Location and General Description
This ecoregion is characterized by a markedly seasonal, monsoonal climate, with a short wet season from November to March, and a long, nearly rain-free dry season for the rest of the year. Rainfall decreases along a north-south gradient, receiving an average annual total of 1200 mm in the north but only 600 mm in the south. Temperatures are high throughout the year and monthly average maxima range from 25o to 35oC.
In contrast to the rugged topography of the ecoregions to the immediate northwest (Kimberley and Carpentaria Tropical Savannas), this ecoregion is typified by extensive plains, of varying geomorphology. The Sturt Plateau occupies most of the eastern half of this ecoregion, and comprises deep red sandy-loam soils on a Tertiary lateritic base. Except for occasional mesas and small lateritic outcrops, it is a largely featureless landscape. The western half of the ecoregion comprises black soil (clay) plains on basalt parent material, intermixed with red loam soils derived from limestone. There are also small areas of sandstone outcrops, outliers of the more extensive sandstone and limestone ranges to the north. The most notable of these outliers is the Bungle Bungle range within Purnululu National Park. This ecoregion forms the upper catchment of many of the largest rivers in northern Australia, including the Fitzroy, Ord, Victoria, Daly, and Roper Rivers.
Vegetation in the ecoregion is strongly associated with soil, geological factors and rainfall (Williams et al. 1996). Woodlands dominated by eucalypts or bloodwood eucalypts (Corymbia spp.) are the most extensive vegetation type, especially on sand and loam soils. Typically, canopy height varies from 5 m to 15 m, and understories are dominated by tall grass species (especially Sorghum, Heteropogon, Themeda, Chrysopogon, Aristida, and Eriachne spp.). Finer-textured soils (clays and clay-loams) support open woodlands dominated by Terminalia and Bauhinia species, or grasslands of Astrebla, Iseilema, Chrysopogon, Aristida and Dicanthium spp. Lancewood (Acacia shirleyi) and bullwaddy (Macropteranthes keckwickii) are found mostly in the east of the region, in scattered pockets of diminishing extent in the west, but do not extend to Western Australia. They form distinctive, mixed or nearly monospecific small stands on lateritic outcrops in the west, but in the east vary from more extensive dense thickets to open forests across a range of substrates, often forming a sharp boundary with eucalypt woodlands (Woinarski and Fisher 1995a). The relatively dense vegetation cover provided by the lancewood-bullwaddy canopy results in a sparse grass cover, while the extensive canopy cover and infrequency of fire provides an environment which supports forbs, small shrubs and vines with rainforest affinities. The rainforest element is so pronounced that Russell-Smith (1991) considered lancewood vegetation to be one of the 16 rainforest types in the Northern Territory.
The smaller sandstone outcrops and sandsheets derived from sandstone support either eucalypt open woodlands with a hummock grass (Triodia spp.) understory or heathlands dominated by Grevillea and Acacia spp. (Woinarski 1992). Riparian strips are another distinctive localised vegetation feature, with vegetation associations varying according to river order, flooding regime and substrate, but typically including river red gum (Eucalyptus camaldulensis), Terminalia platyphylla, Ficus spp., Melaleuca spp., Nauclea orientalis, and/or Pandanus spp. (Woinarski et al. 2000).
There is little endemism associated with this ecoregion because much of its environments and biota grades into neighbouring ecoregions, associated with the gradual latitudinal rainfall gradient across northern Australia (Woinarski et al. 1999a). To a large extent, this ecoregion marks the southern (inland) extent of the biota of monsoonal northern Australia, with some northern incursion of the arid-adapted biota of the central Australian deserts immediately to the south (Woinarski 2000).
This ecoregion does not contain an especially rich nor distinctive biota. The ecoregion includes the most extensive stands of lancewood-bullwaddy found in Australia, but this is generally a fairly depauperate environment (Woinarski and Fisher 1995b). The biota of the ecoregion is not especially well known, as evidenced by the recent discovery of new vertebrate species such as the black-soil skink (Ctenotus rimacola) (Horner and Fisher 1998). Extensive cave systems in limestone and sandstone outcrops in the south of Gregory National Park have not yet been subject to intensive biological investigation, but are likely to harbour an invertebrate fauna including endemic species and/or species with highly disjunct distributions.
The mammal fauna includes unusually abundant populations of the spectacled hare-wallaby (Lagorchestes conspicillatus), especially in the lancewood-bullwaddy thickets (Woinarski and Fisher 1995a), and the northern nailtail wallaby (Onychogalea unguifera), especially in grasslands and open woodlands along the margins between black-soil plains and red loamy soils. Reptiles largely restricted to this ecoregion include Suta ordensis and Varanus kingorum. The agamid lizard Cryptagama aurita has a small distribution, concentrated in this region. Some burrowing frogs, most notably Cyclorana cryptotis and C. maculosus, are largely restricted to the deep red earths of this ecoregion, and may reach extremely high densities when they emerge with the first rains of the wet season.
There is limited endemism associated with some sandstone outliers, notably the Bungle Bungle Range to which the skink Lerista bunglebunglensis and several plant species are restricted (Woinarski 1992). Sheltered gorges remain inaccessible to feral livestock, and provide habitat for relict species of wetter climates, including the fern Taenitis pinnata, resurrection grass (Micraira spp.), and the tall palm Livistona victoriae which reaches heights of 10 m to 20 m (Morton et al. 1995).
The threatened purple-crowned fairy-wren (Malurus coronatus) has a major stronghold in riparian vegetation in this ecoregion, particularly along the Victoria River (Rowley 1993). The grasslands and savanna woodlands support a very rich and abundant assemblage of granivorous birds, including the endangered gouldian finch (Erythrura gouldiae) (Hilton-Taylor 2000).
Much of the region’s conservation value is not localized in any one site, but rather is due to the vast areas maintained in a semi-natural condition, allowing the uninterrupted working of landscape-scale ecological processes.
The environments of this region are largely unaffected by broad-scale clearing. However, more than 90 percent of the ecoregion is devoted to extensive pastoralism. Grazing by livestock and feral stock, principally cattle (Bos taurus and B. indicus) and donkeys (Equus asinus), affects almost all areas of the ecoregion. Feral stock densities may be very high, and in many areas are greater than 10 individuals/km2 (Freeland and Choquenot 1990).
Conservation reserves comprise about 7,000 km2 or 3 percent of the ecoregion. However, the protected areas system is biased so that the areas with relatively high pastoral productivity are generally unrepresented or least represented in the reserve system.
Types and Severity of Threats
The main threats to this region are widespread and far-reaching, rather than localized and acute. The early years of pastoralism, from 1880 to 1910, were accompanied by major environmental degradation, especially around natural water sources (Riddett 1990). Degradation continued in many of the more susceptible environments, especially alluvial flats, with denudation of vegetation leading to the formation of deeply eroded gullies. Such erosion has resulted in annual deposition of 24 million tonnes of sediment into the Ord River (Winter 1990), and some properties have been de-stocked in order to attempt rehabilitation. Vegetation change associated with pastoral impact led to decreases and local extinctions of some bird species, especially those associated with riparian areas, such as white-browed robin (Poecilodryas superciliosa) and purple-crowned fairy-wren (Smith and Johnstone 1977, Boekel 1979, Rowley 1993). Recent grazing gradient studies in this ecoregion have shown strong relationships between grazing levels and the occurrence and abundance of a range of plant and invertebrate species (Ludwig et al. 1999).
Several mammal species, including bilby (Macrotis lagotis VU), northern quoll (Dasyurus hallucatus) and golden bandicoot (Isoodon auratus VU), have declined or disappeared altogether from this ecoregion (Kitchener 1978, McKenzie 1981). These losses may be due to grazing impacts, but may also be related to the spread of feral cats and altered fire regimes.
Across most pastoral properties in the ecoregion, traditional Aboriginal fire regimes (relatively fine-scale, frequent burning particularly in the early dry season) have largely been replaced by attempted fire exclusion. As a result, the incidence of uncontrolled and extensive late dry season fires has increased. This management change has resulted in some pronounced changes in vegetation patterning, most notably with the invasion of some grassland areas by shrubs and small trees, such as rosewood (Terminalia volucris) (Dyer and Mott 1999). In turn, these vegetation changes have led to changes in the relative abundance of many animal species (Woinarski et al. 1999b). Increased incidence of extensive hot fires may also be leading to the reduction of lancewood-bullwaddy environments (Woinarski and Fisher 1995b).
Many exotic plant species have become serious pastoral and environmental weeds in this ecoregion, such as the widespread rubber bush (Calotropis procera). Other weeds include the noogoora burr (Xanthium strumarium), parkinsonia (Parkinsonia aculeata), bellyache bush (Jatropha gossypifolia), and castor oil plant (Ricinus communis), all of which are especially concentrated in riparian and wetland areas.
Justification of Ecoregion Delineation
The Victoria Plains Tropical Savanna ecoregion includes the two IBRAs: ‘Sturt Plateau’ and ‘Ord-Victoria Plains’ (Thackway and Cresswell 1995).
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Woinarski, J.C.Z., and A. Fisher. 1995b. Wildlife of Lancewood (Acacia shirleyi) thickets and woodlands in northern Australia: 2. Comparisons with other environments of the region (Acacia woodlands, Eucalyptus savanna woodlands and monsoon rainforests). Wildlife Research 22 : 413-443.
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Woinarski, J. C. Z., C. Brock, M. Armstrong, C. Hempel, D. Cheal, D., and K. Brennan. 2000. Bird distribution in riparian vegetation of an Australian tropical savanna: a broad-scale survey and analysis of distributional data base. Journal of Biogeography 27: 843-868.
Prepared by: John Woinarski