How can we draw this all together?
The disturbance assessments applied to the ILZ and ELZ are qualitatively distinct and thus have been separately reported as Table 5 and Table 8.
The two sets of figures cannot be simply aggregated.
Nonetheless, we present the following argument to support a combination of these two sets of figures to produce a systematic continental assessment. Further, we then use these figures to compare the status of the Australian continent with global assessments compiled by Hannah et al (1994, 1995).
For the ILZ, we have argued that with impact on biodiversity, the thinned and cleared classes should be aggregated. By far the largest proportion of the indeterminate class area was grasslands or shrublands; disturbed by grazing but not cleared. This class cannot be ignored but it should not be equated to uncleared.
In our opinion, compared with the impact on biodiversity of clearing, the three levels of grazing disturbance assigned for the ELZ are far less severe or permanent. The indeterminate class can by default be added to the slightly disturbed class.
Therefore, we create a combined scale of landcover disturbance with three levels: undisturbed (uncleared, slight), partially disturbed (indeterminate clearing, substantial grazing), and significantly disturbed (thinned and cleared, significant grazing). We stress that this aggregation is based on personal judgement. What is more important, it ignores the threat of feral animals.
Table 12: Combining the ILZ and ELZ disturbance classes to produce a continental aggregate of disturbed area.
Disturbance ILZ ELZ Component Combined Continent
(km2) (km2) (%)
_______________________________________________________________________________
Undisturbed Uncleared 1059741
Slight 2773596
Indeterminate 6189
Other Other 131434 3970960 51.6
Partially Indeterminate 365350
Substantial 655700 1021050 13.3
Significantly Thinned 518223
Cleared 1029640
Significant 1152127 2699990 35.1
From the combined figure in Table 12, more than one third (35%) of the continent was significantly disturbed. From the tenure figures of Table 4, it can be determined that only 81% of the continental area is subject to active landuse. The remainder is allocated to unused (12%) or in conservation reserves (7%).
From this we can conclude that almost half of the landcover subject to landuse was assessed by us as significantly disturbed.
How should we to interpret Table 12 in terms of biotic erosion?
Many concerned individuals regard the land reserved by conservation tenure as the only area worth considering in the difficult task of preventing or slowing biotic erosion: only the National Parks and nature reserves matter, the rest is lost.
In contrast, Pressey (1995), proposes a wider and more useful plan for effective conservation to which we subscribe.
We believe the critical areas to be those which are significantly disturbed. These are the areas most in need of remedial management.
In addition, we propose that for conservation management, the partially and undisturbed areas are equivalent and deserve a lower priority for action than the significantly disturbed areas. However, we note that we are here ignoring the relentless impact of feral species.
Lastly, how do the overall figures for Australia compare with the other continents?
The most recent global figures on disturbance are those compiled by Hannah et al (1994, 1995). Using different techniques, they produce a three-level classification of disturbance that was overall more severe than reported here. In our opinion, their most disturbed category, human dominated, represents a greater level of disturbance and irreversibility than used here. This was shown by their assessment of Australasia being 62% undisturbed, 26% partially disturbed, and 12% human dominated; Table 2 of Hannah et al 1994. The aggregate of their last two categories (38%) was comparable with our most disturbed category (35%).
Using this relationship, we recalculate Table 2 of Hannah et al (1994) to produce a disturbance ranking of all the continents. In relative terms, Australasia and South America have equal lowest proportions of significantly disturbed landcover (38%), with Europe containing the highest (86%).
Disturbance ranking of all the continents
A comparison of the proportion of five continents that is significantly disturbed. The data were recalculated from Table 2 of Hannah et al (1994). Note these figures are for Australasia.
