The introduction of Tasmanian devils to Maria Island halved the population of brushtail possums, according to new research that suggests restoring top predators to ecosystems could help limit the number of overabundant prey.
In 2012, the carnivorous marsupials were introduced to the island off the east coast of Tasmania to create a geographically isolated insurance population free from devil facial tumour disease.
Australian researchers have tracked the impact of the introduction on brushtail possums, finding that “possum abundance fell by more than 50% across Maria Island as a whole”.
Prior to the devils’ introduction, brushtail possums – which had been introduced to the island in the 1950s – had expanded beyond their usual habitat and had become “a pretty significant predator”, according to study co-author Prof Christopher Johnson of the University of Tasmania.
“The place was sort of overwhelmed with possums. They were living in open grasslands a long way from trees,” Johnson said. “They were preying on seabirds – in fact, there were possums living in seabird burrows and coming out at night … and pulling [short-tailed shearwater] chicks out and killing them.”
Brushtail possums are considered omnivores, but in most parts of their geographic range they primarily eat foliage, Johnson said. “They den in tree hollows and they feed in the tree canopy … They will come to the ground to feed but typically they don’t go very far from a tree.”
A founding population of 28 Tasmanian devils was released on Maria Island in 2012 and 2013, growing to an estimated 100 animals by 2016 – the island’s predicted maximum capacity.
Combining data from population surveys and GPS tracking of individual possums, the researchers found that the devils brought about the almost complete disappearance of possums from open grasslands and population decline in dry forest areas. Possum populations “remained stable in wet forests”.
Within five years of the devils being introduced, surviving possums were restricted to forest habitats and using tree hollows as den sites, the team found.
“In one sense, this is a story of what devils do to possums,” Johnson said. “But it’s also an instance of a bigger story about how predators can control the niche that is occupied by other species.
“Predator introductions are pretty rare things still and because the ecological effects of predators in most cases unfold over long periods of time, there really aren’t very many examples like this one.”
Johnson described Maria Island as a test case for other devil introductions. “If someone got serious about introducing devils on to the mainland of Australia, where they used to exist, you would need pretty good data on the impacts of devils on the ecosystem,” he said.
“We’ve really proved in this case … that if management of possums was the goal, we could say that it worked and we could do it again.”
In 2021, a BirdLife Tasmania survey found introduced devils had wiped out little penguins from Maria Island, which had been home to 3,000 breeding pairs of the flightless bird a decade earlier. Previous research conducted by the University of Tasmania scientists also found the devils decimated colonies of the island’s short-tailed shearwaters.
The study was published in the journal Proceedings of the Royal Society B.
