Mediterranean-climate conditions are present in two portions of Australia: southwestern Western Australia, and South Australia around Adelaide. The core Southwestern Floristic Province of Western Australia is about 302 km x 103 km in size, similar to the California Floristic Province. A region that experiences two rainy seasons is often included as well, and almost doubles the core MTE area. While mean annual rainfall commonly ranges from 350 to 800 mm over much of southwestern Australia, it reaches as high as 1500 mm at the extreme southwestern corner of Western Australia and drops to about 250 mm at the eastern edge of the Mediterranean-climate region, where it transitions to arid communities.
The region is relatively level, featuring a few small mountains no more than 1000 m high. Much of the area consists of a low plateau dissected into broad valleys deeply weathered in place. The soils of southwestern Australia, as in the Cape Region of South Africa, are generally very old, highly weathered, acidic, and low in nutrient availability.
In the core Mediterranean-climate regions, the highest rainfall zones receive 800–1200 mm of annual rainfall. These areas support evergreen forests and woodlands dominated by Eucalyptus marginata (jarrah), E. calophylla (marri), E. diversicolor (karri), and E. gomphocephala (tuart). Low Banksia woodlands and coastal heath are also present. At intermediate rainfall regimes of 300–800 mm, the dominant vegetation is a mosaic of shrubby woodlands (mallees) and heathland communities termed kwongan. The vegetation and dominant species are finely tuned to small changes in edaphic conditions that influence nutrient and water availability. Human impacts on these ecosystems have been severe over the past century.
High levels of species diversity and endemism characterize the vascular plant flora, with only moderate diversity and endemism present in most vertebrate groups. The high levels of vascular plant diversity has been related to the development of a complex mosaic of landforms and soils during the Tertiary and Quaternary, the geologic history of oscillating moisture regimes through the Quaternary in the absence of glaciation, isolation of southwestern Australia from the east by the arid Nullarbor Plain, and interactions of gene pools from both paleotropical and temperate assemblages.