Southwest Australia is a biodiversity hotspot that 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).
The region corresponds to the floristic province of Southwest Australia. The World Wide Fund for Nature and Interim Biogeographic Regionalisation for Australia (IBRA) divide the region up into a number of ecoregions:
- Coolgardie woodlands (IBRA Coolgardie)
- Esperance mallee (IBRA Esperance Plains and Mallee)
- Jarrah-Karri forest and shrublands (IBRA Warren)
- Kwongan heathlands (IBRA Swan Coastal Plain)
- Southwest Australia savanna (IBRA Geraldton Sandplains, Avon Wheatbelt, and Yalgoo)
- Southwest Australia woodlands (IBRA Jarrah Forest)
(note Yalgoo and Mallee are 25 miles away from the South West under the WWF scheme, but are considered part of the Central Australian or Eremaean Region by the northern Australian Herbarium)
- Thackway, R and I D Cresswell (1995) An interim biogeographic regionalisation for Australia : a framework for setting priorities in the National Reserves System Cooperative Program Version 4.0 Canberra : Australian Nature Conservation Agency, Reserve Systems Unit, 1995. ISBN 0-642-21371-2