Wild Populations of Tasmanian Devils Continue to Decline, Study Shows

The overall population numbers of wild Tasmanian devils (Sarcophilus harrisii) are continuing to decline, due to the presence of a transmissible cancer, according to a study published in the Journal of Applied Ecology.

A young Tasmanian devil (Sarcophilus harrisii). Image credit: Keres H. / CC BY-SA 4.0.

Tasmanian devils face extinction in the wild because of devil facial tumor disease (DFTD), a rare, contagious cancer found only in devils. It is transmitted from one animal to another through biting, a common behavior among Tasmania devils when mating and feeding.

“Research of Tasmanian devil densities at monitoring sites, together with spotlight counts around the state, show that populations have decreased by about 80% following the emergence of DFTD,” said study lead author Dr. Billie Lazenby, a biologist with the Department of Primary Industries, Parks, Water and Environment in Hobart, Tasmania, Australia.

“While the number of Tasmanian devils in the wild continues to decline and DFTD is spreading, to date, devils are still found in the wild in Tasmania.”

“We are now dealing with very small and potentially isolated groups of devils across Tasmania. The ongoing impact of DFTD, which continues to cause high mortality in devils, could make them vulnerable to other threats.”

The study further indicates that remaining wild populations are showing slight reproductive changes, possibly in response to the challenges posed by the disease.

“Tasmanian devils in diseased areas are now breeding younger and having more pouch young, which has allowed them to persist at low levels in the wild,” said co-author Dr. Mathias Tobler, a scientist with San Diego Zoo Global.

“This research has shown the structure of the wild Tasmanian devil populations in diseased areas has shifted dramatically, with the animals over the age of two being very rare, compared to sites before DFTD emerged. Earlier breeding in young Tasmanian devils means that they are contracting DFTD younger, often as one-year-olds.”

Although these reproductive shifts are allowing the populations of this species to maintain, the overall reduction may indicate that the species is at greater risk of extinction due to other factors.

“While Tasmanian devils have persisted to date in the wild, such large reductions in their numbers and the change in their age structure means their populations are impacted more by other threats, such as roadkill, bushfire, loss of genetic diversity, variation in food availability caused by drought and changes in the ecosystem as it responds to the loss of devils in the wild,” said study senior author Dr. David Pemberton, also from the Department of Primary Industries, Parks, Water and Environment.

“Efforts to manage Tasmanian devils, such as the development of an immunotherapy, are ongoing, but remain in a research-and-development phase.”

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Billie T. Lazenby et al. Density trends and demographic signals uncover the long-term impact of transmissible cancer in Tasmanian devils. Journal of Applied Ecology, published online February 5, 2018; doi: 10.1111/1365-2664.13088