Tuesday, February 5, 2013

[Mammalogy • Extinct] No need for disease: Testing Extinction Hypotheses for the Thylacine Thylacinus cynocephalus using multi-species metamodels | New Study Confirms Humans Caused Extinction of Tasmanian Tiger

Tasmanian tigers at Beaumaris Zoo, Hobart, in 1918
(Tasmanian Museum and Art Gallery, via University of Adelaide)

New Study Confirms Humans Caused Extinction of Tasmanian Tiger
Humans alone were responsible for the demise of the thylacine, an extinct predator also known as the Tasmanian tiger, according to a new study led by University of Adelaide scientists.

The thylacine was a unique marsupial carnivore found throughout most of Tasmania before European settlement in 1803. Between 1886 and 1909, the Tasmanian government encouraged people to hunt thylacines and paid bounties on over 2,000 thylacine carcasses. Only a handful of animals were located after the bounty was lifted and the last known thylacine was captured from the wild in 1933.

“Many people, however, believe that bounty hunting alone could not have driven the thylacine extinct and therefore claim that an unknown disease epidemic must have been responsible,” said Dr Thomas Prowse of the University of Adelaide, lead author of a paper published in the Journal of Animal Ecology.

The study contradicts the widespread belief that disease must have been a factor in the extinction of the Tasmanian tiger.

“We tested this claim by developing a ‘metamodel’ – a network of linked species models – that evaluated whether the combined impacts of Europeans could have exterminated the thylacine, without any disease.”

The mathematical models used by conservation biologists to simulate the fate of threatened species under different management strategies called population viability analysis (PVA) traditionally neglect important interactions between species. The scientists designed a new approach to PVA that included species interactions.

“The new model simulated the directs effects of bounty hunting and habitat loss and, importantly, also considered the indirect effects of a reduction in the thylacine’s prey – kangaroos and wallabies – due to human harvesting and competition from millions of introduced sheep,” Dr Prowse said. “We found we could simulate the thylacine extinction, including the observed rapid population crash after 1905, without the need to invoke a mystery disease.”

“We showed that the negative impacts of European settlement were powerful enough that, even without any disease epidemic, the species couldn’t escape extinction,” Dr Prowse concluded.

1. Population viability analysis (PVA) is widely used to assess the extinction risk of threatened species and to evaluate different management strategies. However, conventional PVA neglects important biotic interactions and therefore can fail to identify important threatening processes.
2. We designed a new PVA approach that includes species interactions explicitly by networking species models within a single ‘metamodel’. We demonstrate the utility of PVA metamodels by employing them to reinterpret the extinction of the carnivorous, marsupial thylacine Thylacinus cynocephalus in Tasmania. In particular, we test the claim that well-documented impacts of European settlement cannot account for this extinction and that an unknown disease must have been an additional and necessary cause.
3. We first constructed a classical, single-species PVA model for thylacines, which was then extended by incorporation within a dynamic predator–herbivore–vegetation metamodel that accounted for the influence of Europeans on the thylacine's prey base. Given obvious parameter uncertainties, we explored both modelling approaches with rigorous sensitivity analyses.
4. Single-species PVA models were unable to recreate the thylacine's extinction unless a high human harvest, small starting population size or low maximum population growth rate was assumed, even if disease effects were included from 1906 to 1909. In contrast, we readily recreated the thylacine's demise using disease-free multi-species metamodels that simulated declines in native prey populations (particularly due to competition with introduced sheep).
5. Dynamic, multi-species metamodels provide a simple, flexible framework for studying current species declines and historical extinctions caused by complex, interacting factors.

Keywords: extinction process; population viability analysis;s pecies interactions; vortex ; metamodel manager

Thomas A. A. Prowse et al. 2013. No need for disease: testing extinction hypotheses for the thylacine using multi-species metamodels. Journal of Animal Ecology. doi: 10.1111/1365-2656.12029