Does overgrazing reduce the ecosystem service values provided by shrublands in semi-arid Australia ?

A project undertaken at the School of Biological, Earth and Environmental Sciences, University of New South Wales and supervised by Dr David Eldridge

Australia’s semi-arid grazing lands have become severely degraded after more than 200 years of overgrazing by domestic and feral herbivores. Overgrazing reduces plant and animal richness, alters soil, water and nutrient flows, and affects key ecosystem processes, resulted in landscape degradation. Landscape degradation in semi-arid Australia is also associated with shrub encroachment, a global phenomenon characterised by an increase in the density of native woody plants. The contemporary view of shrub encroachment as a degradation process, however, contrasts with the extensive body of literature reporting positive effects of shrubs at the individual plant or patch level. Other evidence challenging the degradation view are many reports of neutral or positive effects of encroachment on various ecosystem attributes and functions in several studies worldwide. This putative link with degradation constrains our ability to develop strategies to manage encroached shrublands for alternative landuses as broad as wildlife conservation, ecotourism and carbon farming.

The notion that encroachment is negative and leads to degradation is often promoted in rural Australia. Different perceptions of shrublands by a range of users at different sites could relate to the degree to which sites are grazed, as overgrazing leads to land degradation and the persistence of encroachment. Preliminary research suggest that grazing dampens the positive effects of isolated shrubs on ecosystem processes in some areas. An important consideration, therefore, is how to reconcile the oft-reported negative view of shrubs at large (site) scales with their well-known positive effects at smaller (plant or patch) scales.

In this project we test two predictions about how shrubs and grazing affect shrubland processes and services: (1) that the positive effects of isolated shrubs on ecosystems wane under high shrub densities due to reductions in microhabitat heterogeneity, and (2) that overgrazing, and hence land degradation, drive the relationship between encroachment and degradation. Despite decades of research, no studies have addressed these predictions. We expect our results to help us to refine government policy, which is currently based on shrub density not on intensity of grazing. This is expected to lead to more effective management of semi-arid shrublands and the plants and animals that they support.

 
Figure 1.Mixed shrubland-woodland systems can be highly diverse and productive.

Figure 2.Shrubs are sites of enhanced productivity due to their influence on water flow

Figure 3. Calculating the ecosystem services values of shrublands requires assessment at a range of spatial scales from individual shrubs to landscape levels