Southwest Australia is a biogeographic region in Western Australia. It includes the Mediterranean-climate area of southwestern Australia, which is home to a diverse and distinctive flora and fauna.
The region is also known as the Southwest Australia Global Diversity Hotspot.
Geography
The region includes the Mediterranean forests, woodlands, and scrub ecoregions of Western Australia. 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. Bluff Knoll in the Stirling Range is the highest peak in the region, at 1,099 metres (3,606 ft) elevation.
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.
Climate
The region has a wet-winter, dry-summer Mediterranean climate, one of five such regions in the world. During the winter months, westerly winds bring cool weather, clouds, and rainfall to Southwest Australia. In the summer months, the lower-latitude anticyclonic belt, with generally dry easterly winds, moves southwards, increasing temperatures and decreasing rainfall. Tropical cyclones, off the shore of northern Western Australia during the December-to-March northern wet season, occasionally reach as far south as Perth before moving inland, bringing floods and damaging winds to the west coast and rain to the dry interior.
Rainfall generally decreases from south to north, and with distance from the coast. The highest rainfall is typically in the Karri Forest Region between Pemberton and Walpole, up to 1,400 mm (55 inches) annually.
The region has been experiencing the effects of human induced climate change. Average annual rainfall has declined as much as 20% since the 1970s, declining by 10-20 millimetres each decade. Summertime maximum temperatures have increased by 0.1º to 0.3º C per decade, and the average number of days per year over 40º C in Perth has doubled over the last century. Lower rainfall and higher temperatures have reduced stream flow and inflow into drinking water and irrigation catchments since the 1960s. The summer of 2021/22 was the hottest on record.
Flora
Southwest Australia is recognized as a floristic province.
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.
Fauna
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).
Southwest Australia is an Endemic Bird Area, with several endemic species of birds including the long-billed black cockatoo (Zanda baudinii), western corella (Cacatua pastinator), noisy scrubbird (Atrichornis clamosus), red-winged fairywren (Malurus elegans), western bristlebird (Dasyornis longirostris), black-throated whipbird (Psophodes nigrogularis), white-breasted robin (Eopsaltria georgianus), and red-eared firetail (Stagonopleura oculata). The western rufous bristlebird (Dasyornis broadbenti litoralis), an endemic subspecies of rufous bristlebird, is now extinct.