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 http://dx.doi.org/10.1073/pnas.1115226109

Image by Bobosh_t

More on pythons and other giant snakes:

• Snakes know when to stop squeezing because they sense the heartbeats of their prey
• Meet the Agta, a tribe where a quarter of men have been attacked by giant snakes
• A recipe for growing bigger hearts, found in the blood of pythons
• Titanoboa – thirteen metres, one tonne, largest snake ever.
• Snake proteins have gone through massive evolutionary redesign

Show this to your local climate denialist when they get confused about the distinction between “climate” and “weather.” Not that it will change their minds, but the dog is cute.

The lineage of giant tortoise from the island of Floreana was one of the first permanent casualties. It was extinct by 1835, just 15 years after a certain Charles Darwin visited the Galapagos.

Or was it? A team of scientists has found traces of the tortoises, which suggests that a lost population might still be alive on nearby Isabela Island, the largest of the Galapagos archipelago. The team found neither droppings, nor grainy photos, nor footprints. They found genetic footprints.

In 2008, Adalgisa Caccone found that some of the Isabela tortoises are genetically distinct from their neighbours. Surprisingly, their DNA turned out to be a partial match for the Floreana tortoises, whose genetic material had been recovered from museum specimens. Two years later, Caccone found nine captive tortoises at a captive breeding centre that also had Floreana DNA in them.

The discovery suggested that the Floreana line hadn’t been snuffed out entirely. Pirates and traders often moved the tortoises from one island to another, so it’s entirely plausible that Floreana individuals could have ended up on Isabela. Perhaps they live there still, interbreeding with the locals to produce a hybrid population. If we found them, we could launch a breeding programme to restore their numbers.

Now, Caccone’s team, led by Ryan Garrick and Edgar Benavides, has found more evidence that these lost tortoises are still around. They recently returned to Volcano Wolf, the peak in Isabela where they first discovered the hybrids, and took DNA samples from the locals. They worked hard, collecting samples from 1,669 individuals – a fifth of the estimated total.

They found that 84 of these tortoises are first-generation hybrids. They must all have had a Floreana tortoise as a parent. Garrick and Benavides also found that 26 of these hybrids had Floreana mitochondrial DNA. This small set of accessory genes is only ever passed down from mother to child, so a third of the hybrids have an apparently extinct tortoise for a mother.

Some of the other tortoises on Volcano Wolf also seem to have fainter traces of Floreana DNA in their genomes, suggesting that they are at least fourth-generation descendants. It seems likely that the Floreana immigrants have been mating with the native ones for the last 200 years, ever since they were first brought to Isabela aboard human ships.

Garrick and Benavides estimate that it would have taken at least 38 pure-bred Floreanas to produce the level of genetic diversity seen in the modern hybrids. And there’s a very good chance that these parents are still alive. After all, giant tortoises can live for more than a century, and 30 of the hybrids were less than 15 years old.

Extinct species are sometimes rediscovered, usually because someone finds the actual animal or plant. These resurrected creatures are known as Lazarus species. Likewise, scientists often use genetic studies to show that what people thought was one species is actually two or more – these are known as ‘cryptic species’. But this study marks the first time that scientists have re-discovered an apparently extinct animal by finding its genetic footprints in living descendants. It’s a cryptic Lazarus species!

Caccone’s work suggests two possible ways of saving the Floreana tortoises. Either find the lost parents that apparently still live in Floreana, or use the captive hybrids to start a breeding programme. Either way, it’s worth noting that none of this would have been possible without the existence of museum specimens of the Floreana tortoises.

The team will return to Volcano Wolf in December 2012 to try and find all the hybrids and, with luck, the pure ones. “Obviously, the more we stay up there, the higher the chances to find more animals but the area to search is huge and there are about 7,000 to look for,” says Caccone.

This isn’t just a vain academic exercise. The tortoises are keystone species in the Galapagos, and the biggest plant-eaters around. Their disappearance from certain islands has altered the balance of entire ecosystems. “We will lose what we treasure the Galapagos for,” says Caccone. “It’s a unique place where evolution has played its game of cards in complete isolation with a set of players that arrived by chance and that do not exist anywhere else in the world.”

Reference: Garrick, Benavides, Russello, Gibbs, Poulakakis, Kajdacsi, Marquez, Bahan, Ciofi, Tapia & Caccone. 2011. Genetic rediscovery of an ‘extinct’ Galápagos giant tortoise species. Current Biology. Citation tbc.

PS: The classification of the Galapagos tortoises is an ever-changing beast. There are at least 11 living groups (and four extinct ones). They are either all distinct species, or subspecies of the same one depending on whom you ask. They’re also either members of a distinct genus (Geochelone) or part of a wider one (Chelonoidis) that includes all South American tortoises. This new paper treats the tortoises as separate species of Chelonoidis. It’s all a bit academic and arcane. The key point is that they are the different lineages are genetically diverse, so it’s important that the Floreana ones might still exist.

Images: Tortoise by Claudio Ciofi; Map by Fallschirmjäger

More on the Galapagos: The pink Galapagos iguana that Darwin never saw

More on hybrids:

• Humans and Neanderthals had sex, but not very often
• Raise your pints to the Patagonian fungus that helped us to brew lager
• House mice picked up poison resistance gene by having sex with related species
• One generation, new species – all-female lizard bred in a lab
• Cross-breeding restores sight to blind cavefish
• Holy hybrids Batman! Caribbean fruit bat is a mash-up of three species