Jill Landsberg, Craig D. James, Stephen R. Morton, Trevor J. Hobbs, Jacqui Stol, Alex Drew and Helen Tongway
CSIRO Division of Wildlife and Ecology
Biodiversity Convention and Strategy Section of the Biodiversity Group, Environment Australia, January 1997
ISBN 0 6422 7010 4
Artificial waters are a potential threat to the persistence of many components of native biological diversity in arid and semi-arid Australia. Supplies of water have proliferated in the rangelands since settlement for pastoral purposes. Today, few areas of pastoral rangeland are further than 10 km from an artificial source of water. This widespread provision of water allows large grazing animals – principally sheep and cattle but also kangaroos and feral livestock – to graze virtually all of this rangeland. Our study aimed to determine the effects of the provision of artificial waters and of the grazing it allows, on the native plants and animals inhabiting two of the major biomes of inland Australia.
The study was based on field surveys which sampled biodiversity along gradients in grazing intensity extending out from artificial water sources. Each gradient continued to a reference site remote from all waters, where grazing by stock was minimal. Five further sites were sampled along each gradient, at locations progressively closer to the artificial water. Four such gradients were surveyed in chenopod shrublands and four in acacia woodlands at widely separated localities across the pastoral rangelands of central and southern Australia. At each of the six sites along each gradient systematic samples or counts were collected of understorey plants, overstorey plants, plants in the soil seedbank, birds, reptiles, small mammals, ants, beetles, springtails, grasshoppers and crickets.
The gradients were particularly rich in plant species, a quarter to a half of which were detected only in the soil seedbank and would not have been apparent from once-only field surveys. The most species-rich animal taxa were ants and birds. Most of the invertebrate species collected were new to science. Several new plant species were also identified. Exotic species were generally a minor component of the gradients' diversity, except at the chenopod gradients, where 3-11% of plant species were exotic.
The gradients showed little evidence of severe degradation: plant cover at sites very close to water was little different from cover at water-remote sites. A pilot study of landscape function conducted at two of the gradients found indications of early stages of degradation: landscape patches close to water showed more evidence of tree die-back and may have lost some of their ability to capture, store and utilise water and nutrients.
There were major changes in the composition of biodiversity at different distances from water. The results show that some species in most groups of plants and animals increased in abundance at sites closer to water ("increasers"), but that some decreased at such sites ("decreasers").
- On average, between 15% and 38% of species in different taxonomic groups appeared to be decreasers;
- between 10% and 33% appeared to be increasers; and
- the remaining species did not exhibit any demonstrable response to the artificial water.
Nearly all decreaser species were natives. A small but significant number of the decreaser plant species at each gradient was found only at the gradient's reference site, where grazing intensity was minimal. This is of particular concern because of the extreme reduction that has occurred in the area of pastoral rangeland this far from water. Increaser species included natives and exotics. Buffel grass, Cenchrus ciliaris, was a particularly prominent exotic increaser at one gradient. No exotic bird species were detected, but many of the increasers were species whose ranges have expanded since water has been provided.
Some species of plants and animals were much more abundant in better seasons, but decreaser trends did not abate. For understorey plants at woodland gradients decreaser trends appeared more pronounced in better seasons. At the two woodland gradients surveyed after better seasons, much higher proportions of decreaser plant species were found than at two similar gradients surveyed after drier seasons. The persistence of plant species in the seed bank indicates their potential for seasonal recovery but in this group, too, there were significant proportions of decreaser species.
It is likely that most species of plants and animals show these responses because of the direct and indirect effects of the grazing which radiates out from sources of water. Some species of birds may also be directly advantaged by the availability of drinking water. Regardless of mechanisms, the ultimate cause of the changes detected in most taxonomic groups is likely to be the provision of artificial sources of drinking water.
Given how widespread artificial waters have become throughout the chenopod and acacia rangelands, the results suggest that some 15-38% of species are at risk of declining substantially throughout these lands. Although similar proportions of species are favoured by the provision of water and grazing, and from 36-75% seem unaffected by it, the challenge is to develop strategies that will provide for the persistence of the vulnerable decreaser species.
As a consequence of these results, and in the context of Australia's national commitments to conserve biodiversity, the report makes the following recommendations.
Implement a program of strategic closure of waters
- Implement (with appropriate safe guards) a staged program of strategic closure of artificial sources of water in conservation reserves in pastoral rangelands, in recognition of the small proportion of pastoral lands remaining outside the influence of water and the probable damage to biodiversity stemming from widespread provision of water.
- Promote adaptive management experiments by land managers outside conservation reserves in pastoral regions, to test selective water-closure as a tool for achieving balanced production and conservation goals.
- Determine economic and ecological costs and benefits of alternate scenarios for achieving regional conservation goals including:
- strategic water closures to provide different designs of unwatered areas
- control of grazing areas using other means (fencing, stock management)
- Using this knowledge, devise appropriate incentives for pastoralists to conserve biodiversity. Note that cost-effective monitoring of biodiversity (points 7-9) is necessary for assessment of progress toward its conservation.
Establish a regional perspective
- Undertake a detailed assessment of the spatial distribution of existing artificial waters in the major rangeland bioregions of Australia, to identify bioregions for priority attention.
- Undertake strategic surveys in representative bioregions to determine:
- what proportion of regional biodiversity can be identified as decreaser species, and to what extent this is influenced by variation in landscapes and management history;
- what proportion of the regional area provides suitable habitat for the persistence of decreaser species; and
- what landscapes and habitats within the region need particular conservation emphasis.
Develop cost-effective methods for survey and monitoring
- In monitoring and surveying rangeland biodiversity use cost-effective methods that:
- target the plant groups and animal taxa identified in our study as rich in species, efficient to sample and representative of a wide range of habitats and niches;
- are conducted relative to "reference sites" representing as closely as possible the state of the environment before its management for pastoralism;
- contribute to the development and maintenance of archival collections of lesser known groups, particularly invertebrates, but also understorey plants.
- Undertake research to identify biodiversity attributes for incorporation in assessment and monitoring programs. Based on our results we recommend:
- that classification schemes be developed to group species into "indicator response types" based on biological attributes and response to disturbance. These schemes should complement taxonomic identification in ways that clearly relate to the mechanisms of species persistence and the attributes of species at most risk.
- Undertake research to determine relationships between biodiversity and landscape pattern, in order to identify broad-scale, landscape-based surrogates for monitoring biodiversity.