Most of us are aware of the impact that grazing and trampling by stock and rabbits can have on plant communities across the arid zone. However, little is known about the effects of native grazing animals, particularly medium sized mammals which are now largely extinct across most of arid Australia.
The Arid Recovery Reserve is home to a number of these mammals, including burrowing bettongs, bilbies, bandicoots and stick-nest rats. While establishing populations of reintroduced species within the Reserve has been very successful, we also need to know what effect these animals are having on our plant species.
Since 1997, a number of monitoring sites have been established by Arid Recovery, in collaboration with DEWNR’s Pastoral Branch. Arid Recovery has been monitoring plant cover, condition, diversity and recruitment to determine the effects of introduced herbivores (i.e. rabbits) and reintroduced herbivores (i.e. our bettongs, bilbies, bandicoots and stick-nest rats) on native vegetation communities within the arid zone, and to determine the strongest indicators of change.
Arid Recovery was recently awarded a grant from the South Australian Native Vegetation Council to analyse the data collected as part of the vegetation monitoring, which provided the opportunity to examine hypotheses based on what sorts of changes we expect to see at a landscape scale. A number of researchers at Arid Recovery have also carried out specific studies to investigate whether native herbivores actually provide a benefit to the arid ecosystem, rather than the devastating impacts of introduced herbivores such as rabbits.
The changes in vegetation that we would expect to see include a reduced impact of native browsers on plant recruitment compared to that of introduced species, with higher survival and growth rates of seedlings in areas where rabbits and livestock are excluded. The level of impact, however, is likely to vary depending on the density of individuals within an area.
One Arid Recovery study showed that low to moderate densities of bilbies, bettongs and stick-nest rats have a significantly lower impact on shrub recruitment than low densities of introduced cattle and rabbits. Recruitment of common species such as mulga, silver cassia and sandhill wattle was reduced or arrested by even low densities of cattle and/or rabbits. Evidence from this study suggested that mulga may suffer regional declines if recruitment continues to be suppressed by rabbit and cattle browsing. Another study conducted at Arid Recovery has shown that foraging pits dug by bilbies and bettongs may facilitate seedling recruitment. Bilby digs can trap moisture and minerals, creating nutrient rich soil.
A bilby dig at the AR Reserve, showing how it collects seeds, grasses and plant matter, increasing nutrients in the soil at that point.
Arid Recovery is continuing to monitor vegetation changes in response to the presence of introduced and native herbivores. Continuing analysis of the monitoring data should allow us to determine the most effective indicators of change, not just within Arid Recovery but on a landscape scale. Our research will highlight the importance of establishing indicators of overstocking of native species in fenced areas like Arid Recovery, may also assist with developing novel revegetation strategies such as changing the habitat for direct seeding by mimicking animal digs.