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
Introduction of novel biota into native habitats and communities (continued)
Impact of agriculture on conservation land: edge-effects [L Indicator 4.2]
One of the most pervasive pressures on lands reserved for conservation comes from the incursion of exotic pests and weeds (Saunders et al. 1991). Even where parks are very large there are problems maintaining effective barriers against natural dispersal. An attempt to assess the effect of agriculture on native vegetation in conservation reserves was made in the National Collaborative Project on Sustainable Agriculture (SCARM 1998). A composite indicator related the 'edge-effect' of the boundary length between agriculture and reserves, the area of natural habitat relative to agricultural area, and the intensity of agricultural inputs in a matrix of variables. Low values for each variable indicated that there was a minimal impact of agriculture on conservation reserves, and high values indicated a heavy impact. Results were reported for agro-ecological regions (Figure 45).
Figure 45: Agro-ecological regions of Australia.
Source: SCARM (1998)
The most heavily affected areas (where all three variables have a high value) were in the subtropical slopes and plains of New South Wales and Queensland, while the least affected were in the north-western and northern wet-dry tropics and arid interior.
An unexpected region of high impact was in the semi-arid tropical-subtropical plains. In this region agriculture is almost exclusively based on low-intensity grazing of native pasture, but conservation reserves are few and small, and therefore very vulnerable. This is also a region in which land is being cleared at higher rates than most of the rest of the country.
While this indicator tells us in general terms what is happening to conservation reserves, it cannot assess the status of native habitats that exist outside gazetted areas. Yet in many regions where there may be few parks and reserves there may be a substantial cover of native vegetation actually within agricultural tenures. Increasingly there is a recognised need to preserve more biodiversity in situ and hence in non-conservation tenures (Hale and Lamb 1997).
Further work on the use of this indicator at a regional scale is described in the case study for South Australia.
In South Australia the SCARM indicator has been applied at state and regional scales (Duncombe-Wall et al. 1999). At a state level, South Australian remnant native vegetation is most vulnerable to the impact of agriculture in the low-rainfall cropping region and in the hilly higher-rainfall regions because numbers and areas of reserves are low in these areas. However, this composite indicator did not provide sufficient detail to help identify the threat to remnant vegetation located within larger areas of agricultural land unprotected by conservation status.
More detailed land use studies are needed for this type of analysis, where the intensity of land use is distinguished and ground-truthing is carried out (Baxter and Russell 1997). In a pilot study in the south-east of South Australia, five areas were identified where native, remnant vegetation that exists outside conservation parks is considered to be threatened (see figure 46).
The map shows that much of the surviving remnant vegetation in the region exists in a series of broad sweeping ribs that follow the alignment of the coast. These are Pleistocene and Recent sand-dune ridges, and they have been left uncleared because the land was of low fertility. Grazed, rain-fed pastures predominantly surround the native vegetation in the five threatened areas, with no fencing protection from grazing animals. Two of the threatened areas have a considerable proportion of the land in crops, vineyards or dairy pasture. In all instances the remnant vegetation exists in small patches of less than 1 hectare, with little chance of natural regeneration. Current trends in land use suggest that more of these areas will be converted to crops and vineyards, with very small patches of vegetation cleared, as no permit is needed to clear such tiny isolated remnants.
Figure 46: Threatened habitat areas in south-eastern South Australia, with simplified land use legend and categories.
Source: Primary Industry and Resources, South Australia (unpublished)