Environmental Change in Greater Australia

Article excerpt

Australia, a dry island continent in mid latitude, spans from tropical to cold temperature regions; long isolation has given it its own flora and fauna. Environment changes in the late Quaternary have had their own and special courses in the continent and its several regions. The role of fires set by people is an important issue in the changing `natural' landscape.

Globally, the Pleistocene - Holocene transition incorporated some of the most dramatic environmental changes in earth's history. It extended from the height of the last glacial period, when ice sheets covered about 30% of the earth's surface (Williams et al. 1993 and sea-level was estimated to have been c. 120 m lower than today (Fairbanks 1989), to the attainment of approximately present-day conditions. These changes were achieved in about 11,000 radiocarbon years from c. 17,000 to 6000 b.p.; noted discrepancies between radiocarbon and uranium-series dates on submerged corals around Barbados recently used to date sea-level rise suggest that the transition may have begun at least 2000 years earlier (Bard et al. 1990). However, as most data for this period are based on radiocarbon dates, the conventional radiocarbon time-scale is retained in this paper.

A recent global comparison of various palaeoclimatic data and simulations from general circulation models concluded that the major influences on climates during the transition period were deglaciation and variations in the seasonal and latitudinal distribution of solar radiation caused by changes in earth-sun geometry (Webb et al. 1993). These influences combined to produce marked regional differences in climatic amelioration through their effects on atmospheric and oceanic circulation patterns. Australia, although geographically divorced from the major Northern Hemisphere ice sheets, still experienced substantial environmental change, with the climatic component perhaps augmented by the impact of people. A number of studies indicate that initial burning by Aborigines was sufficient to increase the distribution of fire-promoting vegetation, particularly eucalypt woodlands and forests, and lead to substantially greater environmental instability than experienced in other non-glaciated parts of the world and also during previous climatic cycles within Australia. The effect of this instability, in the form of increased erosion, sand-dune activity, and salinity and greater variability in river flow patterns, would have been greatest towards the end of the Pleistocene when conditions were generally much drier than today (Kershaw 1989). Conversely, the continuation of burning in the Holocene may have prevented the achievement of potential post-glacial amelioration. The period is also distinguished from previous transitions in lacking the impact of a megafauna that may also have succumbed to the activities of people (Flannery 1994).

Present day environments

Australia is characterized by its isolation in the mid-to-low latitudes of the Southern Hemisphere; it has ancient, weathered and low-lying land surfaces, widespread aridity and climatic variability, and a high and widespread incidence of fire. These features have led to the evolution of a vegetation cover dominated by sclerophyllous woodlands and shrublands with eucalypts, casuarinas and phyllodinous acacias, taxa largely confined to the continent, and the more cosmopolitan Chenopodiaceae in saline areas, as major canopy components (Figure 1). Mesic communites are confined to the better-watered and frequently more fertile and mountainous southwestern corner, northern margin and east coast where rainforest communities or elements survive in locally wet or fire-protected patches surrounded by eucalypt-dominated sclerophyll forests. Herbaceous or grassland communities are restricted to heavy soils such as the alluvial plains of the Great Artesian Basin and in parts of the volcanic western plains of Victoria, to the nutrient-poor quartzites of the wet southwest of Tasmania and to small areas of alpine vegetation in the very southeast of the continent. …