Abundance and composition of plant species along grazing gradients in Australian rangelands.
Abstract
The widespread provision of livestock drinking water in previously dry Australian rangelands has supported concomitant increases in cumulative grazing pressure. While the associated impacts on plants of pastoral importance have been well documented, far less is known about the rest of the flora. To address this deficiency, we measured the frequency of occurrence of all plant species at sites along water-centred grazing gradients in commercial paddocks located in the rangelands of central and southern Australia. Four gradients were in chenopod shrubland vegetation, and four in acacia woodlands. Each gradient extended 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 gradient's watering point. Ground-layer species far outnumbered those in upper layers (466 and 134, respectively). Most were geographically localized (72% found at one gradient only) and locally uncommon (46% per gradient occurred with a frequency <5% per site). Species showing trends of decreasing frequency with proximity to water significantly outnumbered those showing increasing trends. Significantly more species that were recorded only once occurred at the sites furthest from water, where long-term grazing pressure was least. Some gradients also showed an overall decline in species richness with increasing proximity to water. Most species were native, and among these there were no clearly identifiable 'global winners' (i.e. no widespread species advantaged by grazing and associated disturbances). In contrast, the majority of the few exotic species, including two of the three most widespread species found, showed increasing abundance with proximity to watering points. Synthesis and applications. The results from this study indicate consistent and substantial changes in plant composition that are probably related to the accumulated long-term impacts of water-centred grazing. The consequences are potentially severe, because artificial watering points are now extremely widespread in the Australian rangelands. Identification and protection of representative, water-remote refugia for the most grazing-sensitive species should therefore be a high priority for regional conservation.