Geography of Western Australia - The Biogeography of Western Australia

The Biogeography of Western Australia

See also: Regions of Western Australia

There are 25 biogeographic regions of 80 in the country, in the Interim Biogeographic Regions of Australia (IBRA) system. The IBRA was developed in 1993-94 under the coordination of Environment Australia by the States and Territories as a basis for developing priorities for the Commonwealth in funding additions to the reserve system under the National Reserve System Cooperative Program, in order to ensure that a balance of representative biogeographic areas were preserved by National Parks and Nature Reserves.

The detailed IBRA analyses addressed:

• The representativeness of the existing reserve system;
• Biases in the existing reserve system;
• Major threatening processes within each IBRA region; and
• Identifying priority IBRA regions in urgent need of further reserve additions.

In Western Australia this involved small changes in the Carnarvon Shark Bay area which was added to the Geraldton Sandplain region, Extension of the Yalgoo Region to include part of the Carnarvon region, transfer of the Salmongum woodland region from the Coolgardie to the Mallee biogeographic region and minor changes to the Central Ranges outliers.

The Avon Wheatbelt This is an area of active drainage dissecting a Tertiary plateau in Yilgarn Craton. The gently undulating landscape of low relief has Proteaceous scrub-heaths, rich in endemics, on residual lateritic uplands and derived sandplains; mixed eucalypt, Allocasuarina huegeliana and Jam-York Gum woodlands on Quaternary alluvials and eluvials. The south eastern boundary has been modified incorporating a small portion into the Mallee region. The whole region has been extensively cleared for agriculture, and rapid dry-land salinity is a major problem.

The Carnarvon Region The Carnarvon region comprises Quaternary alluvial, aeolian and marine sediments overlying Cretaceous strata. There is a mosaic of saline alluvial plains with samphire and saltbush low shrublands, Bowgada low woodland on sandy ridges and plains, Snakewood scrubs on clay flats, and tree to shrub steppe over hummock grasslands on and between red sand dune fields. The Limestone strata are covered with Acacia startii and binervia shrublands outcrop in the north, where extensive tidal flats in sheltered embayments support Mangal vegetation.

The Coolgardie Region Granite strata of Yilgarn Craton characterises Coolgardie, with Archaean Greenstone intrusions in parallel belts. Drainage is arheic and occluded. Mallees and scrubs are found on sandplains associated with lateritised uplands, playas and granite outcrops. Diverse woodlands rich are in K strategy endemic eucalypts, on low greenstone hills, valley alluvials and broad plains of calcareous earths. In the west of the region, the scrubs are rich in endemic Proteaceae, in the east they are rich in endemic acacias.

The Central Ranges There are a high proportion of Proterozoic ranges and derived soil plains, interspersed with red Quaternary sandplains in this region. The sandplains support low open woodlands of either Desert Oak or Mulga over Triodia basedowii hummock grasslands. Low open woodlands of Ironwood (Acacia estrophiolata) and Corkwoods (Hakea spp.) are found over tussock and hummock grasses which often fringe the ranges. The ranges support mixed wattle scrub or Callitris glaucophylla woodlands again over hummock and tussock grasslands.

Dampierland contains four distinct landforms.

1. The Quaternary sandplain overlying Jurassic and Mesozoic sandstones is covered with Pindan and with Hummock grasslands on hills.
2. Quaternary marine deposits are found on coastal plains, with Mangal, samphire - Sporobolus grasslands, Melaleuca acacioides low forests, and Spinifex - Crotalaria strand communities.
3. Quaternary alluvial plains are associated with the Permian and Mesozoic sediments of Fitzroy Trough and support tree savannas of Crysopogon - Dichanthium grasses with scattered Eucalyptus microtheca - Lysiphyllum cunninghamii. The Riparian forests of River Gum and Cadjeput fringe drainages along the Fitzroy River.
4. Devonian reef limestones in the north and east support sparse tree steppe over Triodia intermedia and T. wiseana hummock grasses and vine thicket elements.

The Esperance Plains Proteaceous Scrub and mallee heaths are found on the sandplain overlying Eocene sediments. This area is particularly rich in endemic species. Herbfields and heaths (rich in endemics) are found on abrupt granite and quartzite ranges that rise from the plain. Eucalypt woodlands occur in gullies and alluvial foot-slopes.

The Gascoyne In the Gascoyne region rugged low Proterozoic sedimentary and granite ranges are divided by broad flat valleys. Open mulga woodlands occur on shallow earthy loams over hardpan on the plains, with mulga scrub and Eremophila shrublands on the shallow stony loams of the ranges. For the Carnegie Salient, in the east, it is characterised by extensive salt lake features supporting succulent steppes.

The Gibson Desert The Gibson Desert is basically a lateritised upland on flat-lying Jurassic and Cretaceous sandstones of Canning Basin. Mulga parkland over Triodia basedowii is found on lateritic "buckshot" plains. Mixed shrub steppe of Acacia, Hakea and Grevillea over Triodia pungens occurs on red sand plains and dune fields. Lateritic uplands support shrub steppe in the north and mulga scrub in the south. Quaternary alluvia that are associated with palaeo-drainage features support Coolabah woodlands over bunch grasses.

The Geraldton Sandplain The Geraldton Sandplain is a highly biodiverse region. Vegetation is mainly of proteaceous scrub-heaths, rich in endemics, on the sandy earths of an extensive, undulating, lateritic sandplain mantling Permian to Cretaceous strata. Extensive York Gum and Jam woodlands occur on outwash plains associated drainage.

The Great Sandy Desert The Great Sandy Desert is mainly a tree steppe grading to shrub steppe in south. It comprises open hummock grassland of Triodia pungens and Plectrachne schinzii with scattered trees of Owenia reticulata and Bloodwoods, and shrubs of Acacia species, Grevillea wickhamii and Grevillia refracta, on Quaternary red longitudinal sand dune fields overlying Jurassic and Cretaceous sandstones of the Canning, Centralian, Arunta and Armadeus Basins. Casuarina decaisneana (Desert Oak) occurs in the far east of the region. Gently undulating lateritised uplands support shrub steppe such as Acacia pachycarpa shrublands over Triodia pungens hummock grass. Calcrete and evaporite surfaces are associated with occluded palaeo-drainage systems that traverse the desert; these include extensive salt lake chains with samphire low shrublands, and Melaleuca glomerata - Melaleuca lasiandra shrublands.

The Great Victoria Desert The Great Victoria Desert is an arid active sand-ridge desert of deep Quaternary aeolian sands overlying Permian and Mesozoic strata of the Officer Basin. Between sand-dune ridges there is a tree steppe of Eucalyptus gongylocarpa, Mulga and Eucalyptus youngiana over hummock grassland dominated by Triodia basedowii.

The Hampton Region The Hampton Region comprises Quaternary marine dune systems on a coastal plain of the Eucla Basin, backed by stranded limestone scarp. Areas of marine sand are also perched along the top edge of the scarp. Various mallee communities dominate the limestone scree slopes and pavements, as well as the sandy surfaces. Alluvial and calcareous plains are found below the scarp support eucalypt woodlands and Myall open low woodlands.

The Jarrah Woodlands The Duricrusted plateau of the western edge of the Yilgarn Craton is characterised by Jarrah-Marri forest on laterite gravels and, in the eastern part, by Marri-Wandoo woodlands on clayey soils. Eluvial and alluvial deposits support Agonis shrublands. In areas of Mesozoic sediments, Jarrah forests occur in a mosaic with a variety of species-rich shrublands. This area has been heavily logged for timber, Jarrah here being largely regrowth.

The Little Sandy Desert In this region red Quaternary dune fields have abrupt Proterozoic sandstone ranges of Bangemall Basin. There is a shrub steppe of acacias, Thryptomene and grevilleas over Plectrachne schinzii on sandy surfaces. Sparse shrub-steppe is found over Triodia basedowii on stony hills, with River Gum communities and bunch grasslands on alluvial deposits in and associated with ranges.

The Mallee Region This region has been re-defined to include an area from the Coolgardie Bioregion – the area between Lake Hope, Forrestiana and Mount Holland, which comprises Salmon Gum and Morrell woodlands on greenstone, with smaller areas of mallee and Acacia / Casuarina thicket on sandplains. The south-eastern part of Yilgarn Craton is gently undulating, with partially occluded drainage. Vegetation is mainly mallee over myrtaceous-proteaceous heaths on duplex (sand over clay) soils. Melaleuca shrublands characterise alluvia, and Halosarcia low shrublands occur on saline alluvium. A mosaic of mixed eucalypt woodlands and mallee occurs on calcareous earth plains and sandplains overlying Eocene limestone strata in the east.

The Murchison This arid region of Mulga low woodlands, is often rich in ephemerals, on outcrop hardpan washplains and fine-textured Quaternary alluvial and eluvial surfaces mantling granitic and greenstone strata of the northern part of the Yilgarn Craton. Surfaces are associated with the occluded drainage occur throughout with hummock grasslands on Quaternary sandplains, saltbush shrublands on calcareous soils and Halosarcia low shrublands on saline alluvia. Areas of red sandplains are covered with mallee-mulga parkland over hummock grasslands occurring widely in the east.

The Northern Kimberley This is tissected plateau of Kimberley Basin, covered with savanna woodland of Woolybutt and Darwin Stringybark over high Sorghum grasses and Plectrachne schinzii hummock grasses on shallow sandy soils on outcropping Proterozoic siliceous sandstone strata. Savanna woodlands on Eucalyptus tectifica - Eucalyptus grandiflora alliance over high Sorghum grasses on red and yellow earths mantling basic Proterozoic volcanics. Riparian closed forests of paperbark trees and Pandanus occur along drainage lines. Extensive Mangal occurs in estuaries and sheltered embayments. Numerous small patches of monsoon rainforest are scattered through the district.

The Nullarbor The Nullarbor is a Tertiary limestone plain with subdued arid karst features. Bluebush - Saltbush steppe are found widely in central areas, a low open woodlands of Myall over bluebush is found in peripheral areas, including Myoporum platycarpum and Eucalyptus oleosa in the east and west.

The Ord Victoria Plains Here level to gently undulating plains with scattered hills are found on Cambrian volcanics and Proterozoic sedimentary rocks; vertosols on plains and predominantly skeletal soils on hills; grassland with scattered Bloodwood and Snappy Gum with spinifex and annual grasses. The climate is of dry hot tropical, semi-arid summer rainfall. The lithological mosaic has three main components:

1. Abrupt Proterozoic and Phanerozoic ranges and scattered hills mantled by shallow sand and loam soils supporting Triodia hummock grasslands with sparse low trees.
2. Cambrian volcanics and limestones form extensive plains with short grass (Enneapogon spp.) on dry calcareous soils and medium-height grassland communities (Astrebla and Dichanthium) on cracking clays. Riparian forests of River Gums fringe drainage lines.
3. In the south-west, Phanerozoic strata are expressed as often lateritised upland sandplains with sparse trees. This component recurs as the Sturt Plateau Region in central Northern Territory.

The Pilbara There are four major components to the Pilbara Craton.

1. Hamersley. Mountainous area of Proterozoic sedimentary ranges and plateaux with Mulga low woodland over bunch grasses are found on fine textured soils and Snappy Gum over Triodia brizoides on skeletal sandy soils of the ranges.
2. The Fortescue Plains. This area comprises alluvial plains and river frontages. Salt marsh, mulga-bunch grass, and short grass communities are found on alluvial plains. River Gum woodlands fringe the drainage lines. This is the northern limit of Mulga (Acacia aneura).
3. Chichester. Archaean granite and basalt plains support shrub steppe characterised by Acacia pyrifolia over Triodia pungens hummock grasses. Snappy Gum tree steppes occur on ranges.
4. Roebourne. Quaternary alluvial plains are found with a grass savanna of mixed bunch and hummock grasses, and dwarf shrub steppe of Acacia translucens over Triodia pungens. Samphire, Sporobolus and Mangal occur on marine alluvial flats.

The Swan Coastal Plain This is arguably the most damaged ecosystem in the state. This is a low lying coastal plain, mainly covered with Tuart-Banksia woodlands. It is dominated by Banksia or Tuart on sandy soils, Allocasuarina obesa on outwash plains, and paperbark in numerous former swampy areas. In the east, the plain rises to duricrusted Mesozoic sediments dominated by Jarrah woodland. Three phases of marine sand dune development provide relief. The outwash plains, once dominated by Allocasuarina obesa-marri woodlands and Melaleuca shrublands, are extensive only in the southern part of the region.

The Tanami The Tanami region is of mainly red Quaternary sandplains overlying Permian and Proterozoic strata which are exposed locally as hills and ranges. The sandplains support mixed shrub steppes of Hakea suberea, desert bloodwoods, acacias and grevilleas over Triodia pungens hummock grasslands. Wattle scrub over Triodia pungens hummock grass communities occur on the ranges. Alluvial and lacustrine calcareous deposits occur throughout. In the north they are associated with Sturt Creek drainage, and support Crysopogon and Iseilema short-grasslands often as savannas with River Gum.

The Victoria Bonaparte Phanerozoic strata of the Bonaparte Basin in the north-western part are mantled by Quaternary marine sediments supporting Samphire - Sporobolus grasslands and mangal, and by red earth plains and black soil plains with an open savanna of high grasses. Outcrops of Devonian limestone karst in the west support tree steppe and vine thicket. Plateaux and abrupt ranges of Proterozoic sandstone, known as the Victoria Plateau, occur in the south and east, and are partially mantled by skeletal sandy soils with low tree savannas and hummock grasslands. In the south east there are limited areas of gently undulating terrain on a variety of sedimentary rocks supporting low Snappy Gum over hummock grasslands and also of gently sloping floodplains supporting Melaleuca minutifolia low woodland over annual sorghums.

The Warren The Warren region is of dissected undulating country of the Leeuwin Complex and Albany Orogen with a wet Mediterranean climate, with loamy soils supporting Karri forest, laterites supporting Jarrah-Marri forest, leached sandy soils in depressions and plains supporting paperbark-sedge swamps, and Holocene marine dunes are covered with Agonis flexuosa woodlands.

The Yalgoo Region This region is an interzone between South-western Bioregions and Murchison. It is characterised by low woodlands to open woodlands of Eucalyptus, Acacia and Callitris on red sandy plains of the Western Yilgarn Craton and southern Carnarvon Basin. The latter has a basement of Phanerozoic sediments. This Bioregion has been extended westwards to the boundary of the South-west Botanical Province, so that it now includes the Toolonga Plateau of the southern Carnarvon Basin. Semi-arid to arid, warm, Mediterranean climate. Mulga, Callitris- Eucalyptus salubris, and Bowgada open woodlands and scrubs on earth to sandy-earth plains in the western Yilgarn Craton. Rich in ephemerals.