nature. Many of the world’s worst invasive plant species are hybrids that possess trait combinations unknown in the parental stock. It is very difficult to predict the effects of hybridization between two species of pythons.
Q: If these snakes are active night and day, and if they can live in relatively urban areas, do you expect they will live in cities, such as in South Florida?
Boa constrictors and northern African pythons live in the Miami metropolitan area. The various python species and boa constrictor are often found living in suburban and urban areas in their native ranges. The anacondas live primarily in large wetland areas in their native range and would likely do the same in urban and non-urban areas of the United States if they colonized the U.S. As with alligators, the risk of human attack in urban areas is very small.
Q: Should parents be worried about these wild constrictors?
Currently, the only place where any of these giant constrictor snakes are known to be wild-living in the U.S. is in South Florida. In the Everglades, there is plenty of native prey available for these opportunistic hunters. Based on studies of their behavior and records from their native range, the chance of one finding its way into your home and harming a child is remote. Wild animals rarely enter houses, though some occasionally do so through pet doors, damaged screens or air conditioners, open windows, or other openings. Thus, a sensible approach would be to take the same precautions for these constrictor snakes as one would take for alligators.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://www.fort.usgs.gov/FLConstrictors/FAQ.asp
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