Tasmanian devils—marsupial carnivores the size of a small dog—once actively preyed on possums on Tasmania and nearby islands. Now, the endangered devils are so rare that the tree-living possums frolic freely on the ground and venture into new areas, changing their role in the ecosystem, according to a new study. The finding boosts a hypothesis about how top predators transform the behavior of their prey and has implications for how scientists understand other ecosystems, including that of Yellowstone National Park in the United States.
“It’s good work,” says John Laundré, a predator-prey ecologist at the University of California, Riverside, who was not involved with the study. “Just like in Yellowstone, after the wolves and cougars were removed [in the 1920s] and elk foraged everywhere, these possums can now go anywhere, any time, and eat anything they want.”
But in Yellowstone, controversy has swirled about predators’ effects. The wolves were reintroduced 20 years ago, and some scientists have argued that the canids have also reintroduced an “ecology of fear” in the park, keeping elk and other herbivores vigilant and away from tender saplings along riverbanks; this shift can be tracked in Yellowstone’s recovering vegetation, they say. But others have questioned just how fearful Yellowstone’s elk are, saying that it’s hard to tease out wolves’ impacts on the ecosystem from other factors such as climate change.
The new study examines the ecology of fear in an ecosystem half a world away. Tasmanian devils were once numerous in Tasmania and some surrounding islands, but have been devastated by an aggressive, transmissible cancer and are now endangered. In some mainland areas, the devils have declined by 95%. So Tracey Hollings, a wildlife biologist at the University of Tasmania in Hobart, Australia, set out to explore the devils’ effects on one of their major prey species: the common brushtail possum (Trichosurus vulpecula). As a control, she used an island where the devils were absent at the time of her study (they have since been introduced there)—something not possible in Yellowstone, a one-of-a-kind large terrestrial ecosystem. “Possums are arboreal and eat eucalyptus leaves,” Hollings says, “but they also come to the ground at night to eat fruits and seeds,” sometimes dining in agricultural fields and gardens.
Hollings and her colleagues set out ground feeders stocked with raisins at 18 sites on mainland Tasmania, where devils still roam, and six sites on nearby Maria Island, which was devil-free until 2012 (when a devil sanctuary was established there). The researchers compared the possums’ behavior in areas where devils had declined by 95% with areas with less disease and fewer devil deaths.
For possums, coming down from the safety of the trees (devils aren’t the best climbers) to forage on the ground is a risky endeavor. “Devils pounce on possums that are on the ground,” Hollings says, “so possums prefer open areas” with few bushes where devils can hide. So when a possum discovers a nutritious treat—such as a container of 100 raisins—on the ground, it must decide whether the reward is worth the risk. The scientists made the choice more difficult by mixing pebbles with the raisins and using containers that the animals could only fit their head or paw into. Thus, if a possum wanted to eat raisins, it had to stay put and sort carefully among the stones for the treats.
For 4 nights in a row, the team placed the containers at “safe” sites, with an escape tree nearby; and at “risky” sites, where the nearest tree was 5 to 12 meters away. In the morning, they counted the remaining raisins and collected hair samples from sticky tape hair traps. (Animals other than possums rarely took more than 25% of the food.)
The scientists’ subsequent analysis showed that the possums in areas where devils were few rapidly lost their fear of being attacked. These possums’ behaviors were “almost equivalent to those on Maria Island,” where there were no devils, Hollings says. “They foraged longer and further away from escape trees.” They were also much faster at detecting the containers than possums in areas with devils, the team reports online today in the Proceedings of the Royal Society B.
As predators dwindle, over time prey numbers boom, as elk did in Yellowstone after wolves and cougars were exterminated, Hollings says. But in this case, she’s not sure of the effects on possum numbers, partly because people heavily cull them, and feral cats, owls, and other, smaller predators also kill them. But the study shows that the loss of the devils has already had a behavioral effect: The possums have most certainly lost their fear of being attacked by their natural apex predator. That loss may be a “warning” that Tasmania’s ecosystem is now facing major changes, Hollings says, from the possums feeding heavily on vegetation they may have rarely sampled before.
Indeed, the effects of such a loss of fear “can be stronger than the effects of the loss of direct predation,” says William Ripple, an ecologist at Oregon State University, Corvallis, who was not involved in the study. Laundré agrees. “This has to have some consequence to the ecosystem; not just in terms of numbers of possums, but in how they use the landscape. [The study] is a good demonstration of that shift.”
The study should also serve as a wake-up call about how quickly prey animals’ behaviors can change, notes Oswald Schmitz, a population ecologist at Yale University. The possums altered their feeding behaviors “in less than a decade”—a rapid change predicted by foraging theory but that had not been previously demonstrated, he says.