Sunday, February 26, 2012

Extinctions in Australia

Australia has one of the worst extinction records, not just recently but over the last 50,000 years. Why? And would answering that question shed light on species loss around the globe? Telling a believable history based on paleontology, paleoecology, anthropology, and natural history is crucial to understand the patterns in dynamic ecological processes. Tim Flannery presents a very readable attempt in Future Eaters (1994) but the science at that time was not conclusive and too much of this book is conjecture. Chris Johnson, Professor at University of Tazmania , has finally succeeded with his Australia's Mammal Extinctions: A 50 000 Year History (2006).

Dr. Johnson describes tree major waves of extinction and concludes that new keystone predators caused every one:

1. Human Arrival ~46 Kyr
2. Aboriginal Intensification and Dingo Arrival 5 Kyr
3. White Arrival with Sheep, Rabbits, Fox, and Cats 0.2 Kyr

Australian megafauna that went extinct around 46,000 years ago. A human hunter is shown for scale in the middle.

First Wave Extinctions
46,000 years ago the first humans entered an Australia that would be unrecognizable today: lowered sea levels in the middle of the last glacial (110,000-14,000 years ago) had revealed a vast continent almost twice as large as the one on maps today. It wasn't just the land that as bigger. Dozen of species of giant kangarooos and other marsupials filled every niche from large giraffe-like creatures to marsupial hippos, elephants, and even marsupial lions. On top of all that there were marsupial tapirs and a giant 500 pound burrowing wombat. Not to mention a super-lizard that would dwarf modern komodo dragons.

Extinct Australian Megafauna: A Marsupial Lion, Thylalaceo, attacking a mega-Kangaroo. credit

Dr. Johnson explains the pattern of extinction by noting that these animals all had low birth rates while the remaining animals were fecund enough to tolerate increased human predation.

Figure 6.3 From Johnson (2006). Much smaller and lighter animals went extinct in Madagascar and Australia than in North and South America, but this discrepancy can be explained by showing that all three extinction events had similar Fecundity to Extinction relationships.

Survivors included possums, gliders, koalas, bandicoots, and billabies, the numbat, marsupial moles, and the thylacine (AKA Tazmanian Tiger) and the Tazmanian Devil. All of these species continued to cohabit Australia with early humans up until...

Second Wave Extinctions
In the middle of the Holocene, about 5,000 years ago (and 5,000 years after the end of the last Ice Age), both the thylacine and the Tazmanian Devil went extinct on the Australian mainland. What changed? Perhaps the end of the ice age brought shrinking coastlines and increased crowding of Aboriginals onto less and less good land ("Intensification"). Or perhaps it was the arrival of another keystone predator, the dingo. More info.

Figure 11.4 from Johnson (2006) showing populations of rat-kangaroos and foxes in a) southern, b) central and c) northern New South Wales. Rat-kangaroos solid lines, foxes dashed lines.

Third Wave Extinctions
Whatever the causes of the first two extinction waves, they left humans and dingos as the only keystone predators. When Europeans arrived 200 years ago and began a program of dingo eradication and Aboriginal resettlement, many prey species populations (such as kangaroo and emu) exponentially increased. Unfortunately, at the same time, two new predators were introduced: the fux (Vulpes vulpes) and the house cat. Without keystone predators to control these "mesopredators", the fox and cat swept over the Australian countryside, extincting any native animals that fit into their mouth (key size was about 500g-5kg). Especially hard hit were ground-dwellers in open habitats. The added environmental changes wrought by widespread clearing for sheep ranching and the invasion of rabbits breeding like, well, rabbits, was enough to bring Australia's ecosystems to their knees. Dr. Johnson describes this process as "hyperpredation", when a generalist predator (like fox or cat) can build up to high population levels eating common prey (rabbits) but then turns to rarer species when rabbit population fluctuates.

Figure 12.3 From Johnson (2006) showing interactions that led to Hyperpredation on native mammals.

In general, predators can exert huge effects on ecological communities: "The devastating impact of new predators and the pervasive effects of shifts in the balance of existing predator-prey interactions are the themes that underline the whole history of extinctions in Australia for the last 50,000 years." The surprising conclusion: dingoes control should be stopped so that dingoes, the last remaining top-predator, can control foxes and cats. According to Dr. Johnson, increased dingo population may be Australia's last hope for a balanced ecosystem.

Dr. Johnson considers environmental (climate) change hypotheses, but I wish he had dealt more with the concomitant changes to vegetation communities and ecosystem processes. For example, Loreau and Schmitz both emphasize how changes in herbivore populations can have large indirect effects on nutrient cycling that then feed back into increased changes in vegetation communities. Dr. Johnson does note that people did not seem to change fire frequency/intensity until Aboriginal Intensification, but the data for this seem weak. His Figure 7.1 shows that megafauna extinction removed more than 75% of animal biomass from Australian ecosystems. This must have had a huge effect. But, even today, it can be difficult to document the interactions and feedbacks between herbivores and plants, so the failure to find evidence of this 45,000 years ago is not surprising.

Example of how megafauna are still relevant in Australia today.

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