Eastern Australia mulga shrublands
The Eastern Australia mulga shrublands are characterized by mulga (Acacia anuera) and other Acacia species on very infertile soils in a semi-arid climate. Much of the region retains a large proportion of native species although is subject to severe soil degradation. Of particular importance to biodiversity are the large numbers of ephemeral fresh-saline wetlands that provide important habitat for vast numbers of migratory bird and waterbird species when flooded.
Location and General Description
The region is located within the semi-arid inland Australia. Mean annual rainfall ranges between 450 millimeters (mm) and 650 mm and is highly variable in its distribution although it tends to be concentrated in late summer. Locally this region contrasts sharply with the eucalypt and brigalow (Acacia harpophylla) woodlands on more fertile soils and less arid climate of the Brigalow Belt to the east and the clay plains of the Mitchell Grass Downs to the north. To the west the region merges into the braided floodplains of the "Channel Country" and associated dune-fields of the more arid Simpson Desert region.
Most of the region is dominated by flat to undulating plains divided by low ranges with local relief of less than 100 meters (m). On the plains soils are generally deep but very infertile sandy earths derived from Quaternary sediments deposited over a Tertiary land surface. The low ranges occur on older, weathered geologies and are covered with stony skeletal soils. More fertile and moist soils are associated with floodplains of major rivers and smaller watercourses or from outcropping of more fertile, Cretaceous geologies.
Surface water in the region is drained by a number of southerly flowing river systems. The eastern two thirds of this ecoregion is drained by the Wallum, Nebine, Mungallala, Warrego, and Paroo Creeks/Rivers into the Murray Darling Basin which drains into the southern coast of Australia. The Bulloo River flows across the southwestern part of the region and terminates into lakes, swamps, and overflows of the adjacent Simpson Desert region. The Barcoo River flows across the northwest of the region into the Lake Eyre Basin, another central Australian landlocked drainage system. These river systems and associated floodplains support many claypans, lakes, and other wetland types which are often saline and mainly ephemeral. The unique Mound Springs wetlands are formed around natural outlets from the Great Artesian Basin aquifer that underlies most of the region. These are very limited in extent and occur in the south of the region.
Much of the plains in the region are dominated by low mulga woodlands and shrublands. Poplar box (Eucalyptus populnea), silver leaved ironbark (Eucalyptus melanophloia), and other eucalypts codominate with the mulga canopy in the higher rainfall, eastern portion of the region. The low ranges and hills support sparse Acacia shrublands while the drainage lines and associated floodplains support river red gum (Eucalyptus camaldulensis), coolibah (E. coolibah), and yapunyah (E. ochriophoia) woodlands, Mitchell Grass (Astrebla spp.) grasslands or herblands. The wetlands are often bare when not flooded and fringed by samphire (Halosarcia spp.) or sparse herb, sedge, and grasslands. Brigalow vegetation is found to the east, and gidgee (A. cambagei) woodlands or shrublands are scattered across the region on alluvium or other more fertile clay soils.
At least 747 native plants, 256 birds, 56 mammals, 94 reptiles, and 23 amphibians have been recorded from the region. The eucalypt woodlands associated with riparian areas show the highest species richness per unit area, particularly in the bird and plant taxonomic groups. The Acacia spp. shrublands on ranges show the lowest number of species compared to area, although the microhabitats they provide can support a relatively high number of endemic plant species. The relict alluvial dune fields east of the Warrego River in Queensland support a diverse "heath" flora and several species at the western edge of their distribution such as Eucalyptus melanophloia, E. ammophila, Casuarina inophloia, and Xanthorrhoea johnsonnii.
There are substantial temporal variations in species abundance and distribution associated with seasonal conditions and the alterations in faunal use of assorted habitat types at different times. A high proportion (nearly 30 percent) of the flora is annual and flourishes after the sporadic good rainfall events. Larger marsupials are common across the region. The koala (Phascolarctos cinereus) is found in greatest densities in eucalypt riparian communities but also occur in adjacent upland areas of poplar box and ranges under favorable seasonal conditions.
The many seasonal, and sometimes persistent, fresh to saline wetlands in the region supports vast numbers of birds after flooding, with over 40,000 individuals at one time. Important wetlands recognized in the region include Currawinya Lakes, Lake Bindegolly, Green Creek Swamp, Lake Burkanoko and other wetlands on the floodplains of the Paroo and Warrego Rivers. The Paroo River Catchment is notable because it has been little impacted by irrigation and water harvesting compared to other systems in the Murray Darling basin.
The wetlands are important breeding and habitat areas for several nationally threatened and/or migratory species such as freckled duck (Stictonetta naevosa), musk duck (Biziura lobata), black swan (Cygnus atratus), silver gulls (Larus novaehollandiae), Australian pelican (Pelecanus conspicillatus), great egret (Egretta alba), and glossy ibis (Plegadis falcinellus). The Currawinya Lakes support the largest populations of water birds in arid or semi-arid Australia and have been listed as a wetland of international importance under the RAMSAR convention.
The Mound Springs provide an important refuge for plants and animals in an otherwise dry environment. They provide particularly important habitat for several endemic plant and snail species. In a region dominated by poor soils, more resource rich areas, particularly mesic habitats, may provide important refuge areas to many species, especially during times of drought.
Most of the land in this region is used for cattle and sheep grazing, with smaller areas used for other purposes including conservation, mining, and oil and gas production. The area is sparsely populated. Overall 80 percent of the region retains native vegetation cover. There are relatively few threatened plant species listed for the northern part of the region. Studies in the southern part of the region suggest there has been a decline in a high proportion of mammal and bird species.
Although not widespread across the region, clearing of eucalypt woodlands and Acacia communities is extensive in the higher rainfall eastern fringes of the region. Here 50 to 85 percent of the region has been cleared and impacted by the associated introduction of exotic pasture species, fragmentation, and loss of wildlife habitat. Although less likely to be cleared, much of the western parts of the region are in a degraded condition. A study carried out in the northwestern part of the Eastern Australian mulga shrublands ecoregion in the late 1980's reported that more than two-thirds of the area showed signs of serious land degradation. This degradation is particularly severe within mulga and alluvial land types. The deterioration in diversity, ecological complexity, and functioning of regional ecosystems that has accompanied land degradation is a major cause of biodiversity loss in the region.
A number of protected areas across the northern part of the region have recently been gazetted, following a systematic assessment to delineate a comprehensive and representative reserve system. In this part of the region 47 of 66 (70 percent) regional ecosystems are sampled by the reserve systems which covers less than 3 percent of the total area. National Parks within the region include Currawinya (1,510 square-kilometers [km2]), Hell Hole Gorge (127 km2), Lake Bindegolly (120 km2), Mariala (270 km2), Thrushton (255 km2), Welford (part of 950 km2), Idalia (145 km2), and Culgoa Floodplains (250 km2). The wetlands and lakes contained within Currawinya National Parks are recognized as wetlands of international importance and listed under the RAMSAR convention. There are still gaps in this reserve system, particularly in more fertile and moist areas. These areas are often more highly valued for grazing and are often cleared in the eastern parts of the region but are also likely to contain important drought refuge areas for native wildlife.
Types and Severity of Threats
Vegetation clearance is widespread in the Eastern Australia mulga shrublands ecoregion. Land clearing rates in these areas ranged from over 630 km2 to 845 km2 per annum between 1995 and 1999. However the intensity and potential impact of land clearance on wildlife is complex and varies. The mulga tree is widely used as a fodder source for domestic stock and Mills et al. estimated that over eighty percent of livestock properties west of the Warrego River were suffering from an overutilization of mulga. Clearing to reduce canopy cover and increase the abundance of native grass species in mulga communities may be carried out in a sustainable manner and be at least partly compatible with biodiversity conservation, particularly in the western portion of this ecoregion.
The causes of land degradation are complex and often involve many interacting factors. Artificial watering points, often sourced from the Great Artesian Basin aquifer, and the good forage properties of the ubiquitous mulga Acacia aneura tree allow higher sheep stocking rates to be maintained, particularly in times of drought, than the semi-arid climate would normally support. This high grazing pressure can lead to a self-reinforcing cycle of reduced perennial grass cover, a decrease in natural fire frequency, increased woody shrub cover and increased soil erosion. Many studies have pointed to the strong link between land degradation and the economic viability (and size) of pastoral enterprises. This relationship perpetuates the cycle of degradation.
The unique Mound Springs are threatened by weed infestation, trampling by domestic stock and feral animals, and extraction of water from the Great Artesian Basin aquifer, resulting in the decline and extinction of the springs. Apart from the indirect impacts of grazing on biodiversity through associated land degradation, there are also direct threats to native plants and animals from grazing. Recent studies indicate grazing by domestic animals in mulga communities is associated with an adverse impact on up to 20 percent of all plant and animal species. Predation on and competition with native species from feral animals including rabbits, pigs, goats, foxes, and cats also are major threats to biodiversity in the region. Exotic plant species recorded from the region include rubber vine (Cryptostegia grandiflora), mother of millions (Bryophyllum tubiflorum), and parkinsonia (Parkinsonia aculeata). Many of these weeds currently have relatively localized distribution in the Mulga Lands. However, they have the potential to become widespread and often show a preference for more mesic, better quality habitats which may be critical refuge areas for native biota.
Justification of Ecoregion Delineation
The Eastern Australian mulga shrublands ecoregion is defined by the presence of mulga (Acacia aneura), and consist of one Interim Biogeographic Region of Australia (IBRA), the 'Mulga Lands', although this ecoregion does not include a northwestern outlier that is part of the IBRA unit.
Additional information on this ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
- Anon. 1993. Mulga region. A study of the inter-dependence of the environment, pastoral production and the economy. Department of Lands, Brisbane. ISBN: 0724251898
- Australian Nature Conservation Agency. 1996. A Directory of Important Wetlands in Australia Second Edition. ANCA, Canberra.
- Cameron, J. and R. Blick. 1991. Case Study 2: Pastoralism in the Queensland Mulga Lands. Pages 75 - 116. In J. I. Cameron and J. Elix, editors. Recovering Ground. A case study approach to ecological sustainable rural land management. Australian Conservation Foundation, Melbourne. ISBN: 0858020920
- Dickman, C. R. 1995. Native mammals of western New South Wales: past neglect, future rehabilitation? Pages 81-92 In D. Lunney, S. Hand, P. Reed, and D. Butcher, editors. Future of the fauna of western New South Wales. Transactions of the Royal Zoological Society of New South Wales. Surrey Beatty & Sons, Sydney. ISBN: 0959995196
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- Mills, J.R., E. J. Turner, and T. Caltabiano. 1989. Land Degradation in south west Queensland. Project Report Q089008, Queensland Department of Primary Industries, Brisbane.
- Morton, S.R. 1990. The impact of European settlement on the vertebrate animals of arid Australia: a conceptual model. Proc. Ecol. Soc. Aust. 16: 201-13.
- Neldner, V. J. 1984. Vegetation Survey of Queensland - South Central Queensland. Queensland Botany Bulletin No. 3. Department of Primary Industries, Brisbane. ISBN: 0724239227
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- Wilson, B. A. 1999. Mulga Lands. Pages 6/1 - 6/35 In P.S. Sattler and R. D. Williams, editors. The conservation status of Queensland's Bioregional Ecosystems. Environmental Protection Agency, Brisbane.
- Witt, G. B. and L. Pahl. 1995. Mulgaland communities of south-west Queensland as habitat for Koalas. Pages 177-82 In M. Page and T. Beutel, editors. Ecological research and management in the Mulgalands. Proceedings of Conference held 5-6 July 1994. University of Queensland, Gatton College, Gatton. ISBN: 0909816263
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