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. There were three major results of our study:

  1. Vascular plant richness and abundance were greater under shrub canopies than in the open, but there were no differences in richness, abundance or composition of plants emerging from the soil seedbank. This suggests that the seedbank is relatively resilience, even after decades of overgrazing.

  2. Shrub cover > 50% had no effect on any soil fertility (dissolved C, ammonium, available P, N:P) or soil microbial nutrient (microbial biomass C, N and P, N:P) variables, supporting the notion that soil fertility saturates at high levels of woody cover, and consistent with our recent conceptual model in Eldridge and Soliveres (2014). More nitrogen occurred in soils beneath N-fixing (22.3 mg N kg-1) than non N-fixing (18.0 mg N kg-1) shrubs, irrespective of shrub cover.

  3. Increased grazing reduced biological soil crust richness, but increasing shrub cover increased richness. This suggests a direct increase in the size of the biocrust species pool under shrubs and a homogenisation of crust community with heavy grazing. The effects of grazing and shrub cover in the interspaces were direct, and controlled by environmental filtering. Under the shrubs, however, effects were mediated by intransitivity (the interactions among different soil crust species), where species pools are larger, due to reduced stress.

Overall our study indicates the positive effects of shrubs on a range of ecosystem processes and functions but reductions in function under heavy levels of grazing.


Maestre, F.T., Eldridge, D.J., Soliveres, S., Kéfi, S., Delgado-Baquerizo, M., Bowker, M.A., García-Palacios, P., Gaitán, J., Gallardo, A., Lázaro, R. and Berdugo M. (2016). Structure and functioning of dryland ecosystems in a changing world. Annual Review in Ecology, Evolution and Systematics 47, 215-237.

Delgado-Baquerizo, M., Maestre, F.T., Gallardo, A., Eldridge, D.J., Soliveres, S., Bowker, M.A., . . . Zaady, E. (2016). Human impacts and aridity differentially alter soil N availability in drylands worldwide. Global Ecology and Biogeography, 25, 36-45.

Delgado-Baquerizo, M., Maestre, F.T., Eldridge, D.J. and Singh, B.K., (2016) Microsite differentiation drives the abundance of soil ammonia oxidizing bacteria along aridity gradients. Frontiers in Microbiology, 7, Article no. 505.

Eldridge, D.J., Poore, A.G.B., Ruiz-Colmenero, M., Letnic, M. and Soliveres, S. (2016). Ecosystem structure, function and composition in rangelands are negatively affected by livestock grazing. Ecological Applications 36, 1273-1283.

Maestre, F.T., Eldridge, D.J. and Soliveres, S. (2016). A multifaceted view on the impacts of shrub encroachment. Applied Vegetation Science 19, 369 -370

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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