Q. Where was this study published and who are the authors? What kind of peer-review did this study receive?
The paper, Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park, was published online on Jan. 30, 2012, in the Proceedings of the National Academy of Sciences; the paper can be accessed here. The authors are Michael E. Dorcas, Davidson College; John D. Willson, Virginia Tech University; Robert N. Reed, U.S. Geological Survey; Ray W. Snow, National Park Service; Michael R. Rochford, University of Florida; Melissa A. Miller, Auburn University; Walter E. Meshaka, Jr., State Museum of Pennsylvania; Paul T. Andreadis, Denison University; Frank J. Mazzotti, University of Florida; Christina M. Romagosa, Auburn University; and Kristen M. Hart, USGS.
Both the USGS and the journal, PNAS, require independent scientific review for every publication, and this study met those standards. In the USGS, peer reviewers are selected from experts with no organizational or close professional ties to the authors. Standards require a minimum of two reviews, after which the USGS authors must provide a detailed response to each review that explicitly states how each comment on the document was addressed. Adequacy of the authors’ responses to reviews is assessed by both research managers and independent scientists within the USGS. PNAS also requires independent peer review from experts with no organizational or close professional ties to the authors before it publishes scientific studies in the journal. PNAS is one of the most respected peer-reviewed scientific journals in the world.
Q. What are the principal findings of this research?
Precipitous declines in formerly common mammals in Everglades National Park have been linked to the presence of invasive Burmese pythons, according to this study.
The study, the first to the first to the document the ecological impacts of this invasive species, strongly supports that animal communities in this 1.5-million-acre park have been markedly altered by the introduction of pythons within the 11 years since the snake was recognized as an established invasive species. Mid-sized mammals are the most dramatically affected.
Q. How did the researchers conduct the study?
The researchers collected their information via repeated systematic night-time road surveys within the park and in similar habitats to the north, counting both live and road-killed animals. Over the period of the study, researchers traveled a total of nearly 39,000 miles from 2003 to 2011 and compared their findings with similar surveys conducted in 1996 and 1997 along the same roadways before pythons were recognized as established in Everglades National Park.
The authors supplemented their road-survey data with records of road-killed animals maintained by Everglades rangers and with personal anecdotes collected from individuals with a long history of visiting or working in the park.
Q. What species were most severely affected?
The study revealed drastic declines in the number of mid-sized mammals seen on the roads in Everglades National Park – for several species, the decline exceeded 95 percent. The species most dramatically affected are raccoons, opossums, marsh rabbits, foxes and bobcats.
The most severe declines, including a nearly complete disappearance of raccoons, rabbits and opossums, have occurred in the remote southernmost regions of the park, where pythons have been established the longest. In this area, populations of raccoons dropped 99.3 percent, opossums 98.9 percent and bobcats 87.5 percent. Marsh and cottontail rabbits, as well as foxes, were not seen at all.
These declines coincide with the proliferation of invasive Burmese pythons in this area, and the authors identify predation by pythons as the most likely cause of the mammal declines.
Q. Why do you think the Burmese pythons are responsible for the declines?
The mammal species monitored in this study were common in the park before pythons attained high densities, but this study shows that their numbers have dropped precipitously over the same time as the pythons were becoming more abundant. Researchers compared areas outside the current range of pythons in southern Florida; these areas continue to yield high numbers of mammal sightings for all the monitored species. Given the timing and geographic scope of the severe declines in mammals overlapping with the timing of the establishment of pythons and their geographic range, the authors conclude that python predation is likely responsible for this large change in the mammalian fauna of the Everglades, with largely unknown repercussions for how this ecosystem functions.
The authors found little support for alternative explanations for the mammal declines, such as disease or changes in habitat structure or water management regimes. In order for other large predators, such as Florida panthers and black bears, to have caused the declines, they would have had to considerably increase their populations throughout the Everglades during the last 15 years; no such large population increases have been observed. Mid-sized predators such as foxes or bobcats appear to have declined markedly during this period as well, suggesting that they are not the cause of declines in other mammals.
Q. Are these the only animals that have been impacted by pythons?
Not enough scientific information is available to definitively answer this question. The species that this study documents as apparently declining are those mammal species that are easily observed on roads at night due to their behavioral patterns and abundances. It is important to note that the status of species that are rare (like the Florida panther), patchily distributed, active during the day, or that don’t cross roads was not assessed in this study.
As an example, the round-tailed muskrat is commonly found in python gut contents and appears to have declined in Everglades National Park in recent years, but this species rarely crosses roads and no program is in place to monitor its population status.
Q. Are birds likely to decline due to python predation?
Pythons eat a wide variety of prey (mammals, birds and alligators), and pose a risk to many resources, including threatened and endangered species. Examination of stomach contents of Burmese pythons in Everglades National Park shows that birds make up about one-fourth of pythons’ diet there, but this question cannot be definitively answered with available information. However, one recent (2011) study found 25 species of birds representing nine bird orders from the remains in the digestive tracts of Burmese pythons collected in Everglades National Park; this included the federally endangered wood stork and four species of state (Florida) concern.
Bird populations in the Everglades experience cycles of increase and decrease on a yearly basis as a result of bird migration and changes in water levels and other environmental factors. These fluctuations make it difficult to discern whether or how much python predation is depressing bird populations. Some secretive birds such as rails are masters at staying hidden in marsh vegetation; however, the likelihood of declines in bird species cannot be definitively determined with available information.
Q. How could these large snakes change South Florida’s ecosystems? What about other ecosystems?
Most invasive reptiles and amphibians have not received research attention at a level that would allow in-depth evaluation of impacts. However, the most likely avenue for ecosystem change would be that introduced Burmese pythons or similar species would change food webs by depleting or eliminating vulnerable native species. If enough species are lost, entire ecosystem processes could be changed. For example, on the island of 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 native trees have greatly declined in abundance and may disappear. Similarly, as fish-eating birds have been lost from Guam because of predation by the brown treesnake, the natural nitrogen transport from aquatic and marine systems to bird rookeries has been lost as well, with adverse implications for growth of nitrogen-dependent plants on the island.
Burmese pythons in the Everglades also accumulate mercury in unprecedented amounts, potentially poisoning higher-level predators that might eat the pythons, such as alligators and panthers. Researchers do not yet know how a specific ecosystem in South Florida will be – or is already being -- disrupted by the addition of a novel predator, but from experience with other ecosystems disrupted by introduced snakes, researchers know that serious disruption is a distinct possibility. The severe mammal declines documented in the new study suggest that some degree of ecosystem disruption is probable. However, some disruptions might benefit other species; for example, turtles and some birds might experience greater hatching success of eggs due to the suppression of raccoons, which are regular nest robbers.
In another example, the federally 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. A python on Key Largo has many black rats to eat, but will take native woodrats when the opportunity arises – in fact, several woodrats have been found in the guts of pythons on Key Largo despite the apparently low numbers of pythons that have reached the island. Woodrats may be more vulnerable to predation by large 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.
Q. Where are Burmese pythons or other large constrictors distributed in Florida?
The Burmese python is now distributed across more than a thousand square miles of southern Florida, including all of Everglades National Park and areas to the north such as Big Cypress National Preserve. A number of Burmese pythons have been found in the Florida Keys, but there is not yet confirmation that a breeding population exists in the Keys. A population of boa constrictors has been established for many years in southern Miami, centered on a county park.
Researchers have recently confirmed the presence of a reproducing population of northern African pythons on the western boundary of Miami; this species is both visually and ecologically very similar to the Burmese python. There is as yet no evidence for wild and reproducing populations of the various anacondas or the reticulated python (the longest snake in the world), although representatives of both groups have been captured or sighted in the wild in Florida and elsewhere.
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. Also, the various python species and boa constrictor are often found living in suburban and urban areas in their native ranges. As with alligators, the risk of human attack in urban areas is very small but possible.
Q. Will these snakes – the Burmese pythons and the African pythons -- compete in places where they overlap (e.g., African 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 general 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 since 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. It is unlikely that hybridization would produce a snake larger or more dangerous than either parental species.
Q. How many Burmese pythons inhabit southern Florida?
There are currently no reliable estimates of the total numbers of Burmese pythons in the invasive population. However, from 2000 to mid-October 2011, more than 1,786 pythons were removed from Everglades National Park and adjacent lands. Animals in excess of 16 feet have been captured in the park, including as recently as January 2012. However, even though it seems like such a large snake would be easy to find or see, only a very small fraction of pythons present in the park are ever detected, due to their cryptic coloration; hide, wait, and then ambush behavior; the dense low vegetation that helps conceal them; and seasonal inundation of the landscape, limiting human access.
Based on the geographic extent of the Burmese python population in Florida and knowledge of detection rates for other snakes, experts estimate that a population of at least tens of thousands now live in the wild in Florida, but stress that this estimate is extremely rough. Population size may have dropped somewhat as a result of the severe cold snap of early 2010, but the population is expected to quickly recover from this unusual event.
For most animals, estimation of population size requires ‘mark-recapture’ methods – these methods involve capturing a number of animals, marking them for later identification, and releasing them back into the habitat from which they were removed. By returning to these sites later and capturing another batch of animals, researchers can use the relative number of marked and unmarked animals to estimate population size.
However, in the case of a large invasive snake such as the python in Florida, land managers have decided that removing every python captured is preferable in conservation terms to releasing some animals to gain a clearer understanding of the population size.
Q. Why were the Burmese pythons able to spread so rapidly in terms of the area they have invaded and in terms of how abundant they are now?
Burmese pythons have traits that increased their risk of establishment and that make their eradication difficult. Specifically, Burmese pythons:
Q. A recent paper showed that salt water is not a deterrent to the spread of Burmese pythons. Does this mean that this snake could already be invading other parts of Florida farther away from the Everglades?
A number of Burmese pythons have been found on Key Largo, and a few in the Lower Keys. However, there is as yet no evidence of a breeding population anywhere in the Keys. Because pythons regularly escape or are released from captivity, it can be difficult to determine whether a snake encountered in the Keys arrived there by swimming from the mainland or was a former captive pet on the island. Given the python’s apparent ability to disperse via salt water, however, island residents and resource managers need to stay vigilant so as to be able to detect and eliminate arriving pythons before they become established. See FAQ on what you can do later on in this document.
Q. Is it possible to eradicate Burmese pythons or similar snakes once they have been introduced?
The odds of eradicating an introduced population of reptiles once it has spread across a large area are very low, pointing to the importance of prevention, early detection and rapid response.
That being said, control or eradication of some invasive reptiles and amphibians may be possible, but few control tools are available. In the Everglades alone, state and federal agencies have spent millions of dollars addressing threats posed by pythons; if these species spread to other areas, state and federal agencies in these areas could be forced to spend more money for control and containment purposes.
So far, no one has ever eradicated a snake from any area greater than a few acres in size through purposeful control measures, and few attempts have ever been made to eradicate a large species such as pythons. 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 inhabiting an area 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 may limit python population numbers and help prevent further spread. In the meantime, agencies such as the National Park Service, the Florida Fish and Wildlife Conservation Commission and others are actively testing and applying control and eradication techniques, including trap development, refinement of visual searching methods, and testing detector dogs for locating pythons. See the FAQ on what you can do later on in this document.
Q. What kinds of python control efforts are occurring in Everglades National Park?
Please visit Everglades National Park python site for more information about pythons and Everglades National Park, including efforts to control the species.
Q. What are important next steps to help prevent the snakes from spreading to other areas, and what should I do if I see a python in the wild?
Development of early detection techniques for non-native reptiles is important because prevention is substantially less costly and more effective than control or eradication. 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. The state of Florida has a Nonnative Pet Amnesty Program and an adoption program.
If you see a python in the wild – or suspect that the snake is a python – you should take the same precautions for these constrictor snakes as one would take for alligators: avoid interacting with or getting close to the animal. If you are in Everglades National Park, you can report a python sighting to a park ranger. You can also report the animal via the Ive-Got-1 reporting hotline (888-483-4681), the EDDMapS reporting site or by using a recently developed iPhone application, IveGot1 - Identify and Report Invasive Animals and Plants in Florida. These reporting sites share reports so you only need to report the animal at one of them.
Q. Are large constrictor snakes such as Burmese pythons able to kill people? What is the risk? Would this be in the wild too, or in backyards?
Human fatalities from non-venomous snakes are very rare, probably only a few per year worldwide. All known constrictor-snake fatalities in the United States are from captive snakes; these are split between deaths of snake owners who were purposefully interacting with their pet and deaths of small children or infants in homes where a snake was kept captive as a pet. There have been no human deaths from wild-living Burmese pythons in Florida. Overall, the risk of attack is very low, but would likely be greatest in natural habitats (the wild).
However, we cannot categorically rule out the possibility of an attack in agricultural or suburban areas. Because many suburban areas and even backyards, neighborhoods and parks in Florida include ponds, canals or other bodies of water where large snakes could feel at home, the situation is likely similar to that experienced with alligators: attacks are improbable but possible in any locality where the animals are present and people are also present. The simplest and most sure-fire way to reduce the risk of human fatalities is to avoid interacting with a large constrictor.
Q. Should parents of young children be worried about these wild constrictors?
Currently, the only place where any of these large constrictor snakes are known to be wild-living in the U.S. is in South Florida. In the Everglades, even with the newly documented severe declines in mammals, there is likely still 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.
Q. Are there other invasive reptiles in the country that people should be concerned about?
Invasive animal species are a rapidly increasing environmental and economic problem in the United States. According to U.S. Fish and Wildlife Service records, legal wildlife shipments into the United States between 1999 and 2010 comprised over 2.8 billion individual exotic animals, representing at least 4,200 different species from over 150 countries.
Florida now has the dubious distinction of having the largest number of established non-indigenous reptile and amphibian species in the entire world. Fifty-six are established including 3 frogs, 4 turtles, 1 crocodilian, 43 lizards and 5 snakes.
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 because disposal or release in the wild can do great environmental harm (see FAQ on What Can I Do?). 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, all of its bats and about half of its native lizards. Since some of these animals dispersed the seeds of native plants, the plants, too, are declining.
The python introduction to Florida is so recent that the full ramifications of its introduction on Florida’s unique native plants, animals and ecosystems cannot yet be made, although the new paper suggests that impacts may be pervasive for some animal groups. 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 of native species there.
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 or pet owners, but most of these don’t appear to have established reproductive populations.
It is highly likely that additional species of potentially harmful reptiles (for example, Nile monitors, Argentine tegus) currently established outside the Everglades National Park will invade South Florida national and state parks in coming years. The known densities of these species in their native ranges, combined with the fact that they are novel predators that native species may not sense as dangerous, suggest that they also may exert detrimental effects on native species not adapted to these kinds of predators, as well as harming the ecosystems they live in.
Q. What is the current legal status of Burmese pythons?
On Jan. 23, 2012, the U.S. Fish and Wildlife Service published a rule in the Federal Register that will ban the importation and interstate transportation of four nonnative constrictor snakes (Burmese python, northern and southern African pythons, and the yellow anaconda) that threaten the Everglades and other sensitive ecosystems; these snakes are being listed as injurious under the Lacey Act. The Service will continue to consider listing as injurious the five other species of nonnative snakes that the agency also proposed in 2010 – the reticulated python, boa constrictor, DeSchauensee’s anaconda, green anaconda and the Beni anaconda. Under the Lacey Act, a wildlife species can be listed as injurious because it has been demonstrated to be harmful or have the potential to be harmful to either the health and welfare of people, the interests of forestry, agriculture, or horticulture, or the welfare and survival of wildlife or the resources that wildlife depend upon. To control the introduction and spread of an injurious species, the listing of a species as “injurious” under the Lacey Act means that its importation and interstate transport are prohibited without a permit issued by the Service. This prohibition includes importation or interstate movement of live animals, their gametes, hybrids, and viable eggs. Injurious species may not be transported into or through U.S. territories or states. Permits may be granted for the importation or transportation of live specimens of injurious wildlife for scientific, medical, educational, or zoological purposes. The Lacey Act does not have provisions for the movement of personal pets, and it does not restrict intrastate (within state) transport. For more information about the Lacey Act and the listing of these four constrictors as injurious, please visit this FWS News and Resources site.
Q. Who funded this research?
Support for this research was provided by Davidson College, Duke Energy, the J.E. and Majorie B. Pittman Foundation, Inc., the Center for Forest Sustainability at Auburn University, USGS, and the National Park Service.