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

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.

Publications

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

Zaady, E., Eldridge, D.J., Bowker, M.A., Ochoa Hueso, R. (2016). Effects of local scale disturbance on biocrust. Ch. 21. In: Weber, B., Belnap, B., Büdel, B. (eds.): Biological soil crusts: an organizing principle in drylands. Pp. 429-450. Ecological Studies. Springer.

Rosentreter, R.R., Eldridge, D.J.,Westberg, M., Williams, L. and Grube, M. (2016). Structure, composition, and function of biocrust lichen communities. In: Weber, B., Belnap, B., Büdel, B. (eds.): Biological soil crusts: an organizing principle in drylands. Pp. 121-138. Ecological Studies. Springer.

Bowker, M.A., Belnap, J., Büdel, B., Sannier, C., Pietrasiak, N., Eldridge, D.J. and Rivera-Aguilar, V. (2016). Controls on distribution patterns of biological soil crusts at micro- to global scales. Ch. 10. In: Weber, B., Belnap, B., Büdel, B. (eds.): Biological soil crusts: an organizing principle in drylands. Pp. 173-198. Ecological Studies. Springer.

Maestre, F.T., Bowker, M.T., Eldridge, D.J., Cortina, J., Lazaro, R., Gallardo, A., Delgado-Baquerizo, M., Berdugo, M., Castillo-Monroy, A.P., Valencia, E. (2016). Biological soil crusts as a model system in ecology. Ch 20. In: Weber, B., Belnap, B., Büdel, B. (eds.): Biological soil crusts: an organizing principle in drylands. Pp. 407-428. Ecological Studies. Springer.

Zhang, J., Eldridge, D.J. and Delgado-Baquerizo, M. (2016). Biotic communities cannot mitigate the negative effects of grazing on multiple ecosystem functions and services in an arid shrubland. Plant and Soil, 401, 381-395.

Delgado-Baquerizo, M., Maestre, F.T., Eldridge, D.J., Bowker, M.A., Ochoa, V., Gozalo, B., Berdugo M, Val J. and Singh, B.K. (2015). Biocrust-forming mosses mitigate the negative impacts of increasing aridity on ecosystem multifunctionality in drylands. New Phytologist 209, 1540-1552.

Eldridge, D.J., Beecham, G. and Grace, J.B. (2015). Do shrubs reduce the adverse effects of grazing on soil properties?. Ecohydrology, 8, 1503-1513.

Soliveres S, Maestre FT, Bowker MA, et al. (2015) Functional traits determine plant co-occurrence more than environment or evolutionary relatedness in global drylands. Perspectives in Plant Ecology, Evolution and Systematics 16, 164-173.

Bowker, M.A., Maestre, F.M., Eldridge, D.J., Belnap, J., Castillo-Monroy, A., Escolar, C. and Soliveres, S. (2014). Biological soil crusts (biocrusts) as a model system in community, landscape and ecosystem ecology. Biodiversity and Conservation 23, 1619-1637.

Eldridge DJ, Soliveres S. (2014). Are shrubs really a sign of ecosystem degradation? Disentangling the myths and truths of woody encroachment in Australia. Australian Journal of Botany 62, 594-608

Eldridge DJ, Wang L, Ruiz-Colmenero M. (2014). Shrub encroachment alters the spatial patterns of infiltration. Ecohydrology, 8, 83-93

Soliveres S, Maestre F, Eldridge DJ, Delgado-Baquerizo M, Quero JL, Bowker MA, Gallardo A (2014). Plant diversity and ecosystem multifunctionality peak at intermediate levels of woody cover in global drylands. Global Ecology and Biogeography 23, 1408-1416

Ulrich W, Santiago S, Maestre FT, Gotelli NJ, Quero JL, Delgado-Baquerizo M, Bowker MA, Eldridge DJ (and 45 other authors) (2014). Climate and soil attributes determine plant species turnover in global drylands. Journal of Biogeography 41, 2307-2319.

Delgado-Baquerizo, M., Maestre, F.T., Gallardo, A., Bowker, M.A., Wallenstein, M.D., Quero, J.L., Ochoa, V., Gozalo, B., Garcia-Gomez, M., Soliveres, S.,…. Eldridge, D.J. et al. (2013) Decoupling of soil nutrient cycles as a function of aridity in global drylands. Nature 502, 672-676.

Eldridge D.J., Soliveres S., Bowker M.A. and Val J. (2013). Grazing dampens the positive effects of shrub encroachment on ecosystem functions in a semi-arid woodland. Journal of Applied Ecology 50, 1028-1038.