Southwest Australia

John S. Beard⁴³

The Southwest Australia Hotspot is located in the enormous State of Western Australia, between latitudes 26° and 36°S, and longitudes 114° and 126°E, and originally covered 356 717 km² in extent. The climate is Mediterranean in character, with most of the rain falling in winter. The total average winter rainfall varies from over 1 400 mm in the extreme southwest to 300 mm in the interior, the number of dry months increasing from 3–4 to 7–8 as rainfall declines. Inland, the boundary of the hotspot closely approximates the 300-mm isohyet.

As defined here, the Southwest Australia Hotspot comprises the Southwest Botanical Province of Beard (1980, 1990; Davis et al. 1995), but excludes the neighboring Southwestern Interzone. The vegetation of the Southwest Botanical Province has been mapped in detail to show original natural vegetation before the arrival of Europeans. The vegetation is almost entirely woody, forming forest, woodland, shrubland, and heath, and there are no grasslands. Among the principal vegetation types in this region are Eucalyptus woodlands (formerly covering 25.9% of the Province, but of which 89% has been cleared), and the Eucalyptusdominated “mallee” shrubland (which formerly covered 22% of the Province, but has now been 50% cleared). Kwongan is a term adapted from the Aboriginal Nyungar language to cover the various Western Australian types of Mediterranean shrubland, comparable with the maquis, chaparral, and fynbos of other countries with such systems (Pate and Beard 1984). The principal structural types of kwongan are thicket, scrub-heath, and heath, which together comprise about 30% of the original vegetation. These formations have also been cleared to a large degree: 59% of the once extensive kwongan heath formations have been cleared.

Many of the vegetation units in this Province are of an endemic character. While Eucalyptus-dominated forests, woodlands, and mallee occur in eastern Australia, the dominant species and a majority of the associated species in the west are endemic. Kwongan is a formation unique to Western Australia. Although analogous “heaths” occur in the east, certain structural forms of kwongan such as the Acacia-Casuarina-Melaleuca thickets are endemic. The majority of species in the kwongan flora are also endemic.

In terms of plant diversity and endemism, the Southwest Botanical Province has a total of 5 571 species, of which 2 948 (52.9%) species are entirely confined to this Province; a further 1 462 species (26.2% of species present) extend slightly beyond its borders into the Southwestern Interzone or Eremaea and can still be considered endemic to a more broadly defined concept of southwestern Australia (Beard et al. 2000). The flora of the Southwest Botanical Province comprises 139 families, of which four are endemic: the Ecdeiocoleaceae, Cephalotaceae (represented by the pitcher plant Cephalotus follicularis, VU), Emblingiaceae, and Eremosynaceae. In addition, 87 (12.5%) of 697 genera are endemic. The ten largest families (including the Myrtaceae with 785 species, of which 92% are endemic, and Proteaceae with 684 species, 96% endemic) comprise 61% of the flora, while the number of species per genus averages eight, although the ten largest genera (including Acacia with 397 species, 51% endemic, and Eucalyptus with 246 species, 52% endemic) far exceed this figure (Beard et al. 2000).

Foremost among a wealth of flagship plant species in this region are the Banksias of the family Proteaceae (which are 100% endemic), the “blackboys” of the family Xanthorrhoeaceae (so named because these plants with their tall inflorescences reminded early settlers of aborigines with spears), and the giant Eucalyptus characteristic of the southern portion of this region, including the jarrah (E. marginata), the marri (E. calophylla), and the karri (E. diversicolor), the latter forming a canopy at about 70 m, with some individuals attaining 80 m or more, ranking this southwestern Australian endemic as one of the tallest trees on Earth.

Vertebrate diversity in this hotspot is not nearly as high as that of plants. Mammals number 57 species, of which 12 are naturally endemic to this hotspot. Although overall diversity is low, there are some very interesting higher-level endemics. The honey possum (Tarsipes rostratus), among the few truly nectivorous mammals, is the only living representative of the family Tarsipedidae, and the numbat (Myrmecobius fasciatus, VU) is the only member of the marsupial family Myrmecobiidae. Another appealing flagship marsupial, this time an endemic genus from the kangaroo family, Macropodidae, is the quokka (Setonix brachyurus, VU), a small wallaby confined to the mainland, where it has been declining in numbers, and two small offshore islands (Rottnest Island and Bald Island).

Bird diversity is relatively low, but endemism is a little higher than for most of the other Mediterraneantype systems. Some 285 species are regularly recorded from this region, and 10 of these are endemic. BirdLife International also lists Southwest Australia as one of their Endemic Bird Areas (Stattersfield et al. 1998). Among the birds, flagship species include the black swan (Cygnus atratus), the principal state emblem of Western Australia, and the noisy scrub bird (Atrichornis clamosus, VU), so called because the male is particularly vociferous.

Reptile diversity is quite high, not surprisingly, since Australia, as a country, is considered the world leader in reptile diversity (Mittermeier et al. 1997). A total of at least 177 species are found in the Southwest Australia Hotspot, of which 27 are endemic. The most interesting of these is the western swamp tortoise (Pseudemydura umbrina, CR), a monotypic genus endemic to the region and now found only in one or two small swamps at Bullsbrook, near Perth. Another endemic genus is the short-nosed snake (Elapognathus minor), which is confined to the humid coastal plains of the hotspot. Amphibians are somewhat less diverse, as is usually the case for a dry region, but the 33 species include 19 endemics, four of which represent endemic genera: Myobatrachus gouldii, Metacrinia nichollsi, Arenophryne rotunda, and Spicospina flammocaerulea.

The Southwest Australia Hotspot has a very small amount of freshwater habitat and a correspondingly small fish fauna with only 20 native species. It is, however, one of the most distinctive faunas, 10 (50%) of its species and three of its genera being endemic. Most remarkable is the salamanderfish (Lepidogalaxias salamandroides), which constitutes the one endemic family (Lepidogalaxiidae) that is entirely restricted to this small hotspot, surviving in the harsh conditions of its ephemeral pools and highly acidic peat habitats. Located near the southern limits of the freshwater world, the hotspot's fish fauna is dominated by remnants of ancient Gondwanan groups including the southern lampreys and galaxiids (Allen et al. 2002).

The biggest threat to the unique biota of Southwest Australia has been land clearing for agriculture, as the entire Southwest Botanical Province falls within the Intensive Land-use Zone. This has left a legacy of substantial habitat loss and probably species loss as well. Nearly all the land that can be farmed economically has been now utilized, and expansion of farms into virgin areas has stopped. Clearing of remaining vegetation on existing farms still takes place, though restrictions have been placed on certain areas for environmental purposes such as protection of water supplies. It is estimated that 30% of the native vegetation in this hotspot remains in the Intensive Land-use Zone, which is more or less synonymous with the Southwest Botanical Province; the Swan Coastal Plain, wheatbelt, and mallee regions have been largely cleared, and only patches of original native vegetation remain (Shepherd et al. 2002).

Currently, the most serious threat is the spread of root disease (“jarrah dieback”) caused by the root fungus Phytophthora cinnamomi. This disease was first noticed in the jarrah forests around 1940. Unfortunately, the pathogen responsible was not identified until 1965, by which time thousands of hectares of forest, both trees and associated flora, were affected. By 1974, it was estimated that 2 820 km² had been affected and that the disease was spreading at the rate of 200 km² per year, which turned out to be an overestimate. The disease can be severe on sites where soil drainage is obstructed; most of these have now been impacted so that only isolated trees continue to be attacked by “jarrah dieback.” However, new concern has arisen as Phytophthora has been found attacking kwongan habitats outside the forests, in particular the Stirling Range National Park, where it has caused mortality among susceptible plants like the grass trees (Xanthorrhoea spp.) and members of the Proteaceae, especially Banksias. As indicated above, these are particularly important flagship species and their loss would be tragic.

Conservation in Western Australia is vested in the hands of the State Department of Conservation and Land Management (CALM), which maintains a series of national and state protected areas. Some 10.8% of the hotspot is under some form of official protection, a figure that remains unchanged when one includes only reserves classified in IUCN categories I to IV. An important issue, however, is that of the representativeness of existing protected areas. Biodiversity surveys were never used to determine appropriate sites for protected areas, meaning that there is an uneven representation of different ecosystem types in this region. Many existing reserves are small, and are isolated “islands” of natural vegetation within vast areas of farmlands. Furthermore, many species endemic to the Southwest Province have very restricted ranges and a number of rare species are found only on private land, meaning that the cooperation of landowners has to be sought to ensure their protection.

All things considered, the Southwest Australia Hotspot is one of the least threatened of the hotspots, and has one of the best opportunities to achieve representation of all the region's biodiversity in protected areas. Maintaining the integrity of the existing protected area coverage is clearly one step, but much vacant land that is not good for either farming or pastureland, and unlikely to be disturbed in the near future, could also be conserved. And finally, and perhaps most important, is an increasing desire on the part of the government to look into the possibility of creating new reserves in key pieces of privately owned land in under-represented vegetation types – in order to increase restricted-range species and ecosystem coverage in the region's protected area network. A variety of approaches to creating such private reserves through tax and other fiscal incentives already exist, and could very well be put to use here. If these steps can be taken and succesfully implemented over the long term, the Southwest Australia Hotspot could indeed become one the best-protected hotspots on Earth.

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