The Southwestern Australia floristic region includes about 6800 species present, with large numbers of new species continuing to be described. Including species that extend slightly beyond the Southwestern Botanical Province into a transitional interzone, 79% of the flora has been considered to be endemic, but newer analyses are lowering this figure.
Notable endemics include one monocot order (Dasypogonales) and at least six families,with the monocot Dasypogonaceae, Ecdeiocoleaceae, and Anarthriaceae, and the eudicot 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 high 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 Dryandra about 250 species), 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, Fizgerald River area) and in the sandplains north of Perth (Mt. 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 reaches almost 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 the both nodes of high species diversity in 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.