Our postdoc, Tom Prowse, has just had one of the slickest set of reviews I’ve ever seen, followed by a quick acceptance of what I think is a pretty sexy paper. Earlier this year his paper in Journal of Animal Ecology showed that thylacine (the badly named ‘Tasmanian tiger‘) was most likely not the victim of some unobserved mystery disease, but instead succumbed to what many large predators have/will: human beings. His latest effort now online in Ecology shows that the thylacine and devil extinctions on the Australian mainland were similarly the result of humans and not the scapegoat dingo. But I’ll let him explain:
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‘Regime shifts’ can occur in ecosystems when sometimes even a single component is added or changed. Such additions, of say a new predator, or changes such as a rise in temperature, can fundamentally alter core ecosystem functions and processes, causing the ecosystem to switch to some alternative stable state.
Some of the most striking examples of ecological regime shifts are the mass extinctions of large mammals (‘megafauna’) during human prehistory. In Australia, human arrival and subsequent hunting pressure is implicated in the rapid extinction of about 50 mammal species by around 45 thousand years ago. The ensuing alternative stable state was comprised of a reduced diversity of predators, dominated by humans and two native marsupial predators ‑ the thylacine (also known as the marsupial ‘tiger’ or ‘wolf’) and the devil (which is now restricted to Tasmania and threatened by a debilitating, infectious cancer).
Both thylacines and devils lasted on mainland Australia for over 40 thousand years following the arrival of humans. However, a second regime shift resulted in the extinction of both these predators by about 3 thousand years ago, which was coincidentally just after dingoes were introduced to Australia. Dingoes are descended from early domestic dogs and were introduced to northern Australia from Asia by ancient traders approximately 4 thousand years ago. Today, they are Australia’s only top predator remaining, other than invasive European foxes and feral cats. Since the earliest days of European settlement, dingoes have been persecuted because they prey on livestock. During the 1880s, 5614 km of ‘dingo fence’ was constructed to protect south-east Australia’s grazing rangelands from dingo incursions. The fence is maintained to this day, and dingoes are poisoned and shot both inside and outside this barrier, despite mounting evidence that these predators play a key role in maintaining native ecosystems, largely by suppressing invasive predators.
Perhaps because the public perception of dingoes as ‘sheep-killers’ is so firmly entrenched, it has been commonly assumed that dingoes killed off the thylacines and devils on mainland Australia. People who support this view also point out that thylacines and devils persisted on the island of Tasmania, which was never colonised by dingoes (although thylacines went extinct there too in the early 1900s). To date, most discussion of the mainland thylacine and devil extinctions has focused on the possibility that dingoes disrupted the system by ‘exploitation competition’ (eating the same prey), ‘interference competition’ (wasting the native predators’ precious munching time), as well as ‘direct predation’ (dingoes actually eating devils and thylacines).
Unfortunately for the dingo, most people have overlooked that about the same time as dingoes came along, the climate changed rather abruptly, and aboriginal populations were going through a major period of ‘intensification’ (that is, human population growth and technological advances). On mainland Australia after 5 thousand years ago, the climate shifted to a drier, more El Niño-dominated state. At about the same time, the archaeological rock-shelter records demonstrate that the density of humans increased. Humans competed with and hunted the native carnivores, and these pressures would have strengthened as the human population grew. Ecological regime shifts (including loss of species) can be triggered by slowly changing variables once a threshold is exceeded (see figure), and the viability of thylacines and devils on the Australian mainland might have been compromised once human density surpassed such a threshold.
Possible ways ecosystem equilibrium states can vary. In a and b, only 1 equilibrium exists for each condition. However, if the equilibrium curve is folded backwards (c), 3 equilibria can exist for a given condition (modified from Scheffer et al. 2001 Nature 413:591)
To investigate these competing or interacting (synergistic) proposed causes of the thylacine and devil extinctions, we built a complex mathematical model system to recreate the dynamic interaction between the main drivers (dingoes, climate and humans), the long-term response of herbivore prey, and the viability of thylacine and devil populations. We designed our models to include the key stressors that are implicated in the Holocene extinctions by including predatory interactions, competition between predators, as well as the influence of climate on vegetation and prey population dynamics. We asked whether the dingo invasion is supported as the main and most probable extinction driver after human population growth is taken into account.
Challenging popular belief, our simulations show that although dingoes might have hastened these extinctions, human intensification, possibly made worse by simultaneous climate changes at that time, is the most likely extinction driver. We successfully simulated the thylacine and devil extinctions (with greater than 50 % probability) for scenarios in which human density grew to reach at least 0.55 individuals km-2. This is a realistic estimate of the actual density for a technologically sophisticated hunter-gatherer society. This result assumes human attack rates on the native carnivores that equal one thylacine and devil killed per human every 7 and 0.17 years, respectively. Again, these rates are ecologically reasonable given that Aborigines hunted both thylacines and devils for food and ceremonial purposes.
Our multi-species models support recent claims that the dingo’s role in Australia’s Holocene extinctions has been overstated. In our simulations, dingoes could reduce thylacine and devil populations through different types of competition, as well as direct predation. However, when we included the dingo introduction as the sole extinction driver, dingoes rarely drove thylacines and devils to extinction in the time required. Our results support the notion that thylacines and devils persisted on Tasmania not because the dingo was absent, but because human density remained low there and/or because Tasmania is less affected by El Niño dynamics.
The mechanisms generally involved in the mass extinction of the Australian megafauna (human predation and competition) were also sufficient to exterminate smaller thylacines and devils once the human population had attained a sufficiently high density. Human intensification during the late Holocene parallels in some ways the development of agriculture on other continents, but in a culture that retained a hunter-gatherer economy, and would similarly have impacted negatively on the wildlife exploited for human use. It seems probable that human impacts on the structure and composition of Australian biodiversity were not limited to the late Pleistocene but extended into the late Holocene.
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Prowse, TAA, CN Johnson, CJA Bradshaw, BW Brook. In press. An ecological regime shift resulting from disrupted predator-prey interactions in Holocene Australia. Ecology doi:10.1890/13-0746.1
