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50,000 years of mammal extinctions


 
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Professor Chris Johnson, from the School of Tropical Biology at James Cook University, discusses some of the important findings in his book that traces the environmental history of Australian mammals, in particular mammalian extinctions.

    

Having studied wallabies, kangaroos, wombats and rat kangaroos for some 20 years, Professor Johnson is well acquainted with the many spectacular and unusual features of present day Australian mammals. However, some mammals with particularly spectacular and interesting features are no longer with us, having become extinct.

There was, as Professor Johnson explains, a particular period 45,000 years ago when some 50 mammal species became extinct. These include mega fauna such as the marsupial lion, the marsupial tapir, a large echidna, the diprotodon (the largest marsupial that ever lived), as well as giant birds.
This period of extinctions coincided with the arrival of people on mainland Australia. While some authors argue that the extinctions resulted from climate change or the burning of fires that altered the environment so much that mega fauna habitat was lost.


 
Professor Johnson, on the other hand, sees the link between humans and extinction, accelerated by a human survival practice – hunting.     
Key to his evidence is the fact that, like whales, the extinct mammals had very low reproduction rates.
A diprotodon, for example, would have a baby about once every six years, meaning the natural replenishment was very low. It has been estimated that for every person at that time, one kill of a diprotodon over ten years would be enough to cause extinction.

 
This picture of natural hunting is a far cry from others that suggest human onslaught and specialisation on large mammals. Professor Johnson believes that the hunting regime was normal, as there is evidence that the Aborigines survived with little impact on the environment.    
It is very likely that the death rate for these mammals overtook the birth rate, resulting in declining populations over a thousand period before extinction came. The mega fauna would have shaped the vegetation, just as they do now in Africa, where the browsers and grazers (giraffes, antelopes, elephants) hold sway. Remnants of that vegetation still exist in Australia. For example the Acacia Peuce or Wadiwood tree (shown below and left) starts out as a small prickly shrub but when it grows to about three metres high it produces a central stem with soft foliage, well beyond the reach of mega fauna.

Despite the extinction of its predators, the plant is almost extinct itself, unable to redevelop its leaf and growth forms. The extinction of the mega fauna meant that the top predators were a marsupial lion, the Thylacine (Tasmanian Tiger) and the Tasmanian Devil. They survived the first impact of humans but became extinct on the mainland in the last 10,000 years, making humans the dominant predator.

Although the dingo can be found in fossil records about 3,000 years ago, Professor Johnson argues that the Thylacine, for instance, was extinct before the advent of the dingo. There is evidence of Thylacine hunting in the rock art of northern Australia, suggesting that once again hunting was responsible for the extinction. It was about that time that spear throwers appeared, making long range hunting much more efficient.

 
Above: Plate 13 from Australia's Mammal Extinctions, photos by Jon Luly. Juveline plant defences via spiny foliage. (a) juvenile Acacia peuce with its defensively spiky hummock shape and (b) the pungent tips of its stiff individual phyllodes (c) formidable spiky foliage of the related Acacia pickardii.
Below: Plate 14 from Australia's Mammal Extinctions, photos by Jon Luly. (a) a half grown tree with spiny defensive dense spiny foliage (b) the mature form is a tall, erct tree with undefended foliage.
    

 
These two very different extinction events, one based around a weak replenishment rate and one based on the introduction of new hunting technology are a timely reminder of the consequences, albeit unintentional, of humans on our natural environment.
Lessons, it appears, we have yet to learn from.

Images from Chris Johnson from his book Australia's Mammal Extinctions: A 50,000 Year History published by Cambridge University Press

Text: V.B. December 2008

 

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