Releasing the Devil
Conservation trade-offs: Island introduction of a threatened predator suppresses invasive mesopredators but eliminates a seabird colony (2020) Scoleri et al., Biological Conservation, https://doi.org/10.1016/j.biocon.2020.108635
Invasive species are a nightmare for local wildlife wherever they are, but on islands they’re even worse. Introduced predators can wipe out entire populations of species, as Tibbles the cat and his fellow feral buddies demonstrated in the extreme when they drove the Lyall’s wren extinct. On coastal islands this is a recurring theme. An invasive ‘mesopredator’ – like the American Mink in Europe or the cat in Australia – is introduced and quickly goes to work, often on small mammals, birds, reptiles and amphibians alike.
Sometimes, but not always, introducing a top predator to an area can suppress the activities of the mesopredator. They can outcompete the mesopredator for resources, or begin to prey on them. The problem is, that if that top predator goes after the same food as the mesopredator, the local prey species suffer either way.
What They Did
Maria Island, off the east coast of Tasmania, was selected for reintroduction of the endangered Tasmanian devil in 2012. The island is home to the short-tailed shearwater (Puffinus tenuirostris). Playing the role of the mesopredator on the island are an unlikely couple: the feral cat, constant pain in the arse to Australian biodiversity, and the brushtail possum, which to my surprise will actually go after small birds and their chicks.
The researchers set up camera traps at two sites of shearwater breeding colonies, looking for attacks of all three species on the shearwaters. The cameras were set up for the same six week period every year from 2013 to 2016, during which parents raise chicks and then depart the nest. They compared activity of the predators to burrow occupancy of the birds to see if they could pick up any trends in predation over time.
Did You Know: Facial Tumours
Whilst a Tasmanian devil might not strike you as a ‘top predator’, their aggression mean that they play the part of top predator on Tasmania and it’s surrounding islands. The benefits of introducing the devil are twofold. It could help suppress mesopredator activity, but perhaps more importantly, over the last few decades Tasmanian devil populations have crashed, due largely to a facial tumour. The tumour is a rare transmissible cancer, and is often spread when the notoriously aggressive devils bite each other. Finding new areas for uninfected populations is therefore crucial right now.
What They Found
The biggest factor in predator activity was time, with possum and cat activity decreasing every year as the Tasmanian devil population increased. While this sounds good for the birds, the decrease in predation by cats and possums was almost matched by the increase in devils preying on the birds, so that predator activity only marginally decreased over the entire period.
The biggest problem here though was that unlike the possums and cats, the devils would actively burrow to find shearwater chicks and eggs. This behaviour meant that sadly by 2016, the shearwaters were extinct at both sites.
The problems here and the same ones that face any research that occurs on an island, especially when the question has such obviously applicable connotations – would things happen the same way on the mainland? On the Tasmanian mainland shearwaters co-occur with devils, and these results are only applicable to one island, so at this point we can’t really know.
Talk of the introduction of Tasmanian Devils back to mainland Australia has been floating around for a long time now, and research like this is important for obvious reasons. In this case the potential impact on shearwaters were known, but the potential for positive impact on Tasmanian Devil populations outweighed those concerns. Reintroduction of shearwaters could be possible, with both the removal of cats and possums potentially enough to help the birds co-occur with the devils.
There is hope for both species though, with devil populations showing signs of recovery from and resistance to the disease. If devil populations rebound, the need to reintroduce them to islands may vanish.
Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is still angry at the Looney Tunes for their portrayal of a Tasmanian Devil. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.