The region includes the Mediterranean forests, woodlands, and scrub ecoregions of Western Australia. The region has a wet-winter, dry-summer Mediterranean climate, one of five such regions in the world. The region covers 356,717 km2, consisting of a broad coastal plain 20-120 kilometres wide, transitioning to gently undulating uplands made up of weathered granite, gneiss and laterite. Desert and xeric shrublands lie to the north and east across the centre of Australia, separating Southwest Australia from the other Mediterranean and humid-climate regions of the continent.
Vegetation in the region is mainly woody, including forests, woodlands, shrublands, and heathlands, but no grasslands. Predominant vegetation types are Eucalyptus woodlands, eucalyptus-dominated mallee shrublands, and kwongan shrublands and heathlands, which correspond to the chaparral, matorral, maquis, and fynbos shrublands found in other Mediterranean-type regions. The region has generally nutrient-poor sandy or lateritic soils, which has encouraged rich speciation of plants adapted to specific ecological niches. The region hosts a great diversity of endemic species, notably among the protea family (Proteaceae).
Southwest Australia is home to many endemic carnivorous plants, including more than half the world's species of sundews (Drosera), the bladderwort subgenus Polypompholyx, the Byblis gigantea complex of rainbow plants (composed of two species, Byblis lamellata and B. gigantea), and the pitcher plant Cephalotus follicularis, sole species in the plant family Cephalotaceae.
The honey possum (Tarsipes rostratus) is a tiny marsupial endemic to Southwest Australia that feeds on nectar and pollen, and is an important pollinator for several southwestern plants including Banksia attenuata, Banksia coccinea, and Adenanthos cuneatus. Other mammals endemic to Southwest Australia are the western brush wallaby (Macropus irma) and the quokka (Setonix brachyurus).
The World Wide Fund for Nature and Interim Biogeographic Regionalisation for Australia (IBRA) divide the region into six ecoregions and ten biogeographic regions:
- Coolgardie woodlands (IBRA Coolgardie and Hampton)
- Esperance mallee (IBRA Esperance Plains and Mallee)
- Jarrah-Karri forest and shrublands (IBRA Warren)
- Southwest Australia savanna (IBRA Geraldton Sandplains, Avon Wheatbelt, and Yalgoo)
- Southwest Australia woodlands (IBRA Jarrah Forest)
- Swan Coastal Plain scrub and woodlands (IBRA Swan Coastal Plain)
The transitional Coolgardie, Hampton, and Yalgoo regions are generally drier than the rest of the Southwest. They considered part of Southwest Australia by the WWF, but are considered part of the Central Australian or Eremaean Region by the Western Australian Herbarium.
Southwest Australia has several permanent rivers and streams, including the Swan–Avon system, the Blackwood River, and other short rivers. The perennial rivers drain from the interior plateau and Darling Range across the coastal plain. Their flow is strongly seasonal, corresponding to the Southwest's wet winter–dry summer weather pattern. The perennial streams extend from east of Esperance on the south coast to the Arrowsmith River north of Perth, most often in areas with 700 mm or more of annual rainfall.
Arid regions separate Southwest Australia's freshwater habitats from Australia's other year-round rivers. As with its terrestrial flora, Southwest Australia's Mediterranean climate and biogeographic isolation has given rise to a distinct freshwater ecoregion with many endemic species.
There are fifteen freshwater fish species, including nine exclusively freshwater species, three estuarine species adapted to brackish water, and three diadromous species that spend part of their life-cycle in the sea. The exclusively freshwater species are endemic to Southwest Australia, as are two estuarine species. The salamanderfish (Lepidogalaxias salamandroides) is the sole species in the endemic family Lepidogalaxiidae. Salamanderfish can aestivate during the summer months, an adaptation to the region's dry summers. Other endemic species are the nightfish (Bostockia porosa), western mud minnow (Galaxiella munda), black-stripe minnow (Galaxiella nigrostriata), Balston’s pygmy perch (Nannatherina balstoni), western pygmy perch (Nannoperca vittata), and western galaxias (Galaxias occidentalis).
Southwest Australian varieties of the diadromous common galaxias (Galaxias maculatus) and spotted galaxias (Galaxias truttaceus) have adapted so they can live their life-cycle and reproduce in fresh water.
- Wardell-Johnson, Grant; Wardell-Johnson, Angela; Bradby, K; Robinson, Todd; Bateman, Bill; Williams, K; Braun, K; Beckerling, J; Burbridge, Michael (2016), Application of a Gondwanan perspective to restore ecological integrity in the south-western Australian global biodiversity hotspot, Department of Environment and Agriculture, retrieved 11 October 2016
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