Table 22 Summary of vegetation condition Resource condition derived from traverse assessments PA Waddell Perennial Proportion (%) of One objective of rangeland surveys in Western vegetation condition traverse assessments Australia is the assessment of grazing impacts Very good 8 on natural resources, focusing on the use of Good 58 land for pastoralism. Other land use impacts Fair 26 have not been addressed in a similarly detailed Poor 7 manner primarily because pastoralism is the Very poor 1 most extensive land use. Land uses such as mining activity were found to be localised. With these findings it is important to consider In the context of this report resource condition the irreversible alteration of vegetation commu- has been determined by assessing the bio- nities due to various ecological disturbances. physical indicators: perennial vegetation and Some vegetation communities have undergone soil surface condition, both of which provide ecological changes so dramatic the original measures of ecosystem change. There are perennial species have been replaced by an three methods of assessing resource condition: annual component. In the 1974 Western • visual traverse assessments at kilometre Australian Nullarbor Plain survey Mitchell, intervals McCarthy and Hacker (1979) describe many land systems as having chenopod shrubland • collection of ecological data during field work communities; this information supports findings from inventory sites from Beard’s (1975) Nullarbor vegetation • aerial photo interpretation and ground survey. During 2005–2007 whilst re-traversing verification. various routes of the 1974 survey there was an absence of chenopod shrubland in areas where Summary of traverse assessments it had been previously recorded as dominant. The elimination of large areas of chenopod Traverse assessments were used to assess shrubland from areas in which it formerly resource condition in the survey area, as occurred is likely the combination of ‘drought’, described in the Methodology chapter. Range fire and rabbit impact. Past biological surveys in condition was assessed within a 50 m radius the region (Beard 1975, 1990; McKenzie & around the vehicle at 1 km intervals along pre- Robinson 1987) comment on the effect of rabbit selected traverse routes. These assessments grazing in altering the landscape. Accepting were visual subjective ratings using objective these changes as permanent (Beard 1975; assessment criteria. The rating system is Davey 1978; Mitchell, McCarthy & Hacker described in the Methodology chapter. 1979; Westoby, Walker & Noy-Meir 1989) some In the survey area a total of 6997 traverse areas now in irreversible transition were points were described and assessed for assessed on their present form rather than various biophysical parameters; these are speculating on their former state. This has summarised in Table 22. Within the boundaries resulted in some habitat types describing the of pastoral leases 6276 traverse points were present features and composition of an area as assessed for range condition. the stable state, rather than considering it as a former state in poor condition. A total of 91.6 per cent of all traverse assess- ments indicated vegetation was in the very It is also necessary to consider these findings good, good or fair categories. This may be in the context of a biased sampling technique. regarded as acceptable condition. The In attempting to achieve a representative remaining 8.4 per cent of regular traverse coverage across the survey area large portions assessments showed obvious signs of pastoral of some leases were traversed through overgrazing resulting in a poor or very poor relatively undeveloped country. Lease develop- condition assessment, with considerable loss of ment is strongly influenced by the nature of the palatable perennial plants or general loss of terrain which limits suitable dam sites or the perennial plants or marked increases in cover presence, quality and reliability of aquifer water. by unpalatable species. In terms of the overall condition of the native vegetation, the bias may be considerable as 323 Mixed chenopod shrubland: pearl bluebush The same land unit patterns where the chenopod (Maireana sedifolia) dominates the stony plain whilst cover has been replaced by the dominance of bladder saltbush (Atriplex vesicaria) dominates the speargrass (Austrostipa scabra). low-lying corridors between the stony rises. many areas not developed for pastoral use are pastoral induced erosion cells. Forms of traversed, particularly when attempting to use erosion away from water points were assessed routes that did not require backtracking. In on their origin—anthropogenic or karst addition 721 of the 6997 traverse points were induced—and this was taken into account assessed outside pastoral lease boundaries when determining a condition rating. Where with the majority of these considered in good or erosion was attributed to pastoral activities this very good vegetative condition. contributed to downgrading of a site’s rating. Reconnaissance trips and early traverse Wind rather than water is the primary ratings recognised a high level of erosion mechanism driving accelerated erosion where across many habitat types with the associated vegetation cover has reduced through grazing dilemma of attributing the cause. In some or fire. Wind scalds were the most common locations pastoral activities have contributing to type of erosion observed, regularly exposing overgrazing resulting in accelerated erosion, fresh rock faces and root bases. Extensive soil especially the large piospheres radiating from loss has occurred around some water points. In water points. However, away from water points localised areas scalding and microterracing it is difficult to differentiate natural erosion cells was also common particularly near sinkhole that are a feature of the Nullarbor karst against entrances at the karst surface. Around some water points there has been wide- Surface disturbance through continual grazing spread soil loss through wind erosion. The lack of pressure has resulted in extensive soil redistribution; surface weathering and lichen on freshly exposed rocks perched on residual soil pedestals provide rock surfaces indicates the recent nature of the some indication of the severity of the erosion. erosion. 324 Patterns of soil and vegetation soil to support perennial vegetation, leaving condition surfaces bare during dry seasons or dominated by annual species. To prevent this form of Plant growth is restricted by the availability of permanent damage land users should monitor water with distribution influenced by land unit the condition of the soil surface and crypto- relief, soil depth and salinity. Soil erosion and gamic crusts as well as preserving vegetation vegetation condition are often closely related. A cover. Gillieson, Wallbrink and Cochrane (1996) reduction in total shrub and/or grass coverage state that at least 15 per cent projected foliar results in a decline in vegetation condition cover is necessary to prevent wind erosion. leaving soil surfaces increasingly exposed to the erosive effects of wind and water, unless the Good condition areas surface is inherently resistant. Due to factors Figure 28 shows the distribution of traverse controlled by geology and climate Nullarbor soil assessments within pastoral lease boundaries development is limited with both nutrients and of very good or good vegetation condition. A stored seed only found in the top 20–30 mm of large proportion of these points occurred in the soil profile (Gillieson, Wallbrink & Cochrane habitat types which are either largely unaffected 1996). In such a wind-blown landscape the by grazing, such as eucalypt woodlands in the importance of vegetation cover and cryptogamic west and south-west of the survey area, or in crusts are paramount in reducing erosion. undeveloped or very recently developed Since the introduction of pastoralism the pastoral areas, such as myall woodlands of the Nullarbor has experienced widespread soil calcrete Nyanga Plain. On the stony Nullarbor erosion (Gillieson, Cochrane & Murray 1994), Plain away from the summer grazing radius of with considerable soil losses recorded through stock vegetation condition was also generally the droughts of the early 1970s and 1980s. considered to be in good condition. Areas Whilst vegetation recovery may take decades, outside of present day pastoral lease the replacement of lost soil is unlikely to occur boundaries were assessed during the 1974 in such a short geological period, particularly if Western Australian Nullarbor Plain survey the soil loses its ability to support regeneration. (Mitchell, McCarthy & Hacker 1979) and are not Wind erosion has reduced the capacity of the included in Figure 28. Figure 28 Traverse assessments of very good to good vegetation condition with no erosion 325 Poor condition areas occurs in areas characterised by alluvial plains, river plains and wash plains. Such areas are Figure 29 shows the distribution of traverse highly productive and historically overgrazed. assessments within pastoral lease boundaries Accelerated erosion in these land types is of poor or very poor vegetation condition. In exacerbated by the incised nature of the drain- regional terms the areas most adversely age in these regions. However as an areic impacted by grazing are in the different forms drainage system the Nullarbor region of karst depressions which form corridors of predominantly experiences surface water loss linear depressions