Since pythons invaded, Florida’s mammal populations have crashed

By Ed Yong | January 30, 2012 3:00 pm

It turns out that if you unleash giant snakes into a place that didn’t previously have giant snakes, the other local animals don’t fare so well. That seems obvious, but you might be surprised at just how badly those other animals fare.

Since 2000, Burmese pythons have been staging an increasingly successful invasion of Florida. No one knows exactly how they got there. They normally live in south-east Asia and were probably carried over by exotic wildlife traders. Once in America, they could have escaped from pet stores or shipping warehouses. Alternatively, overambitious pet owners could have released when they got too large for comfort. Either way, they seem to be thriving.

With an average length of 12 feet (4 metres), the pythons are formidable predators. They suffocate their prey with powerful coils, and they target a wide variety of mammals and birds. The endangered Key Largo woodrat and wood stork are on their menu. So are American alligators (remember this oft-emailed photo?). Conservationists are trying to halt the spread of the giant snakes, out of concern that their booming numbers could spell trouble for local wildlife.

Michael Dorcas from Davidson College thinks they are right to be concerned. In the first systematic assessment of the pythons’ impact, Dorcas has found that many of Florida’s mammals have plummeted in numbers in places where the snakes now live.

Raccoons, for example, used to be one of the most frequently seen animals in Florida’s Everglades National Park. Between 1996 and 1997, you’d see one every 35 kilometres on the local roadsides. That’s no longer the case. In the last few years, Dorcas and his team have driven over 57,000 kilometres of Everglades tracks, counting animals as they went. They worked between sunset and sunrise on 313 separate nights. Their roadside census showed that since 2003, when the python populations really took off, raccoon sightings have fallen by 99.3 per cent. Opossum numbers have fallen by 98.9 per cent. There are 87.5 per cent fewer bobcats. They didn’t see a single rabbit.

This could, of course, be coincidence, but the numbers fit in both time and space. The mammal populations have suffered the greatest losses at the southern end of the park where the pythons first staged their invasion. At the further corners, where the snakes have only been recently found, the mammals’ numbers haven’t fallen quite as far. And mammal sightings were even more common in two areas outside the park, where pythons have never been seen.

It’s possible that some other factor has simultaneously triggered the decline of these mammals, but it’s hard to think what that might be. There’s no evidence that they’ve been hit by a new disease, and they all hail from diverse groups, which makes the possibility of a shared infection less likely. Hunting is unlikely too. It’s banned in the Everglades National Park. While some inevitably happens, it’s hard to imagine that it would occur at the scale necessary to bring about the falls that Dorcas saw, especially in the park’s remote southern area.

This is probably just the tip of the iceberg. Raccoons and opossums are easy to spot; there may be dozens of other species, including local birds, which are also being affected in less detectable ways. But beyond Dorcas’ hard statistics, it’s difficult to predict what impact the pythons would have. They could eat some species to extinction. They could outcompete other predators for food. They could allow mid-tier animals to boom in numbers, by getting rid of predators.

Regardless, Dorcas’ results should give added urgency to attempts to control the invasive pythons. We’re actually in a fortunate position of having identified a problem a mere decade or so after it began. Other parts of the world haven’t been so fortunate.

Shortly after World War II, the brown tree snake was introduced to the Pacific island of Guam. It slowly went about exterminating the native species. It took more than 30 years to work out what the snake was up to, and by then it was too late for many species. Thanks to the snake, the Guam rail and Micronesian kingfisher only survive in zoos. The Guam flycatcher has disappeared. The rufous fantail is no more. Hopefully, the Everglades will avoid the same fate.

Reference: Dorcas, Wilson, Reed, Snow, Rochford, Miller, Meshaka, Andreadis, Mazzotti, Romagosa & Hart. 2011. Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park. PNAS

Image by Bobosh_t

More on pythons and other giant snakes:



Comments (25)

  1. Thomas

    The last para. in the Nat Geo article: “Habitat depletion, continued demand for Burmese pythons in the pet trade, and hunting for their skins and flesh have landed these graceful giants on the threatened species list.” Seems they may have found a reprieve in the Everglades, unfortunately for the natives.

  2. Bobby LaVesh

    I can’t help but be curious how far North they can travel. Are they limited to areas that never see frost- or could they survive occasional mild cold spells like central Florida experiences?

    Shame they don’t eat fire ants.

  3. They suffocate their prey with powerful coils
    The show “Inside Natures Giants” claims that it’s not suffocation that kills their prey, (I forget exactly what does but) it’s about the heart and the constriction (high pressure?) stops it.

  4. I don’t believe that pet owners are to blame for this “invasion.” The snakes being found in the wild don’t match popular color morphs in the pet trade, and considering how expensive large snakes are and how easy it is to find someone who will take on off your hands, nobody would just dump them and few people would lose them. Certainly not often enough to create a situation like this.

  5. Bogleech, could be that first generation feral pet color morphs produce offspring with varied pigmentation and those with the best camouflaging do best and produce more offspring. Thus returning the python population rapidly to its more common and successful wild colouration. I suspect many of the pet colour morphs are caused by homozgous recessives so these would disappear pretty quickly if different colour morphs mated. Ah natural selection, gotta love it.

  6. Bobby LaVesh

    #4; I don’t understand Python pattern genetices so forgive me if this couldn’t happen but…

    Say we call your “pet-trade desirable” pattern “A” and your undesirable pattern “B”. (It probably isn’t simple genetics like this… but play along)

    If two parent snakes with the A pattern bred- but were both carriers for pattern B- a certain % of the offspring would likely also have pattern B.

    The As would find homes in the pet-trade. The Bs would not. Therefore- who is to say that a snake breeder that could not find a home for the undesirable Bs would not let them loose rather than kill them.

    Could still very easily be from the pet trade (and often the pet trade is the cause of this type escape) these could be the offspring of the unwanted young.

  7. signtist

    Don’t blame the pythons, blame the idiotic FL rednecks who 1) think pythons make cute pets and 2) release them into the Everglades after they realize that a 200lb reptile with the crushing power of a hydraulic press is no pet

  8. I can see the argument from Bogleech regarding pet releases, but sadly I am inclined to suspect that unwanted pets accounted for much of the initial establishment of the burmese pythons in Florida simply because it is specifically burmese pythons that are the primary snake of interest. Other large constrictors that are less common in the pet trade seem not to show up (reticulated pythons, blood pythons, etc) and much smaller snakes, which were also imported to Florida in vast numbers, have not established in the same way (presumably because fewer individuals tire of them and release them).

    So, for my fellow reptile keepers out there: please be responsible and never release a pet into the wild. It gives us all a bad rap and can cause major environmental disturbances.

  9. MattK

    Boblech, (and to second Alex)

    Feral hogs, feral rabbits, feral mink, feral goldfish, feral carp, feral pigeons, etc don’t match the popular colours in captivity, and yet…

    This paper analyses some historical scenarios about the establishment of Burms

    “blame the idiotic FL rednecks”… I suppose you are not particularly familiar with the reptile trade but “rednecks” make up a pretty small proportion of those who keep large snakes in my experience. As large snakes go, Burms are probably the most amenable to captivity being mild tempered and complacent relative to say, a Retic or the Rock Pythons, and they have been bred extensively and are often sold by unscrupulous pet dealers to any 14yo kid who can cough up 80 bucks. This is not usuasual to Burms. Many pet stores sell cute little 4cm red-tailed catfish while failing to impress upon potential buyers some basic facts about RTCF biology.

    Two things about this are interesting. One, it’s nice that they’ve actually looked for evidence of a negative effect – it is important to demonstrate that exotic species are actually invasive (have important negative effects on other organisms in the ecosystem) rather than just established. There are some cases where exotic species become widespread yet don’t seem to have serious negative effects and there have been serious misalocations of conservation resources because of this sort of confusion (e.g. Purple Loosestrife). Also, it causes confusion in the public and eventual backlash from ignorant anthropologists when discussions about invasive species are not evidence based. Demonization of invasive species is a lazy but relatively effective (in the short term which is how such things are evaluated) way to communicate a simple message to the public, but this approach has downsides.

    The second interesting point is that Burms have several characteristics that make them different from most other invasive species. Like all large bodied reptiles they have relatively slow growth, late age at maturity, and low reproductive/recruitment rate which means they have a life history strategy that is more common amongst endangered species (turtles are the epitomy of this, but whales, crocodiles, great apes and Patagonian toothfish are also examples) than successful invaders. Now most articles will call them “fast growing and fast reproducing” but this is not true in a meaningful way (more of a way of playing into the public’s preconceived notions of invasive species) since almost all organisms exhibit startling rapid population growth when survival of all age classes is high (Darwin’s elephant thought experiment is relevant here). So I would be interested to know exactly how Burms are successful despite their life history limitations.

  10. @MattK – Fantastic comment. Regarding life history limitations, the paper doesn’t address this directly, but there’s a bit that might hint at an answer:

    “Most reptiles are predators that, as ectotherms, can direct large proportions of assimilated energy to growth, storage, and reproduction (9), often allowing them to persist at high densities and pose major risks to native wildlife. Most reptiles are predators that, as ectotherms, can direct large proportions of assimilated energy to growth, storage, and reproduction (9), often allowing them to persist at high densities and pose major risks to native wildlife (10).”

    Maybe it’s not so much about fast growth or reproduction, but energy efficiency.

  11. MattK: I quite enjoyed your commentary. It is worth noting, as well, that while animals such as large pythonids and turtles are indeed “slow” with regards to some life history traits (time to maturity, rate of growth, lifespan) they are quite fast in terms of potential offspring per year (I grant that burms do not lay enormous clutches, but clutch sizes of 20+ are not uncommon; some of the large turtles do significantly better). In most locations, juvenile mortality of long-lived reptiles is very high, and so the overall population growth rate is not terribly fast. However, it is likely that burmese python neonates experience relatively low mortality in Florida. If most of the offspring survive for at least a few years rather than die early, it is plausible that a rather substantial impact could be generated even in the short term (given that even a neonate burmese is at least capable of consuming a large songbird). In the long term, as Ed Yong’s followup suggests, low metabolic demands provide the possibility of maintaining high adult densities.

  12. Having moved to Florida in the last couple years by no real choice of my own I’d be the first to complain about it, but the “idiotic FL rednecks” comment is pretty ignorant.

  13. MattK

    Thanks Ed, yes I noticed that section with interest. I think that may play into it, but of course all non-avian reptiles (everything except birds and mammals really) posses that advantage yet large bodied reptiles and most especially turtles are among the most endangered groups of vertebrates. For an overview see The Global Decline of Reptiles Deja vu Amphibians. As you note, even Burms are in some trouble in their native range. So I don’t think that explanation alone is sufficient.

    Ectothermic efficiency in converting energy to offspring seems to manifest itself more obviously in terms of population density so that an ectothermic (cold blooded) predator of size X can exist at 10x higher density than a similar sized endothermic predator – I don’t have the relevant literature at my finger tips but this is the usual reason cited for the ability of Komodo Dragons to exist at high densities on small depauperate islands while similar sized mammalian predators can’t maintain a viable population over the long term. This relationship has also been used as an indirect way to assess the metabolism of predatory dinosaurs – the ratio of predators to prey should be much lower (fewer predatory dinosaurs) if the predatory dinosaurs were warm blooded. The other interesting angle of this phenomenon is that each individual eats less and so it takes more individuals to have an equal effect on prey populations – so a hypothetical invasive comparable mammalian predator could do similar damage with a population density several times lower

    I think that Michael Habib has what I would hypothesize is right idea, that it must be linked to increased survival of young snakes. (by the way, I believe that the mean reproductive output of everglades Burms is somewhere around 40 eggs every second year). It just seems counterintuitive that the survivorship would be so high since many animals prey on small/medium sized snakes. Even venomous snakes, such as Rattlesnakes, are fairly frequently killed and eaten by a variety of predators – just about any radio telemetry study of snakes demonstrates this. The intriguing possibility is whether there is feedback between the surpression of midsized predators by the adults and juvenile survival. With the magnitude of the effect reported here that seems likely. Unfortunately, as with many animals and reptiles (perhaps) especially, the juvenile period is a black box in most cases.

  14. serpentpox

    Put a $200 State bounty on python heads, and let them keep and sell the hide
    to the leather industry, and the “idiotic FL rednecks” will clean up this problem
    in a year or two.

  15. MattK

    python population size 100 000, annual recruitment could be around 50 000 adults per year (nobody knows for sure but this is well within the wide range of estimates), so just to keep the population steady, never mind “cleaning up this problem” might cost in the neighbourhood of $10,000,000.00 annually. In order to actually decrease the population, one would need to spend even more, at least initially. As the population got smaller the effort per snake would need to be greater and the price of the bounty would need to increase. They would never be eliminated (not even close, have you been to the everglades?) so there would have to be a constant maintanance expenditure in bounties paid in perpetuity. I’m not saying that sort of thing won’t play a role in some sort of solution, but “cleaning up this problem”…ain’t gonna happen, I guarantee.

  16. Ron Kaminsky

    Anyone know whether Shawn Heflick’s statements at URL are based on real evidence? He gives lots of interesting extra data (for example, he claims that the source was probably a reptile facility which was destroyed by Hurricane Andrew in 1992).

  17. MattK

    Ron, the paper (unfortunately behind a pay wall) that I linked to in my first comment, evaluates the “Andrew Hypothesis” and suggests that it is unlikely. The authors suggest that using realistic assumptions for growth and survival, the establishment of the population would have needed to begin earlier, probably in the 80s. However, there is a lot of uncertainty around the estimated parameters.

    It is also important to understand the politics/ethos of herpetology. There are herpetologists that take a favourable view of the pet trade in general – most herpetologists developed their interest in part through keeping exotic reptiles and amphibians. Many, if not most, currently keep various exotic reptiles at home. They acknowledge problems with the trade but prefer to deal with them through promoting “responsible pet ownership” and “responsible pet dealers and breeders” rather than through restrictive regulations that “punish responsible pet owners” because of the actions of a few ‘bad apples’. For some reason, many feel that a major release of pets during a disaster reflects less poorly on the industry than multiple releases over time by individual pet owners. They also feel (rightly in my opinion) that the stated rationales for the recent ban on some large constrictors reflects a prejudice against snakes and political convenience that exploits public misconceptions rather than an objective approach to managing invasive species. The major sources of invasive species is transoceanic shipping, horticulture, the sport fishing industry, and even other parts of the pet trade. If you go to the everglades now you will see habitat dominated by Tilapia and Walking Catfish (aquaculture), Oscars, Mollies, several species of armoured catfishes (aquarium trade), Water Hyacinthe, Brazillian Pepper Bush (horticulture), and Melaleuca (actually imported for the purpose of destroying the glades) Brown Anoles, several species of Hemidactylus house geckoes, Greenhouse Frogs, Cuban Treefrogs (stoways from shipping), etc. So the focus on large constrictors does come across as odd, especially a nation-wide shipping ban. So anyway there is a lot of general pushback and a lot of cynicism towards U.S. Fish and Wildlife (part of that fueled by a potential distribution map for *Python molurus* that most think is ill-conceived, inacurate, and some suggest was politically motivated). What I am trying to get at, is that I don’t have anything particular to say about Heflick but in general when reading about invasive Burms it’s best to be pretty skeptical. Hopefully it will all shake out with the collection of more and better data.

  18. Calli Arcale


    There are some cases where exotic species become widespread yet don’t seem to have serious negative effects and there have been serious misalocations of conservation resources because of this sort of confusion (e.g. Purple Loosestrife).

    This comment piqued my interest. Purple loosestrife is considered a major pest in my neck of the woods, and I’m wondering — are you saying that purple loosestrife does not actually have serious negative effects? Because the local cattail populations would beg to differ; where it becomes established, the cattails disappear from all but the wettest margins. Since there are animals which depend on the cattails, this is more than just a botantical problem.

  19. MattK


    Because the local cattail populations would beg to differ; where it becomes established, the cattails disappear from all but the wettest margins.

    You mean these cattails?

    Purple loosestrife is considered a major pest in my neck of the woods

    But is it?

    The implications of accepting untested hypotheses: a review of the effects of purple loosestrife (Lythrum salicaria) in North America
    Is purple loosestrife (Lythrum salicaria) an invasive threat to freshwater wetlands? Conflicting evidence from several ecological metrics
    Wetland vegetation before and after experimental purple loosestrife removal
    Relationship between the abundance ofLythrum salicaria (purple loosestrife) and plant species richness along the Bar River, Canada

    The first paper, especially is worth reading. Also note that I am not pretending to have done anything like a thorough lit review. Anyway, the real question is, is it enough of a pest, and are control measures effective enough to justify the expense of trying removing it? My hearsay understanding is that the hunter advocacy organisation Ducks Unlimitted, in an effort to rebrand themselves as a conservation focused organisation (not to knock them, they do a lot of conservation work) hit upon Loosestrife as a flagship issue to advocate and they were instrumental in getting the ball rolling on that. Other organisations of various stripes took up the cause and billions of dollars were spent in promoting and pursuing Loosestrife control. My understanding is that there were/are probably much better uses for all that money and volunteer effort.

  20. Calli Arcale

    All I know for sure is that huge swaths of what used to be cattail in my area are now loosestrife. It sure *looks* invasive. I’ll have to read the papers you cited (though I’m not paying $35 to read the one on your “you mean these cattails?” link), because the assertion definitely seems counterintuitive to me. I’ve been watching loosestrife march across my area (the Twin Cities in Minnesota) over the past fifteen years or so; whether Ducks Unlimited had picked the best thing for them to be concerned with or not, my gut feeling is that it is a legitimate concern. In fact, I suspect the main reason one might think money was wasted was not that it’s not harmful but that control may well be hopeless. Sort of like the Asian carp and the milfoil; it’s may be a case where there *are* “serious negative effects”, but control is perhaps not possible.

  21. MattK

    Here’s another URL for the abstract of the Cattail article. I didn’t realize the abstract wouldn’t be visible at the other link.

  22. Calli Arcale

    Thank you! I’ve read the abstract now. It’s discussing two different species of cattail and their relative abundance. One may be an introduced invader, but cattails have been in North America for a very long time, and the article clearly agrees with that (stating that their pollen shows up in the fossil record right back to the Pleistocene). So I’m still not really clear on what your point is with respect to that article.

    I read the first article you linked. If it represents the current state of research, then better research is indeed justified. The article discussed earlier research on the foraging habits of muskrats, but that research doesn’t sound like it was very conclusive, and I have to wonder whether any attempt has been to explore whether it impacts their lodge-building. The lodges I see are generally made predominantly of cattail, and I know loosestrife stems don’t have the same sort of structural properties. But it doesn’t really refute the idea that loosestrife is damaging. It just calls into question the level of research showing that it’s damaging.

    What I’ve seen in the Minnesota Valley National Wildlife Refuge makes it difficult to take seriously the idea that loosestrife is not invasive and doesn’t impact cattails. I remember when those fens were solid cattail. They’re not now, and the loosestrife does block out cattails; you can tell because they green up at different rates in the spring, and look very different in the winter when they’ve died back. There are big swaths of loosestrife, with a thin fringe of cattail around the edges.

    I do agree with the article’s conclusion that we should be very cautious about biological control. I tend to be very wary of biological control in general, because past history has shown that it tends to follow the law of unintended consequences.

  23. DaveP

    About the releases, not all of them are voluntary. Hurricane Andrew let a bunch of pets out when it leveled the owner’s house. Also, there are the escapes to deal with.

  24. Not possible to blame neither pet owners nor any others.. Take seaspecies for instance.. one type of crab from China not native to the US finds its way into a ships ballasttanks. Ships arrieve LA, empties its ballast tanks and BOOM.. Same thing goes for seeds, insect eggs and other things that can find their way aboard an airplane.
    We can then of course discuss wether a species is invasive or simply evolving.. ? When Homo Sapiens crossed into alaska and spread out through the american continent, was the invasive? Or simply exploiting oppertunities to spread? Nice article though and interesting topic!

  25. Scott

    I’ve heard that the unusually cold winter we had last year (2011) killed off a lot of the python and iguana populations. I hardly ever see iguanas anymore.


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