Location and General Description
The Gulf of Carpentaria forms a low, arid division between the wetter "Top End" of Arnhem Land and the Cape York Peninsula. The Pellew and Wellesley Islands lie close to the southern coast of the Gulf and are included in the Carpentarian Tropical Savannas. The marginally higher rolling clay plains of the Mitchell Grass Downs lie to the south of the ecoregion. To the northwest is the Proterozoic sandstone plateau underlying the Arnhem Land Tropical Savannas and the Cretaceous sandstones of the eastern Victoria Plains tropical savannas. Eroded highlands and plateau of the Einasleigh Upland Savannas rise above the eastern edge of the plains. Climatically the region differs from the wetter and less seasonal rainfall patterns of the Cape York Tropical Savannas that back the Gulf’s northeastern shores.
Annual rainfall is low, approximately 750 mm at the coast, decreasing to less than 500 mm inland, with pronounced summer seasonality, high rates of evaporation, and an accompanying dominance of grasses as the tallest stratum. Rainfall around the eastern margins of the Gulf is over 900 mm a year, and ranges between 1000 mm to 1200 mm for most of the Northern Territory portion of the ecoregion. Average summer maximums in November or December are between 36o and 39oC, and annual average maximums are above 32oC, reaching over 34oC in the northwest.
Queensland sections of the ecoregion are almost entirely found on the Cainozoic deposits of Karumba basin, which overlies the Jurassic-Cretaceous Carpentarian Basin. Deposits of the Karumba Basin form the large plains to the south of the Gulf and marine deposits occur around the margins of the present day Gulf. The distributary channels and alluvial fans at the mouths of the Flinders, Mitchell, Gilbert, Leichardt, McArthur, and Roper Rivers have extended the seaward edge of the plains. These rivers form fairly extensive freshwater and saline wetlands along their lower courses and around the edge of the Gulf.
Deposits of the Carpentarian Basin are exposed in the east of the ecoregion in areas bordering the Einasleigh Uplands. These often form dissected plateaus capped by laterites, with extensive outcrops in the southeast of the ecoregion near its junction with the Einasleigh Upland Savannas and Mitchell Grass Downs. Drainage through this region is local, and weathering has produced a distinct region of earthy sands bounded by clays in the west. In the north there are massive red earths derived from laterized surfaces, Cretaceous sediments, and sediments carried by rivers draining from the Einasleigh Uplands. The Gulf Fall, which dominates the western portion of the ecoregion, is an area of dissected Proterozoic sediments and metamorphics backed by extensively laterized Proterozoic hills with shallow stony soils. The scarps and gorges of the Gulf Fall grade into the Mount Isa Inlier (part of the Mitchell Grass Downs ecoregion), outliers of which are located in the nearby plains. Outwash from the highlands and marine deposition has produced a small coastal plain with sandy soils that features similar saline and freshwater wetlands to the more extensive plains further east. Red earths occur along the Roper River in the far north of the region.
Vegetation varies markedly throughout the Carpentaria Tropical Savannas ecoregion. Mangroves are common along the coastline. Wetlands in the vicinity of the coast contain a variety of communities, with Acacia stenophylla the prominent component. Coastal plains of the Northern Territory below the Gulf Fall and Uplands are vegetated by woodlands of Darwin stringybark (Eucalyptus tetrodonta), while the Fall is covered by open eucalypt woodlands including Darwin box (E. tectifica) and long-fruited bloodwood (Corymbia polycarpa). Spinifex (Plectrachne pungens) hummock grasses are common in the understory of the Fall and coastal plains.
Some areas of the uplands have a tall grass (Sorghum spp.) understory similar to those that dominate the wetter savannas of Arnhem Land to the northwest. Grasslands of bluegrass (Dicanthium spp.) dominate the flat clay plains around the Gregory, Leichardt, and Flinders Rivers and form a wide band reaching southwards to plains dominated by Mitchell grasses (Astrebla spp.). Grassland communities of the Gulf Plains are regarded as depauperate, but that may be due to a lack of survey effort. West of the grassland plains a narrow band of eucalypt and Melaleuca woodlands with a Chrysopogon fallax-dominated understory skirts the north of the Mount Isa Inlier. Eucalypt woodlands occur on the plains in the east of the region between the Gulf and Einasleigh Uplands, and Melaleuca spp. form extensive low woodlands with a mixed grass component dominated by Aristida spp. and Chrysopogon fallax. Lancewood (Acacia shirleyi) forms significant stands among the dominant ironbark (Eucalyptus crebra) and bloodwood (Corymbia spp.) woodlands on the higher plateau remnants in the southeast of the region.
Small patches of monsoon rainforest occur in fire-protected and/or unusually moist areas and support a biota notably distinct from that of the surrounding eucalypt woodlands, with many species occupying very limited fragmented ranges.
The semi-arid lowlands around the head of the Gulf of Carpentaria form a notable disjunction in the distribution of many vertebrate groups in northern Australia (Ride 1970). The "Carpentarian barrier" separates vertebrate populations in Cape York from those of the Top End of the Northern Territory due to the arid climate found here.
Dicanthium grasslands of the Gulf Plain in western Queensland are one of only two sizeable regions in Australia where grasses form the dominant stratum (AUSLIG 1990). The other is the Astrebla grasslands of the adjacent Mitchell Grass Downs. Unfortunately, the grassland fauna is relatively poorly known. The endangered Julia Creek dunnart (Smithopsis douglasi) (Hilton-Taylor 2000) has been recorded in the far south of the grasslands (Woolley 1992). Another endangered animal found in this region, the golden-shouldered parrot (Psephotus chrysopterygius) has been recorded from Staaten River National Park.
The Pellew Islands are located in the southwestern Gulf and retain undisturbed remnant vine thickets and mangroves. At least four plant species are considered to be endemic to the Pellew Islands (Johnson and Kerle 1991), including the Corchorus rostricephalus plant. The Carpentaria lerista (Lerista stylis) is found mainly on the Pellew Islands, the adjacent mainland, and Groote Eylandt to the north (Cogger 1992). Both the Pellew and Wellesley island groups have major conservation significance. Both shelter species which have disappeared from the adjacent mainland, such as the brush-tailed rabbit-rat (Conilurus penicillatus), with the Wellesley Islands the only occurrence of this species in Queensland. Other species with notable disjunct distributions on the Pellew Islands include the canefield rat (Rattus sordidus) (the only known occurrence in the Northern Territory), the vulnerable Carpentarian pseudantechinus (Pseudantechinus mimulus) (Hilton-Taylor 2000), brush-tailed phascogale (Phascogale tapoatafa) and northern quoll (Dasyurus hallucatus) (Johnson and Kerle 1991). In addition, many of the islands of the Pellew and Wellesley group, and nearby coastal areas, are of international significance as major breeding sites for colonial seabirds and marine turtles (Chatto 2001).
Parts of the Gulf Fall are regarded as "extremely significant" refuges by Morton et al. (1996). The region contains many species restricted to sandstone substrates and gorges. Sandstone associated species with isolated populations in the Gulf Fall include the sandstone antechinus (Pseudantechinus bilarni) (Fisher et al. 2000). The Carpentarian grasswren (Amytornis dorotheae) is largely restricted to sandstone escarpment of the Gulf Fall. Remnant rainforest pockets are particularly important, and four of these pockets near the Northern Territory-Queensland border support the only known populations of the critically endangered Carpentarian rock-rat (Zyzomys palatalis) (Trainor et al. 2000). A gecko (Gehyra borroloola) and at least four plant species are endemic to these rainforest pockets and more endemics are expected to be discovered (Morton et al. 1996).
The gorge systems of Lawn Hill National Park support extensive aquatic and wetland communities and are bounded by rainforest remnants. Nineteen of the wetland areas formed along lower river channels and around the Gulf’s margin are regarded as nationally significant (Environment Australia 2001). Morton et al. (1996) regarded the Gregory-Nicholson wetlands system to be a significant refuge area due to variety of habitats it presented and its importance to migratory species. Wetlands near the mouth of the Roper River in Limmen Bight may be among the most important in the Northern Territory due to large numbers of birds that aggregate in the area (Storrs and Finlayson 1997). River systems in this ecoregion support two recently described endemic freshwater turtle species, Elseya lavarackorum and Emydura worrelli.
Natural vegetation still dominates the region (Morgan 1999, Thackway and Creswell 1995). Clearing of Acacia cambadgei woodlands near the boundary with the Mitchell Grass Downs has occurred in Queensland, but has not been widespread (Morgan 1999). Conservation reserves in this ecoregion include Staaten River National Park (4,700 km2), Lawn Hill National Park and adjacent resource reserves (less than 1,000 km2), Mitchell-Alice River National Park, and part of Bulleringa National Park. Several major reserves are concentrated in Queensland sections of the ecoregion, particularly in the northeastern Gulf Plains. The Northern Territory remains relatively unprotected in reserve systems. A recently declared large national park in the Limmen Bight region will partially address this lack of reserves in the Northern Territory. One of the Pellew Islands is protected in Barranyi National Park.
Near-coastal and coastal wetlands are poorly reserved areas that make a significant contribution to the biological diversity of the region and are important to migratory birds. Lawn Hill National Park protects large areas of the Queensland section of the Gulf Fall, which is unreserved in the Northern Territory. Substantial survey efforts are required to identify areas of the central Gulf Plains that might provide significant refuge areas suitable for reservation. The unprotected Dicanthium grasslands of the region are one its most distinctive features.
Many native mammal species have declined or been lost from the mainland parts of this bioregion (Johnson and Southgate 1990, Johnson and Kerle 1991).
Types and Severity of Threats
As in adjacent ecoregions, the major threats to natural systems of the Carpentarian Tropical Savannas stem from grazing practices (Morgan 1999). Continued overstocking and introduction of fodder species is affecting two of the outstanding features of the region, the community composition of native grasslands (Tothill and Gillies 1992), and riparian and wetland habitats (Morgan 1999). Altered fire regimes and utilization of remnants by cattle also degrades monsoon rainforests (Russell-Smith and Bowman 1992). Changed fire regimes (principally to more frequent, more extensive late dry season fires) are also altering vegetation characteristics of the eucalypt woodlands and hummock grasslands, to the detriment of the restricted range Carpentarian grass-wren (McKean and Martin 1989). The potential effects of these processes are compounded by a lack of knowledge of the natural systems of the region.
Exotic weeds, some of which were originally introduced as fodder, are threatening communities across all ecosystems. Rubber vine (Crytostegia grandiflora) smothers natural vegetation and is a serious threat, particularly to riparian vegetation. Infestations are common along major rivers and intervening habitats in the east of the Gulf Plains (Morgan 1999). Prickly acacia (Acacia nilotica) is a serious threat to grassland and riparian communities and major infestations occur around the southern boundary of the Carpentarian savannas with the Mitchell Grass Downs (Anderson 1993). The largest island in the Wessel group, Vanderlin, is currently being degraded by a recently introduced and rapidly increasing population of feral goats.
Justification of Ecoregion Delineation
The Carpentaria Tropical Savannas ecoregion consists of three separate IBRAs: ‘Gulf Fall and Uplands’, ‘Gulf Coastal’, and ‘Gulf Plains’. The lower rainfall received in this region separates the region from neighboring Arnhem Land and Cape York Tropical Savannas ecoregions.
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