Location and General Description
The Mitchell Grass Downs run in a southeasterly direction for 1,500 km from the center of the Northern Territory to central southern Queensland. The region is predominantly located in tropical semi-arid habitats (from approximately 350 mm to 750 mm annual rainfall), and borders desert habitats that lie to the west, south, and southwest. Rainfall is influenced by the summer monsoon in the north, with seasonality decreasing towards the southeast. Summer (January) maximums are hot and often reach 40°C, but winter (July) minimums are cold, and frost can occur over much of the region, particularly in central western Queensland.
Included within the ecoregion, as described here, are three distinct physiographic regions. The majority of the region is dominated by undulating plains of brown and grey clays that commonly overlie Cretaceous shales in western Queensland, and calcareous Cambrian deposits on the Barkly Tableland in the Northern Territory. The Barkly Tablelands consist of Tertiary clay deposits overlying calcaeous geology while the Queensland portion is mainly clay soils formed in situ from underlying Cretaceous shales (most of which are marine deposits which is why these areas are full of salt at depth) These plains skirt the western and southern edge of the older, highly folded and mineralized Proterozoic highlands of the Mount Isa Inlier, while the Desert Uplands in the east mark the limit of the Mitchell Grass Downs ecoregion. The climate transitions to seasonally arid tropical savannas to the east (in the Brigalow Belt) and to the north (Einsleigh Uplands). The boundary between Cretaceous sediments and Tertiary deposits of the Gulf Plain roughly marks the northern limit of the Mitchell Grass Downs in Queensland, with grasslands to the north generally dominated by bluegrasses (Dichanthium spp.). Ranges of Triassic sandstone, and Tertiary surfaces of the Desert Uplands contrast strongly with the flat plains further west. Most of the Uplands are covered by Quaternary sand plains with massive red and yellow earths interspersed with lateritic escarpments with lithosol soils. Mesas capped by Tertiary duricrusts also occur in Western Queensland near Kynuna and Winton, and lateritic surfaces remain in areas of the Barkly Tableland and southeastern Queensland downs. The Mount Isa Inlier has skeletal, often stony soils, with areas of alkaline duplexes and massive earths. To the southwest of the inlier is an area of calcareous earths derived from the Cambrian Georgina Limestone, one of only two such occurrences in northern Australia.
Gentle upwarping has caused the western Queensland Mitchell Grass Downs to form the divide between northern drainage to the Gulf of Carpentaria via the Flinders River and southwestern drainage via the Diamantina River and Channel Country into Lake Eyre. In the east, the crest of the Great Dividing Range passes through the center of the Desert Uplands, and drains westward into the Flinders catchment, eastward to the Belyando-Burdekin, or into the small internal catchment of Lake Buchanan. The Mount Isa Inlier is drained by the Leichardt River to the north and by streams running into the Gregory River. Most drainage on the Barkly Tableland is internal, and runoff is low due to the porosity of the underlying limestones. To the south, limited runnoff from the Inlier and Tableland enter the Georgina River, which disappears into the sands of the Simpson Desert.
The undulating plains that typify this ecoregion are commonly devoid of major tree cover. Large tracts of Mitchell Grass pasture grazed by sheep and cattle separate scattered Acacia woodlands, usually dominated by gidgee (Acacia cambagei). Occasional drainage lines possess mixed communities of red river gum (Eucalyptus camuldulensis), coolibah (E. coolabah), and paperbarks (Melaleuca spp.). Ephemeral lakes of the Barkly Tableland support communities of bluebush (Chenopodium auricomum) and/or coolibah (E. coolibah) low open woodlands.
In the Kynuna-Winton area of Queensland, mesas formed by resistant tertiary duricrusts are vegetated by stands of lancewood (Acacia shirleyi) and low eucalypt communities with a spinifex (Triodia spp.) groundcover. Stony areas of outwash from these mesas support chenopod scrublands. In the southeast of the region lateritic scarps and surfaces area associated with areas of gidgee and brigalow (Acacia harpophylla) woodlands separated by Mitchell grasses. Spinifex communities (predominantly Triodia pungens) are common groundcover in the open woodlands of the Mount Isa Inlier and Desert Uplands. Desert uplands woodlands are dominated by E. populnea, E. melanophlia and E. similas while the Mt Isa region has generally low open woodlands dominated by snappy gum (E. leucophloia), Cloncurry box (E. leucophylla), and silver box (E. pruinosa) low open woodlands.
The distinctive grasslands of the downs harbor several endemic reptiles: the gecko Gehyra minuta, skinks (Ctenotus schevilli, C. agrestis, and C. joanae), an agamid lizard Pogona henrylawsoni, and a monitor Varanus spenceri. This ecoregion also forms the main distribution for numerous other reptiles, including many large venomous elapid snakes (Pseudonaja guttata, P. ingrami, and Pseudechis colletti) (Horner and Fisher 1998). Although the invertebrate fauna is generally poorly known, many ant species are known to occur only in this ecoregion (Fisher 2001). This ecoregion has also yielded the only recent record of the critically endangered night parrot (Geopsittacus occidentalis) (Hilton-Taylor 2000) (Boles et al. 1994). The endangered Julia Creek dunnart (Sminthopsis douglasi) is endemic to the downs, Desert Uplands, and restricted areas of the Gulf Plains immediately north of the downs (Strahan 1998). A recent survey of the Desert Uplands region resulted in the discovery of two new reptile species, Ctenotus rosarius sp. nov, and Lerista sp. nov, as well as the discovery of animals outside their previously known home ranges, indicating that further research may yield a fuller picture of the region’s fauna (Kutt 2001). The vegetation is also distinctive, with Mitchell grasslands and associated communities supporting at least 10 endemic plant species (Fisher 2001).
While the downs are generally considered fairly depauperate (Wilson 1999), this region does contain a unique and distinctive fauna, adapted to the region’s seasonality and variability in yearly rainfall. One of the most distinctive features of the Mitchell Grass Downs are the extraordinary irruptions of its two most characteristic and pivotal animal species, the flock bronzewing (Phaps histrionica) and the long-haired rat (Rattus vilosissimus). These two species encapsulate the apparent simplicity but striking dynamism of this system. No other Australian ecoregion possesses this trait so markedly.
The flock bronzewing, a large ground-foraging pigeon, formerly occurred in flocks of hundreds of thousands, dispersing widely across the Mitchell Grass Downs in response to rainfall variation. They declined rapidly in response to pastoral settlement in the period 1850-1900 and were feared extinct by about 1950. They have since recovered somewhat, and can still be seen in flocks of tens of thousands (Higgins and Davies 1996). The long-haired rat has a similar "boom-bust" cycle, irrupting in vast plagues following favorable rainfall (Carstairs 1974). Early descriptions catch some of the immensity of these irruptions: "The numbers of the rats were incredible … fifty thousand square miles were occupied by these animals and one rat to every ten square yards would not represent anything like their number … they devoured everything edible that came in their way, and destroyed what they did not devour … they swarmed to such an extent that it was almost impossible to sleep, for the rats invaded the blankets of the sleeper in order to find a meal. Cases in which a man’s fingers, toes or ears were nibbled were common" (Wood Jones 1923-25).
Irruptions continue to occur at irregular intervals, with the rats then extending from their core refuge areas of the Mitchell Grass Downs to spread across much of inland semi-arid Australia. In their wake, many birds of prey, most notably the largely nocturnal letter-winged kite (Elanus scriptus) also reach unusually high population densities and extend their geographic range well beyond their normal center in the Mitchell Grass Downs.
Another distinctive feature of this region is the response of much of the fauna to regular seasonality. The clay soils of the Mitchell grasslands dry and crack widely during the long dry season. Above ground the environments are simple, lacking trees and offering little shelter, so much of the fauna utilizes the deep fissures and cracks. Small carnivorous marsupials, typically including the long-tailed planigale (Planigale ingrami) and the stripe-faced dunnart (Sminthopsis macroura), are among the most abundant mammals using this subterranean shelter. The shrew-like long-tailed planigale is one of the world’s smallest mammals with an adult weight of less than 6g, and has a remarkably compressed head, ideal for probing among the crack network. Very high densities of specialized skinks and large snakes also find refuge in this fissured environment. With the coming of the annual rains, the fissures close and much of the landscape becomes waterlogged, encouraging the emergence of vast numbers of burrowing frogs (Palmer and Pidcock 2001).
Wetlands in the ecoregion also provide important habitat for aggregations of animals. The freshwater lakes of the Barkly Tableland are intermittently important breeding grounds for tens of thousands of waterbirds. Lake Woods is one of the most important breeding grounds in inland northern Australia (Fleming et al. 1983) and the coolibah swamps of Lake Tarrabool are the largest in tropical Australia (Storrs and Finlayson 1997). Lakes Buchanan and Galillee in the Desert Uplands are important seasonal breeding grounds for waterbirds and are considered rare type of wetland in Australia and the world (ANCA 1996). Lake Buchanan is the only site at which the plant Lawrencia buchananensis has been recorded. Lake systems across the ecoregion are regarded as significant refugia by Morton et al. (1996). The wetland edges and grasslands of this ecoregion provide critically significant habitats for many migratory shorebirds, most notably the little curlew (Numenius minutus) and oriental pratincole (Glareola maldivarum) (Chatto and Whitehead 1996), for which a large proportion of the total world population aggregates during the non-breeding season.
Artesian, or mound springs, are a unique wetland type formed around artesian springs. They arise from natural outlets in the Great Artesian Basin aquifer that underlies the western half of the ecoregion. Mound springs in the Great Artesian Basin are often home to endemic communities (Morton et al. 1996), and Edgbaston, or Aramac Springs, are especially well known for their high concentrations of endemic plant, fish, and snail species (ANCA 1996). Endemic fish include the red-finned blue eye (Scaturiginichthys vermeilipinnis), which is restricted to two springs in the Edgbaston complex of the eastern downs (Wager and Jackson 1993). Ponder and Clark (1990) documented a radiation The largely undisturbed vegetation and varied topography of the Mount Isa Inlier creates an oasis of diversity inserted into the flat clay plains. The Selwyn Range is considered to be a highly significant refuge for flora and fauna (Morton et al. 1996), and, like the plains, the inlier has its own endemic gecko (Gehyra robusta) and skink (Ctenotus striaticeps) (Cogger 1992). One of the most distinctive eucalypts of the region, the Cloncurry box, is also endemic, a situation mirrored in the Desert Uplands where E. whitei is a common dominant, but restricted to the region. In general, individual plants in this region are not threatened: Morgan (1999a, b) and Wilson (1999) list only one endangered plant species for the Queensland sections of the Mitchell Grass Downs.
Major habitats within the Mitchell Grass Downs ecoregion are relatively intact and dominated by natural ecosystems (Orr and Holmes 1984, Thackway and Creswell 1995). Three species of vertebrates are extinct within the ecoregion and a further twelve are endangered. Twenty-two of 152 regional ecosystems are considered endangered within Queensland (Morgan 1999a, b, Wilson 1999), 17 of which are located in the Desert Uplands (Morgan 1999a). Nearly all are naturally restricted to limited areas of habitat, and their endangered status is not solely the result of dramatic alteration of natural systems. Mitchell Grass Downs are poorly represented within conservation reserves, particularly in the Northern Territory. However, recent land acquisition in Queensland, most notably the establishment and recent expansion of Astrebla Downs National Park, has begun rectifying this situation. The park protects important habitat for an isolated population of the vulnerable bilby (Macrotis lagotis).
Very little of the Mount Isa Inlier is protected within the National Parks. Parts of Lawn Hill National Park are located within the region, however, the majority of the folded Proterozoic hills and woodlands that dominate the Mount Isa Inlier are not represented within conservation reserves, despite the highly significant status of the region as a refuge. Undulating plains carrying Eucalyptus/Triodia woodlands with outcropping cavernous dolomite are protected in Camooweal Caves National Park. Relict Tertiary surfaces and their associated communities are partly reserved within Bladensburg National Park and White Mountains National Park. Sandstone ranges of the Desert Uplands are well protected in White Mountains National Park. None of the significant wetlands of the region are adequately protected. Recent surveys have located populations of the endangered Julia Creek Dunnart in Moorinya National Park and Bladensburg National Park
Species now extinct in the Mitchell Grass Downs include the western quoll (Dasyurus geoffroii VU) and northern quoll (D. hallucatus). In addition, there has been marked reduction in the range of the Carpentarian antechinus (Pseudantechinus mimulus VU) and brush-tailed possum (Trichosurus vulpecula).
There are seven notable conservation reserves which fall at least partially within the Mitchell Grass Downs; some of these are shared between ecoregions. In their entirety, these areas range in size from Connells Lagoon, the smallest at 260 km2 to Diamantina NP, at 4700 km2. However, most of the Diamantina area falls in the Simpson Desert ecoregion to the south, so Astrebla Downs National Park is the largest in the ecoregion at 1,740 km2. The habitat types within these reserves range from eucalypt woodland, chenopod shrubland, bluebush, lancewood, gidgee, and grasslands to Acacia woodlands. The amount of area that these conservation reserves share with Mitchell Grass Downs varies. From smallest to largest area in the ecoregion, protected areas include: part of Bladensburg, Connells Lagoon, Moorinya, Lawn Hill, part of Diamantina, and White Mountains protected areas.
Types and Severity of Threats
Although the region is not subject to intensive land use, there are serious concerns about high rates of land clearance in the Desert Uplands and gidgee communities in southeastern part of the Mitchell Grass Downs. Grazing is another serious concern (Wilson 1999). There are large areas with recent and ongoing vegetation clearance in the Desert Uplands portion of this ecoregion. For example, Jericho Shire, which covers much of the southern part of the Desert Uplands, has one of the highest clearing rates of any local shire in Queensland, which in turn has by far the highest clearing rate of any Australian State. Seven percent, or 1,350 km2, of the Shire was cleared from 1992 to 1995 (McCosker and Cox 1996), and the total rates of clearing across the Desert Uplands have remained roughly level (State Land and Trees Study 1999, 2000).
Cattle are contributing to degradation of limited riparian and wetland habitats across the ecoregion (Storrs and Finlayson 1997, Wilson and Purdie 1990) and particularly favor bluebush communities around lakes on the Barkly Tableland (Learmonth and Learmonth 1971). Grazing by sheep and cattle has a weak but significant detrimental effect on community composition of Mitchell Grass communities (Fensham et al. 2000). The spread of the exotic prickly acacia (Acacia nilotica) is facilitated significantly by cattle, and the species is estimated to have infested over 5,000 km2 since its introduction as a fodder tree (Anderson 1993). The bulk of infestations occur on the clays of the Mitchell Grass Downs and have the potential to transform previously treeless grasslands into savannas or grasslands with scattered tree cover and dense riparian infestations (Wilson 1990). Government sponsored "conservation" farming strategies are still encouraging introduction of exotic fodder plants, principally buffel grass (Cenchrus ciliaris) in the Desert Uplands part of this ecoregion to increase productivity (Sturtz and Chapman 1995). Feral pigs (Sus scrofa) are present in this region, as are cats (Felis catus). Recent research with cats indicates that they are having a tremendous impact on native fauna (Kutt 2001).
Justification of Ecoregion Delineation
Justification for Ecoregion Delineation: The Mitchell Grass Downs ecoregion comprises three IBRAs, the ‘Mitchell Grass Downs’, the ‘Desert Uplands’ and the ‘Mount Isa Inlier’.
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Prepared by: Fred Ford
Reviewed by: Bruce Wilson, John Woinarski