Core mediterranean-climate conditions are present in southwestern Western Australia and in South Australia around Adelaide. The core Southwestern Floristic Province of Western Australia is about 302 x 103 km2 in area, similar in size to the California floristic province. A broader region with transitional rainfall patterns with biseasonal distribution is often included as well, almost doubling the core MTE area. While mean annual rainfall is commonly 350–800 mm over much of southwestern Australia, it reaches as high as 1500 mm at the extreme southwestern corner of Western Australia and drops to a low of about 250 mm at the eastern edge of the mediterranean-climate region in a transition to arid communities.
Topographic heterogeneity is very low over this region, with elevations of the few small mountains reaching to no more than 1000 m. Much of the region is a low laterized plateau dissected into broad valleys with deep in situ weathering. Soils of southwestern Australia, as in the Cape Region of South Africa, are generally very old, highly weathered, acidic, and low in nutrient availability. For the true mediterranean-climate regions, the highest rainfall zones with 800–1200 mm of annual rainfall 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.