Re: How many animal species have become extinct in australia since colonisatio

I assume that by 'colonization,' you mean European colonization.  There 
have actually been two "colonizations" of Australia.  The first was 
approximately 35,000 - 40,000 years ago, when the aborigines arrived on the 
continent.  They brought dingoes with them, a new predator that is thought 
to have had a devastating effect on wildlife there.  The second was around 
1800, when Europeans arrived in Australia, bringing dogs, guns, rats, cows, 
goats, agriculture, rabbits, and foxes, among other things.

Extinctions from the first colonization are difficult to determine, because 
fossil records are always spotty at best.  There is a lot of debate over 
whether extinctions in the last 30,000 years were caused by man 
(overhunting) or by other factors, such as climate.  About 50 animal 
species are thought to have gone extinct between the first and second 
colonizations.  Most of those are mammals, with a few reptiles and bird 
species, including the large flightless moa.  In that same time, North 
America was colonized by humans, and some three dozen large animal species 
went extinct--again, there is a lot of debate over whether humans were the 
sole cause, only a partial cause, or not involved at all.

By contrast, most of the extinctions since European colonization can be 
traced more or less directly to the hand of man.  In Australia, about 22 
mammal species, but no bird species, have gone extinct since Europeans 
arrived there.  It is interesting to note that although Australia is often 
held up as an example of how man can drive creatures extinct, the same 
number of mammal species (22) have gone extinct in North America since 
colonization in about 1600, plus eight bird species.

The most alarming part of this is to compare the rates of extinction: in 
Australia, about 50 species over 30,000 years--and then nearly half that 
number in the last 200 years.  Note, too, that none of these figures 
include extinction of insect species, which we have no reliable way of 
tracking, because fossil records and historical records are so poor for 
them.

(A valuable source of information for this answer was Jared Diamond's 
article: "Historical Extinctions: A Rosetta Stone for Understanding 
Prehistoric Extinctions" in Quaternary Extinctions, Paul Martin and Richard 
Klein, eds., 1984, along with other articles in that publication.)