By Alaric Fisher, Parks and Wildlife Commission of the NT
From Managing for Healthy Country in the VRD, Tropical Savannas CRC 2000.

Graph 1 depicts the relationship between paddock size, waterpoint configuration and total population of a decreaser species on black soil country

Figure 2 shows the relationship between distance from water and species abundance for a decreaser, the lizard Ctenotus rimacola

Figure 3 shows the effect watering points have on an increaser, the brown songlark, Cincloramphus cruralis

Vertebrate abundance

Work in the VRD and Barkly region has shown that while the effects are relatively muted, grazing on black soils will affect the composition of vertebrate species found in these areas. These effects are most pronounced for reptiles, for which the total number of animals (abundance) and the number of species found (species richness), decreases as grazing pressure increases.

Indicator species

"Decreaser" and "increaser" vertebrate species have been identified. Decreaser species are sensitive to grazing effects and are more abundant further away from watering points. The decreaser vertebrate species also tend to be largely confined to the black soil habitat, such as the lizard Ctenotus rimacola. Increaser species, such as the brown songlark, increase in abundance as grazing pressure increases or are more abundant close to watering points. The establishment of many watering points has allowed these species to spread widely across a landscape where they previously probably occurred only sparsely.

Black soil country

The implications of this are that uniform heavy stocking across a large area of black soil country will reduce or even eliminate some endemic vertebrates. Maintaining lightly grazed patches of country with a high level of perennial cover will safeguard against this situation. It follows that retaining areas that are distant to water should be a priority. Comparisons of paddock size, the positioning of water points and the total population of decreaser (grazing sensitive) species suggest that the population of these species is higher in paddocks with two corner water points than in paddocks with one central water point, because there is more water-remote country.

Red soil country

Work done in red soil country, on the other hand, suggests that current management regimes are not adversely affecting biodiversity. No trends in the abundance or richness of vertebrates were found along the grazing gradient at Kidman Springs and there was little difference between the exclosures and grazed sites. It must be stressed, however, that sampling was undertaken within a series of very good seasons and that management during poorer seasons is more likely to be critical for biodiversity conservation, land condition and production.