Changes in Vegetation

The essential management problem in attaining a sustainable cattle-based range system is to match a variable grazing capacity with cattle stocking rates. Erratic or uneven rainfall, lack of accurate carrying capacity estimates, and a mix of cultural and economic factors usually complicate the situation. Extending the available grazing area by developing boreholes tends to allow for a larger sustainable herd and this is a common solution in a majority of rangelands. This widespread provision of water allows large grazing animals – principally livestock such as cattle and sheep, but also wildlife – to graze virtually all of the rangeland. | Pressure State Response Further reading

Pressure

Increased grazing and trampling pressure around the boreholes causes vegetation zonation with a species poor zone in the immediate vicinity of the boreholes, followed by thickets and transitional zones leading to vegetation that is closer to the "natural" state at the maximum distance to which livestock travel from the artificial water supply. There is an increase in soil nutrients near the boreholes due to concentrated deposition of dung and urine, the amount dependent on livestock density and borehole age. The increased soil fertility is reflected in the nutrient content of the herbs and trees near the boreholes.

Changes in the ecology and soil nutrient status, the contribution of boreholes to vegetation change (and environmental degradation) needs to be viewed in terms of:

- The density of boreholes,
- Stocking levels,
- the radial nature of range usage - giving rise to marked spatial variations in impact

However, in many cases sensitivity to degradation is complex and may be affected by factors other than simply time and stocking levels.

Artificial water sources form a potential threat to the persistence of many components of native biological diversity in arid and semi-arid areas. Boreholes have proliferated in many rangelands and in some rangelands (e.g. in Australia, Botswana) few pastoral areas are further than 10 km from an artificial source of water.

Impact on native plant species varies. Studies in Australia indicate that some species react to the grazing pressure by a decrease in density, whereas others may increase. It is likely that those species which are either more palatable to livestock, and/or more nutritious, will suffer a decrease in density. Results of the Australian study indicate that some 15 to 38% of species are at risk of declining substantially throughout the rangelands as a result of the provision of artificial sources of water for livestock.

State

State can be characterised by:

- Numbers and density of boreholes;
- Change in livestock population numbers;
- Change in vegetation species composition in areas close to artificial water sources, together with different changes in areas at a distance from the water sources;
- Changes in the distribution and densities of wildlife species, especially in relation to distance from artificial water sources;
- The area of arid and semi-arid land that can be regarded as being close to artificial sources of water - e.g. less than 10 km distant; and.
- Rates of construction of new boreholes (e.g. as a result of pressure from the livestock sector).

Response

Response is then characterised by the following requirements and activities:

- Undertake a detailed assessment of the spatial distribution of existing artificial water sources to identify regions for priority attention. Integration of data from literature, maps, borehole records, Global Positioning System (GPS) receivers, field observations, and remote sensing platforms are essential in creating a GIS-based hydrogeologic model of the study area adequate to support strategy development;
- Undertake strategic surveys in representative areas to determine the changes in abundance of plant species with distance from artificial water sources;
- Develop cost-effective methods for survey and monitoring, using classification schemes that group species into "indicator response types" based on biological attributes and their response to disturbance;
- Conduct research into and promote adaptive management by land managers to use water closure as a tool for achieving a balance between production and conservation goals;
Implementation of a programme of strategic closure of artificial sources of water on rangelands – to enable selected areas to recover from the effects of high grazing pressure, trampling and artificially increased levels of nutrients, and to enable the preservation of some pastoral lands remaining outside the influence of artificial water sources.

Further Reading:

Perkins, J.S., Thomas, D.S.G., Allison, R.J. 1993. Environmental responses and sensitivity to permanent cattle ranching, semi arid western central Botswana. Landscape sensitivity. 1993, 273 286

Landsberg, J., James, C.J., Morton, S.R., Hobbs, T.J., Stol, J. Drew, A. & Tongway, H. 1997. The Effect of Artificial Sources of Water on Rangeland Biodioversity. Final Report to the Biodiversity Convention and Strategy Section of the Biodiversity Group, Environment Australia. Environment Australia and CSIRO. Commonewalth of Australia.

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