Biodiversity Theme Report

Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Dr Jann Williams, RMIT University, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06749 3

The Meaning, Significance and Implications of Biodiversity (continued)

Australia's biodiversity: A unique heritage

  • Geological history
  • Droughts and flooding rains
  • -->

    Australia's biodiversity has been especially shaped by the features of the Australian landscape, and long periods of evolutionary isolation. Compared with other parts of the world and because of the age and deep weathering of the landscape, Australia has low relief, low and variable rainfall and low nutrient soils. Long periods of isolation from other landmasses have resulted in the evolutionary radiation of groups of terrestrial plants and animals such as eucalypts, wattle, hummock grasses and marsupials (Strahan 1983; Flora of Australia 1999). The aquatic environment also hosts a great diversity of plant and animal species, although this element of the Australian flora and fauna often receives less attention despite its ecological significance (Flora of Australia 1999).

    Geological history

    All the present continents coalesced into a single supercontinent, Pangaea, about 230 million years ago. Within Pangaea, the land masses of Australia, New Guinea, South America, Africa, Madagascar, Antarctica, India, New Zealand, Arabia and parts of South-East Asia were close together and formed the continent of Gondwana. Pangaea began to break up about 160 million years ago. Australia and Antarctica parted only 35 million years ago. At that time, the climate of Gondwana was humid and temperate and rainforest covered much of the landscape. Australia drifted north and collided with the islands in the Pacific about 15 million years ago. During this period of isolation, the continent was exposed to dramatic climatic changes. The circum-Antarctic current formed when Australia and Antarctica separated, resulting in substantially reduced temperatures at the southern pole and the formation of the Antarctic ice cap over the next 20 million years. Most of the Australian landscape was stable during the isolation of the last 30 million years, resulting in today's ancient, weathered surfaces (Archer et al. 1998).

    These geological events and the associated landscape and climate changes have created a unique legacy of flora, fauna and landforms on the Australian continent. In general, Australia's flora and fauna are a mixture of the original Gondwanan stock, modified over millions of years of separation, and the more recent arrivals from Asia. For example, in northern Australia, several plant and animal groups are shared with the islands of New Guinea and Indonesia. Many taxa (families and genera) are also shared between southern Africa, South America and Australia because of their common origins. For example, marsupials are common in Australia, South America and New Guinea, but not in Asia. The rainforest trees Nothofagus and Araucaria occur in South America and Australia. The flora of south-west Australia shares many relatives with the flora of southern Africa.

    Droughts and flooding rains

    The major components of the Australian terrestrial flora and fauna have diversified into a rich spectrum of species adapted to the vagaries of the Australian environment. The droughts, fires and floods that are a part of the Australian environment are closely tied to regional climate patterns, especially the El Niño-Southern Oscillation (ENSO) phenomenon. Rainfall, especially for eastern Australia, has a very high interannual variability. Extensive and often prolonged droughts, sometimes with severe fires in forested regions, occur during El Niño years, whereas flooding is common in La Niña years. The widespread rainfall associated with La Niña events affects the abundance of plants and animals through the filling of water bodies such as Lake Eyre and indirectly through its effect on fire regimes and pest outbreaks.

    Disturbance and climatic cycles are also important for biodiversity in marine systems. Natural agents of change in marine communities include tropical cyclones and temperate severe storms, outbreaks of predators and ENSO effects on currents and coastal ocean productivity.

    Summer wildfire in dry sclerophyll forest on Black Mountain, ACT

    Summer wildfire in dry sclerophyll forest on Black Mountain, ACT.

    Source: AM Gill, CSIRO Plant Industry.

    Source: AM Gill, CSIRO Plant Industry