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.
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The Southwestern Australia floristic region includes about 6,800 species, with large numbers of new species continuing to be described. If species that extend slightly beyond the Southwestern Botanical Province into a transitional interzone are included, 79% of the flora has been considered to be endemic, although newer analyses are lowering this figure.

Notable endemics include the monocot order Dasypogonales, the monocot families Dasypogonaceae, Ecdeiocoleaceae, and Anarthriaceae, and the eudicot families Cephalotaceae, Emblingiaceae, and Eremosynaceae. Other high-level endemic monocot clades regarded by some as families include Haemodoraceae subfamily Conostylidoideae, Baxteria and Calectasia of Dasypogonaceae, and Hopkinsia and Lyginia of Anarthriaceae. Recent estimates suggest that 92 (13%) of the 711 genera present are endemic.

Woody perennials in four families—the Myrtaceae, Proteaceae, Fabaceae, and Ericaceace (Epacridaceae)—dominate the flora. Much of the species diversity within these families is due to extensive adaptive radiation within a few large genera. Large genera for the region include Acacia (400 species), Banksia (with about 250 species of Dryandra), Eucalyptus (246 species), Grevillea (200+ species), Stylidium and Melaleuca (150+ species each), and Hakea and Caldenia (100+ species each).

Nodes of unusual species diversity are present along the south coast of Western Australia (Stirling Range, Fitzgerald River area) and the sandplains north of Perth (Mount Lesueur area). As in the fynbos of South Africa, large numbers of endemics with highly local patterns of distribution also characterizes kwongan. Local scale plant diversity is nearly as high as areas in the western Cape Region of South Africa. Vascular plant diversity in a sample of 0.1 ha stands of heathland in southwest Australia exhibited a range of 43–103 species, while jarrah forests and mallee stands had a lower range of 17–55 species.

At the regional level, southwestern Australia exhibits major differences in centers of highest diversity among the most important woody genera. Some genera— as for example, Banksia (Proteaceae), Adenanthos (Proteaceae), Leucopogon (Epacridaceae), and Eucalyptus (Myrtaceae)— are most speciose near the south coast or in southern kwongan and mallee communities. Other large genera have their highest diversity in northern kwongan—Grevillea (Proteaceae), Conostylis (Haemodoraceae), and Lechenaultia (Goodeniaceae).

Finally, a large group of genera show bimodal patterns of diversity reflecting   nodes of high species diversity in both northern and southern kwongan—Calothamnus (Myrtaceae), Melaleuca (Myrtaceae), Hakea (Proteaceae), Darwinia (Myrtaceae), and the Dryandra clade of Banksia (Proteaceae). Two other large genera, Acacia (Fabaceae) and Verticordia (Myrtaceae), are most diverse in the inland transition area of rainfall.

The vertebrate fauna of Australian Mediterranean-type ecosystems are not unusually diverse, nor are many endemic. Instead, the majority are populations of more typically arid or mesic habitat species whose ranges extend into Southwestern or South Australia. The majority of the vertebrates have relatively broad ecological niches rather than specialized requirements unique to the Mediterranean-climate regions.

The Mediterranean-climate mammal fauna includes only 57 species, 12 of these endemic. Interpreting the patterns of distribution of large mammals is difficult, however, because of the strong impact of both Aboriginal and European peoples. The extinction of a large megafauna in the late Quaternary left the Australian continent without large grazers or predators.

Bird diversity is also relatively low, with 285 species regularly present. Ten of these are endemic. Most notable among these endemics is the black swan (Cygnus atratus), shown on the state emblem for Western Australia.

Diversity is high among reptiles in southwestern Australia, an evolutionary consequence that stems in part from the low mammal diversity. Of the 177 native species, 27 areendemic. Amphibians include 33 native species, with 19 endemic species and four endemic genera.

With only small areas of riverine or freshwater habitat, fish diversity in Southwestern Australia is low. Only 20 native species are present but half of these are endemic. These include three endemic genera and one endemic family.

Although aboriginal people arrived in Australia at least as far back as 40,000 years ago, European settlement of southwestern Australia did not occur until the early 19th century. Agricultural expansion, grazing, and deforestation over the past century have had profound ecological effects and have fragmented the natural landscape. Introduced animal species such as rabbits, foxes, cats and ferrets have had a dramatic influence on the unique native biota of this region.