This document outlines the objectives, strategy, findings and limitations of a continent-wide assessment of landcover disturbance undertaken by CSIRO staff on behalf of the Biodiversity Unit of DEST.
The assessment had two objectives: to use contemporary (1990-1992) satellite image data to map the type, severity and extent of landcover disturbance across the continent; and to integrate these data with existing understanding to explore the implications of the landcover disturbance for biotic impoverishment, the loss of biodiversity.
In the formulation of these two goals, it was argued that landcover is a tangible surrogate for habitat for all terrestrial life, and that disturbed landcover represents a change in habitat suitability that was usually a deleterious change.
The principal findings were three. First, within the intensive landuse zone of the continent, we found that as of 1990, a total of 1, 550, 000 km2 of forests and woodlands have been cleared or thinned over the preceding 200 years. This represents 52% of this landuse zone and 20% of the entire continent.
These figures are unexpectedly high, but nevertheless plausible. Assuming a worst-case error analysis, the minimum figure was at least 1, 240, 000 km2. Either of these two values revise the current estimates of clearing upwards. Within this landuse zone, individual landcover types have levels of clearing that range between 30% and 99%, with an overall mean value of 47%. Almost everywhere within this landuse zone, the degree of fragmentation of the remaining landcover was high.
Second, within the remaining 4, 708, 000 km2 of the continent, the extensive landuse zone, 37% of the continental area was assessed to be slightly disturbed, 9% was substantially disturbed, with 15% interpreted as significantly disturbed. This distribution of disturbance more strongly reflects the resilience of different landcover types than the standard of pastoral management.
Combining these figures indicates that more than one third (35%) of the continent was significantly disturbed out of the total 81% that was subject to active landuse. The remainder was allocated to unused (12%) or conservation reserves (7%). Thus almost half of the landcover subject to landuse was significantly disturbed. A global comparison indicates that Australasia and South America have equal lowest proportions of significantly disturbed landcover (38%), with Europe containing the highest (86%).
While the landscapes of Australia are not as disturbed as those of Europe, the extent of clearing is pervasive. The view from space of the agricultural areas of this continent reveals the little native vegetation that is left with which to conserve the remnant biodiversity for the future.
By comparison, in the extensive landuse zone, the Australian Outback, the prospect is far less daunting. Though disturbed, the landcover remains relatively intact, and the standard of land management can always be improved.
Considered together, the disturbance of clearing and grazing has been, and will continue to be, a significant driver of the process of biotic erosion; perhaps the most significant driver. The landcovers that have been most disturbed were the richest country; those that enjoy the highest rainfall and therefore have the highest productivity. Between them, the landuses of agriculture and pastoralism have 'taken the eyes of the land'. While we do not yet have the quantitative functional relationships to support this conclusion, we speculate that it must be so. The case studies, which cover the bulk of the continent, support this view.
Third, by examining the protection offered by the different tenure types across the continent, we conclude that the vulnerability to future disturbance is high for all landcover types. Looking to the future from the landscapes of today, it is evident that conservation management by means of tenure protection, ie the dedication of land in reserves or parks to preserve biodiversity, is a limited strategy. Effective, off-park conservation strategies are required to integrate the conservation of biodiversity with other present and future landuses.
While it will be difficult, the broad scale integration of conservation and landuse in the future is possible. Landuse is a social activity and is therefore manageable by social means. Unfortunately, one significant threat to the success of future conservation efforts remains. Analysis of the distribution of four feral species (rabbit, cat, fox, pig) thought to be significant drivers of biotic erosion suggests that their presence exacerbates the effects of landcover disturbance. While cats were mostly evenly distributed across the continent, the highest densities of rabbits, foxes and pigs coincide with the most severe and extensive landcover disturbance, clearing.
The acceptance of the findings presented here is largely determined by the credibility of the interpretation of landcover disturbance from satellite and its linkage with biodiversity. The interpretation that was finally used was one of many that were exhaustively tested during this project. We strove to minimise errors of both commission and omission, and where possible, we tested our findings against other estimates to indicate the direction and magnitude of error. Nonetheless, uncertainty remains; most critically with the assessment of clearing. .
The results are presented in a pragmatic spirit of applied scientific research - to present the best understanding possible at this time. This was the same spirit and intent that motivated the publication of the Atlas of Australian Resources series; in particular, the most recent Vegetation volume (AUSLIG 1990). Australia is, as the result of considerable past efforts, a well-mapped continent. This raft of resource knowledge supports not only government policies but also public debate about those policies. We hope that this compact disk, and its summary in printed format, Graetz et al, (1995), will also contribute to a better understanding of the Australian continent.
