Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Accelerated erosion and loss of surface soil (continued)
Grazing is the single greatest pressure on the two-thirds of Australia that is occupied by agricultural lands. Data from the most recent agricultural census (ABS 2001: 1996-97 data) indicate that pastoral lands occupy 61%, cropping 3.8%, improved pastures 4.2%, and rough grazing lands on farms another 8%. In addition, large tracts of privately owned forests (42 million ha) and multiple use forests (13 million ha) are also grazed, comprising some 6.5% of the land area. Although domestic animals are excluded from national parks, conservation reserves and defence lands, these areas are also grazed by native and feral vertebrate herbivores.
The combination of fluctuating climatic and vegetational regimes and the buildup of feral and domestic herbivore populations and subsequent crashes has been documented many times in the tropical and subtropical environments of Australia. The cumulative long-term effect of these oscillations has not been, as pastoralists were inclined to believe, a return to the pre-drought condition. Instead there tends to be a slow and inexorable deterioration and loss of certain critical parts of the vegetational composition of each ecosystem (Ludwig and Tongway 2000, Tothill and Gillies 1991,Wilcox and Cunningham 1994). In particular, understorey herbs and grasses are lost, and the soil surface becomes hard, losing plant debris and topsoil. Restoration of this vital component of the ecosystem, so necessary for nutrient cycling, microbial biodiversity and native faunal habitat, is always uncertain and at worst impossible (Hacker 1989).
Two scenes of rangeland at Leonora, Western Australia.
Source: Ann Hamblin
Figure 4 shows that the SOI oscillates irregularly over a 3-6 year period, and that there have been 36 El Nios (strongly negative SOIs) and 25 La Nias (strongly positive SOIs) in the period from 1876 to 2000. Particularly severe droughts occurred in the 1890-1900s, the early 1940s, early 1980s, and much of the 1990s. Conversely, a buildup of stock numbers and rabbit plagues followed lush grass growth after exceptionally rainy seasons occurred in the 1920s, early in the 1950s, and in the 1970s.
Figure 4: Fluctuations in the Southern Oscillation Index from 1876 to 2000.
Source: Bureau of Meteorology 2001
The ability of rangelands to recover depends on the removal of the pressures that caused the degradation and the time to the next sustained wet phase-often more than two or three decades. Long-term exclosure studies demonstrate that removal of larger vertebrate herbivores will achieve regeneration over a period of decades, but without other methods, such as trapping sediment and litter in cut brush, the time for restitution may be many decades (Ludwig and Tongway 2000). In Figure 5, desertification is exemplified by an expansion of denuded, exporting regions to low-lying, accumulating patches wherever there is a transfer of surface soil and nutrients, seeds and insects.
Figure 5: Trigger-transfer-reserve-pulse sequences in rangeland rehabilitation.
Source: after Ludwig and Tongway (1997)