Since the 1990s, Tasmanian devils, Sarcophilus harrisii, have been ravaged by a gruesome, infectious cancer known as devil facial tumor disease. Some populations have declined by over 90%, though no local extinctions have been reported yet. This severe and sustained, disease-induced decline of an apex predator is changing the dynamics of Tasmania’s ecosystem from the top down. Possums have become much more relaxed and are unafraid to spend time looking for food away from the safety of their trees. The work was published in Proceedings of the Royal Society B this week.
Predators are really good at creating landscapes of fear, forcing prey animals to balance the need to eat with the need to avoid being eaten. The common brushtail possum, Trichosurus vulpecula, is the major prey species of devils. They live in trees and eat mostly leaves, but they also forage for fruits and seeds on the floor. They tend to stay close to trees as devils aren’t the best climbers. “Devils pounce on possums that are on the ground,” Tracey Hollings from the University of Melbourne tells Science, “so possums prefer open areas,” with few bushes in which devils can hide.
Hollings’ team wanted to see how these possums are responding to the changing landscape of predation risk in Australia’s island state. They picked multiple sites in Tasmania and six on devil-free Maria Island to the east. These areas represent the full spectrum of predator risk: from places where devil facial tumor disease has been around for over a decade, to places exposed for under a decade, to a few pristine areas that are still disease-free. The team used hair traps (or PVC pipes with double-sided tape) that had been baited with food and they also set up artificial food patches with sultana grapes at various distances from escape trees. Smooth river pebbles were mixed with 100 sultanas in a container, with a hole big enough for either a paw or a head but not both. This meant that the possums would have to spend time sorting through them for treats, and had to decide if it was worth the risk.
Brushtail possums, the team found, have been increasingly spending more time on the ground, discovering food patches more quickly and foraging farther from safety since the outbreak of the devil disease and the resulting devil decline. In fact, possum anti-predator behavior in areas with over 90% devil decline is similar to that of possums living on a devil-free island.
This behavioral change is one of the most rapid responses from a prey species to predator decline ever documented. And it may be an early indication of cascading effects to come.