Environment - Pythons hunt Florida mammals to brink of extinction

Florida's ecologists can be forgiven one pet hate: Burmese pythons discarded by their owners have eaten many Everglades mammals practically to extinction.

"This is the first documentation of impacts on animal populations, and they are dramatic, with 99 per cent declines in some cases," says Michael Dorcas of Davidson College in North Carolina.

Dorcas and his colleagues scoured 57,000 kilometres of roads in the Everglades between 2003 and 2011, recording all the animals they saw, dead or alive. For comparison, they had data from Everglades park staff recording roadkills between 1993 and 1999. Burmese pythons were recognised as an established species in the region in 2000.

The analysis showed that since 2000, observations of raccoons have crashed by 99.3 per cent, opossums by 98.9 per cent and bobcats by 87.5 per cent. Rabbits have vanished completely.

Significantly, the declines were not so sharp in areas yet to be fully colonised by the pythons, and there were no declines at all in python-free areas.

"The places where the prey mammals have declined most are where the pythons are most numerous," says Dorcas. Case reports describe finding the remains of declining species in pythons' stomachs, adding to the evidence that the snakes are responsible for the slaughter.

A further problem, says Dorcas, is that the prey species are unfamiliar with large snakes as predators, and so don't take steps to avoid them. "It has been millions of years since there was a large snake predator in Florida, so the animals are naive to detecting or recognising snakes as large predators," he says.

The bad news is that there are probably tens of thousands of the pythons at large in a remote wilderness area covering thousands of square kilometres, and despite their size they are extremely difficult to find and kill.

Last week, the US Fish and Wildlife Service banned imports and interstate transportation of the Burmese python – but reversing the invasion will be problematic, says Dorcas.

Dorcas strongly discourages pet owners from releasing pythons in the wild. He says that in Florida, there are "amnesties" where owners can hand in unwanted snakes for euthanising. "They are cool animals, but now they are a real problem," he says.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1115226109.