Southwest Australia

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.

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). [Read more…] about Plant Diversity

The vertebrate fauna of Australian MTEs shows no unusual level of high diversity of endemism. The majority of the fauna in these MTEs are populations of more typically arid or mesic habitat species whose ranges extend into Southwestern or South Australia. Thus, the majority of the vertebrates appear to have relatively broad ecological niches rather than specialized requirements unique to the MTEs.

For mammals, the mediterranean-climate fauna includes only 57 species, 12 of these being endemic. Interpreting the patterns of distribution of large mammals is made difficult, however, by the strong impact that both Aboriginal and European populations have had on this fauna. 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 in part of the low mammal diversity. There are 177 native species, with 27 of these being endemic. 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.

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.