Western Australian mulga shrublands
Near the goldfields, Western Australia Photograph by Alec Holm
The Western Australian mulga shrublands is an ecoregion in the southwest of Australia within the Deserts and Xeric Shrublands biome. The dominant tree of this arid region is the mulga (Acacia aneura), a species whose height rarely exceeds seven metres. The areal extent of this ecoregion is approximately 177,800 square miles.
Ecology
Mulga trees and shrubs are not well adapted to regular fire; moreover, the species in mulga communities vary in their ability to survive wildfires. The mulga tree itself has a severely limited ability to resprout subsequent to fire disturbance, and relies instead on mechanisms of seed production for species survival. Many plants produce hard, woody fruits or seeds, which can not only survive intense heat, but also may require the stimulus of fire to scarify and promote germination. Long-lived seedbank stores in the arid soils is common in these mulga shrublands.
The red kangaroo, a resident of the Western Austalian mulga. CC-sa Mulga has developed extensive adaptations in this ecoregion; in particular, the taxon has thick-skinned phyllodes, which structures are optimised for low water loss performance, combined with a high oil content, sunken stomata, and a large number of minute hairs to reduce transpiration water loss. During prolonged arid intervals, the mulga loses much of its foliage, creating a further mulch layer, from which soil nutrients can be recycled.
Like most Australian Acacia species, mulga is thornless. The needle-like phyllodes protrude erectly, eschewing much of the midday sun and capturing the milder and cooler morning and evening sunlight. The scant precipitation is channeled down the phyllodes structures and is collected in the surface soil immediately adjacent to the trunk, providing the tree with a threefold increase in effective rainfall. Mulga roots penetrate deeply into the soil, seeking out sub-surface moisture. The roots also harbor bacteria capable of fixing atmospheric nitrogen and thus nutrient-poor soils
Biodiversity features
Australian false vampire bat. Source: Alan McBeth There are at least 374 taxa of macro-animal species (e.g. mammals, birds, reptiles) present in the West Australian mulga shrublands, including three endemic bird species and three endemic reptile species.
Mammals in the ecoregion include the western grey kangaroo (Macropus fuliginosus), the red kangaroo (Macropus rufus), the Vulnerable greater bilby (Macrotis lagotis), the Vulnerable Australian false vampire bat (Macrodema gigas)
Reptiles of the ecoregion include the endemic Wilson's spinytail gecko (Strophurus wilsoni), the endangered woma python (Aspidites ramsayi), the endemic and Vulnerable status yinnietherra dragon (Ctenophorus yinnietharra), and the endemic Kennedy's lerista (Lerista kennedyensis).
Birds in this ecoregion include the Near-threatened star finch (Neochmia ruficauda), which taxon is threatened by overgrazing, which activity reduces essential cover as well as food sources. other notable avafauna include the Near-threatened bush thick-knee (Burhinus grallarius), the Near-threatened Alexandra's parrot (Polytelis alexandrae) and the Near-threatened Australian bustard (Ardeotis australis).
Floristically mulga associations are generally poor with respect to biodiversity, and the mulga of western Australia are generally low in biomass density and are comprised of lower growing shrublike forms than mulga units in Queensland for example. Major genera of plantlife in the mulga are typically: Cassia, Acacia, Bassia, Aristida, Eremophila and Eriogrostis. Myoporaceae are prominent as secondary floristic components.
Conservation status
The conservation status of the Western Australian mulga shrublands is Vulnerable. This ecoregion does not have a G200 designation. The G200 designatiion is a system created by the World Wildlife Fund to establish conservation priorities among the world's ecoregions. Thus the G200 designation is intended for those ecoregions deemed most critical in terms of conservation threat, and also those ecoregions which harbor great species diversity and endemism.
Types and severity of threats
Chief threats to the ecoregion include the potential for mining expansion and some practises of overgrazing by domesticated livestock. The risk of overgrazing is accentuated by the palatability of the mulga tree for livestock; this fact is particulary significant in that the mulga has strong adaptation for survival in the hottest and driest of years. The mulga provides approximately twelve percent protein yield.
Justification of Ecoregion Delineation
The Western Australian mulga shrublands ecoregion comprises two full units under Interim Biogeographic Regionalisation for Australia (IBRA): namely Gascoyne and Murchison (Thackway and Cresswell 1995). The ecoregion is generally characterized by tall mulga shrubs.
References
- R.Thackway and I.. Cresswell. editors. 1995. An Interim Biogeographic Regionalisation for Australia: a framework for establishing the national system of reserves, Version 4.0. Australian Nature Conservation Agency, Canberra
- D.E.Boyland. 1973. Vegetation of the mulga lands with special reference to southwestern Queensland. Tropical Grasslands. vol.7, number 1
- Environment Australia. Revision of the Interim Biogeographic Regionalisation for Australia (IBRA) and Development of Version 5.1 - Summary Report. Department of the Environment and Water Resources, Australian Government.
- G.N.Harrington, D.M.D.Mills, et al. 1984. Semi-arid woodlands. Management of Australia's Rangelands. G. N. Harrington and A. D. Wilson. Melbourne, CSIRO Publishing.
- K.C.Hodgkinson. 1991. Shrub recruitment response to intensity and season of fire in a semi-arid woodland. Journal of Applied Ecology 28: 60-70
- A.A.Mitchell and D.G.Wilcox. 1994. Arid Shrubland Plants of Western Australia, Second and Enlarged Edition. University of Western Australia Press, Nedlands, Western Australia. ISBN 1-875560-22-X.
A portion of the Justification of Ecoregion Delineation portion of this paper was prepared by Angas Hopkins of the World Wildlife Fund
