Interpreting History

We don't know the precise combination of factors that led to most of South Australia's extinctions. Most historical extinctions were not documented at the time they were happening.

However, as our knowledge of living species increases, we can develop hypotheses that appear to fit the facts available. Below are three examples of the sorts of interacting problems that are believed to have caused extinctions, especially of our mammal species.

Declining resources and habitat fragmentation

All animals must be able to find enough food and the right sort of shelter. When habitats became reduced to small pockets, animal populations that remained would often have been trapped in patches of habitat that were too small to ensure their long-term survival. They became vulnerable to predation, loss of food supply and competition. The isolation of each patch of habitat prevented recolonisation, and one by one the populations disappeared until the species was extinct.

Why mammals?

Why have mammals been so hard hit? One reason may be the high energy needs of mammals, which must find large amounts of food to fuel their 'warm blooded' bodies (endothermic). Birds are also endothermic animals, but compared to mammals most are smaller and more mobile and can often move from patch to patch to get enough to survive. Reptiles, amphibians and fish are 'low energy' animals. Their body warmth comes from their surroundings or the sun (ectothermic) and they can get by on much less food than mammals of similar size. As a result, even quite small habitat remnants can continue to support reptiles or amphibians.

The arid zone

Australia, particularly South Australia, is a very dry place. Many of the mammal species that have become extinct lived in the arid zone and were medium-sized (75g to 5 kg body weight). One theory, which attempts to explain why so many were affected, is as follows.

The infertile environment of inland of Australia made it difficult for medium-sized mammals to get enough food. As a result and especially during drought, they were restricted to pockets of more fertile habitat. With the arrival of grazing stock and the rabbit, the refuge areas became so degraded that they could no longer support native mammal populations during hard times. Their refuges gone, their populations dwindled to the point where other factors, such as altered fire patterns, introduced predators and severe droughts, drove them to extinction.