Since its discovery in the 1990s, a contagious cancer called devil facial tumor disease has wiped out 85 percent of the Tasmanian devils on the Australian island of Tasmania. The animals have yet to completely disappear from any one area of the island, but in some places, as many as 95 percent of the devils are now gone.
For the creatures that the devils prey on — such as brushtail possums — the loss of a predator is good news. And the possums, scientists have found, are losing some of their wariness. In areas where the tumor disease has been present the longest — and the devils are at their lowest numbers — possums have become more comfortable venturing onto the ground from their arboreal homes, scientists report June 16 in the Proceedings of the Royal Society B.
Brushtail possums spend most of their time up in the trees, where they are safe from ground-dwelling predators, such as devils and cats. But they venture down to the ground to feed. Tracey Hollings of the University of Tasmania and colleagues wanted to know whether the possums would change their ground behavior as the devil population disappeared. So they surveyed possum populations at 30 sites on the north side of Tasmania and six sites on Maria Island. That island, off the east coast of Tasmania, has not had devils for at least 10,000 years. (Devils were reintroduced last year, after this study.) Brushtail possums were introduced 50 years ago, so it is a good place to see how possums act without one of their major predators.
The team started by looking at animal hairs, which helped them confirm that areas where the devil disease has been present the longest have the fewest number of devils. Those areas were also where more possums were active on the ground.
The researchers then set up experiments at many of the research sites: In the evening, they set out a container filled with rocks and 100 sultanas at the base of a tree. The next morning, the scientists counted how many sultanas were left. The more bits of food that had been eaten, the more comfortable the possums had been in descending from their arboreal homes. Again, the scientists found that the possums tended to eat more of the sultanas in spots where there were fewer devils.
The possum behavior changed quickly as the devils disappeared, the researchers note. “As devil abundance declined, possums increased their terrestrial activity, foraging for longer, further from refuge,” they write. And in spots where the devil population had declined by 90 percent or more, possums foraged in similar ways to animals on devil-free Maria Island.
The altered possum behavior “may be a forewarning of significant ecological changes in Tasmania, as devil populations are likely to remain at very low levels for decades,” the team writes. How those changes will manifest, though, will be difficult to predict.
There are programs under way to help the devil species combat the disease and plans to one day reintroduce the animals, cancer free, to Tasmania. And it could be easy to worry that possums that grow up in the absence of the predators could be doomed when the devils return. But Hollings and colleagues note that the possums will still have to deal with several other predators, such as cats and the spotted-tailed quoll. And prey species, they say, can react pretty quickly when a new predator starts hunting in the neighborhood.