Biological diversity in Southwest Australia
Contents
Overview Map of Southwest Australia. (Source: CI (Biological diversity in Southwest Australia) / CABS)
The Southwest Australia Hotspot occupies an area of about 356,717 km2 on the southwestern tip of Australia, in the state of Western Australia. As defined, this hotspot comprises the Southwest Botanical Province, but excludes the neighboring Southwestern Interzone. This hotspot is one of five Mediterranean-type ecosystems in the world. Most rain occurs in this area during the winter months, while the summers are characteristically dry. A broad coastal plain, 20-120 kilometers wide, grades into gently undulating uplands, with weathered granite, gneiss and lateritic formations. Further inland, rainfall decreases and the length of the dry season increases.
Native plants are well adapted to the nutrient-poor sandy and lateritic soils, which also support broadacre cropping and sheep grazing. Vegetation in the province is mainly woody, comprising forests, woodlands, shrublands, and heaths, but no grasslands. Principal vegetation types in this region are Eucalyptus woodlands, and the Eucalyptus-dominated 'mallee' shrubland. Kwongan is a term adapted from the Aboriginal Noongar language to cover the various Western Australian types of shrubland, comparable with the maquis, chaparral, and fynbos of other countries with Mediterranean-type systems. The principal structural types of Kwongan are thicket, scrub-heath, and heath, which together comprise about 30 percent of the original vegetation. A number of vegetation units are endemic, including some types of Eucalyptus forests and some forms of kwongan.
Unique and Threatened Biodiversity
Impressive plant endemism in Southwest Australia is attributed to millions of years of isolation from the rest of Australia by the country's vast central deserts. Extreme climate shifts and poor soils also promoted specialization of the region's flora.
Plants
Nearly 80 percent of the plant species in Southwest Australia are found nowhere else in the world, including many of the brightly-colored members of the genus Banksia. Banksia coccinea is pictured here. (Source: © Densey Clyne/Auscape) Of more than 5,570 species of vascular plants found here, nearly 2,950 are endemic (about 53 percent). A significant number of genera are also endemic: 87 of 697 genera (12.5 percent) are found nowhere else in the world. Additionally, four families are endemic: Ecdeiocoleaceae, Emblingiaceae, Eremosynaceae and the monotypic Cephalotaceae, which is represented by the Albany pitcher plant (Cephalotus follicularis, VU), a carnivorous plant that traps insects in its modified leaves.
The ten largest families in the hotspot (including the Myrtaceae with 785 species, of which 92 percent are endemic, and Proteaceae with 684 species, with 96 percent endemism) comprise 61 percent of the flora. The number of species per genus averages eight, although the ten largest genera far exceed this figure: for example, the wattles of the genus Acacia (Mimosaceae, Fabales) possess 397 species (51 percent endemic), while the gums, or Eucalypts, of the genus Eucalyptus (Myrtaceae) possess 246 species (52 percent endemic) .
The Banksia plants of the family Proteaceae are among the most distinctive of the flowering plants found in this hotspot. These brilliantly-coloured flowering plants range in form from trees to small prostrate plants, one of which even has underground stems. Kangaroo paws (Anigozanthos spp., Haemadoraceae) are another brightly colored flower from the region: the unopened cluster of flowers are thought to resemble the paw of a kangaroo.
The region's flagship tree species include three eucalypts: jarrah (Eucalyptus marginata), marri (E. calophylla), and karri (E. diversicolor). While jarrah and marri grow to only about 20-30 meters in height, some karri forests have canopies up to 70 meters high, and individual trees may grow as high as 80 meters, ranking the karri as one of the tallest trees on earth.
Vertebrates
Birds
Over 280 native bird species occur in the region, 12 of which are endemic. The level of endemism is slightly higher than in other Mediterranean-type hotspots, and the region is considered an Endemic Bird Area (EBA) by BirdLife International.
The region is home to 22 parrot species (three endemics), including Carnaby's black-cockatoo (Calyptorhynchus latirostris, EN). The noisy scrub-bird (Atrichornis clamosus, VU), which earned its name through the loud vocalizations of its males, was presumed extinct until a small population was rediscovered in 1961. Several other bird species are near threatened or rapidly declining in the face of habitat loss, modification and fragmentation or inappropriate fire regimes.
Mammals
The quokka (Setonix brachyurus, VU) is a member of the kangaroo family (Macropodidae). This endemic marsupial faced extinction on mainland Australia, but it is currently making a come-back in some areas. The largest population is on Rottnest Island, off the coast of Southwest Australia. (Source: © John Cancalosi) This hotspot is known to be the home of about 60 species of native mammal, 12 of which are endemic: among these is included the mouse-sized, nectarivorous honey possum (Tarsipes rostratus), the only representative of the family Tarsipedidae, and an animal that is confined to the coastal plain heaths. Another interesting endemic is the quokka (Setonix brachyurus, VU), a small, wallaby confined to three areas: the mainland, where it has been declining in numbers, and two small offshore islands (Rottnest Island and Bald Island), where it appears to be doing well.
Some mammal species have become de facto endemics to Southwest Australia through range contraction, having been extirpated from the rest of their natural ranges. The squirrel-sized numbat (Myrmecobius fasciatus, VU), is one example- the marsupial equivalent to the anteaters of South America and also known by the names of walpurti or banded anteater, the numbat is the only member of the family Myrmecobiidae. It has also been chosen as one of the State symbols for Western Australia. Besides this species, four other threatened mammals are endemic to the hotspot, including the most threatened species in the hotspot, Gilbert’s potoroo (Potorous gilberti, CR), thought to be extinct until its rediscovery in the Two People’s Bay Nature Reserve in 1994, which is still the only place from which it is known to survive.
Reptiles
As might be expected in a country that leads the world in reptile diversity, there are a wide variety of reptile species in Southwest Australia. Of the 175 or so species present, about 30 (i.e. 15 percent) are endemic to the hotspot. The western swamp turtle (Pseudemydura umbrina, CR), a member of an endemic, monotypic, genus, is the most threatened reptile in Australia and is one of the 25 most threatened freshwater turtles in the world. The wild population stands at less than 100 and is now only found in one or two swamps near Perth.
Other threatened reptiles include the Yinnietharra rock dragon (Ctenophorus yinnietharra, VU), an understudied lizard species that is confined to two granitic rock outcrops in the region and does not inhabit outcrops of different origins, and the Lake Cronin snake (Echiopsis atriceps, VU), whose very limited range includes unprotected private lands.
Amphibians
There are more than 30 species of amphibian recorded for this hotspot, nearly two-thirds of which are endemic, including four species that represent endemic genera: the turtle frog (Myobatrachus gouldii), Nicholl's toadlet (Metacrinia nichollsi), the sandhill frog (Arenophryne rotunda), and the harlequin frog (Spicospina flammocaerulea, VU). Besides the last-mentioned species, which was only described in 1997 and is known to have a limited area of occurrence, two other frog species are considered threatened, one of which, the yellow-bellied frog (Geocrinia vitellina, VU), is represented by six populations confined to a 6.3 km2 area east of the Leeuwin-Naturaliste Ridge.
Freshwater Fishes
The Southwest Australia hotspot has very little freshwater habitat, which leads to a low fish diversity. Thus, only about 20 species of native freshwater fish have been recorded in the area. However endemism is comparatively high among these species, with about half of these species being endemic (including three whole genera). The most remarkable of these species is the salamanderfish (Lepidogalaxias salamandroides), which is the only member of the hotspot’s single endemic family (Lepidogalaxiidae).
Human Impacts
The numbat (Myrmecobius fasciatus, VU) is a marsupial 'anteater' that is the only member of its entire family. The species is now restricted to the hotspot, although it once had a much wider distribution. (Source: © John Cancalosi) The greatest human impact in Southwest Australia has been the clearing of native vegetation for agriculture. Agricultural development began in 1829, with the arrival of the first European settlers to the region. However, because of the poor soils, development progressed slowly until the 1890s, when phosphate fertilizers were introduced. Today, most usable private land in the region is farmed, although it requires the application of phosphate, as well as zinc, copper, cobalt, and molybdenum. Because of the region's long dry seasons, bush fires have traditionally been used for hunting and clearing land. Although native plants are highly adapted to fire, the alteration or intensification of burning regimes can dramatically change the composition and condition of the natural vegetation.
One of the most serious current threats to the natural vegetation of Southwest Australia is the spread of root disease, or "jarrah dieback" caused by the root fungus Phytophthora cinnamomi. The disease was first noticed in the jarrah forests in 1940 but not identified until 1965. By that time, thousands of hectares of forest had been infected and killed. Root disease is now spreading to other habitats, including kwongan shrublands, and in particular Stirling Range National Park, where it has caused mortality among susceptible plants like the peculiar grass trees (Xanthorrhoea spp.) and members of the Proteaceae, especially the banksias (Banksia spp.).
Large-scale mining for bauxite is increasingly a threat to Southwest Australia's ecosystems; the region is one of the largest producers of alumina in the world. Open-pit mining destroys habitats and pollutes waterways. However, recent reclamation efforts have been successful at establishing native plants in abandoned mine pits, a technique that holds promise for land management in the region.
Introduced alien species, especially foxes (Vulpes vulpes) and feral cats (Felis 'sylvestris'), threaten native fauna and have caused major declines in species like the numbat. Land managers have successfully poisoned these alien species with sodium flouroacetate, also known as 'poison compound 1080'. Amazingly, native mammals have evolved a natural tolerance to the poison because the compound is naturally prevalent in the leaves of many native legumes.
Today, of the principal vegetation types found in the region, 89 percent of the Eucalyptus woodlands have been lost, while 50 percent of the Eucalypt-dominated mallee and 59 percent of the Kwongan heath formations have been cleared. In total, only 30 percent of the original vegetation remains in more-or-less pristine condition.
Conservation Action and Protected Areas
Karri forest in Western Australia. The karri (Eucalyptus diversicolor) is one of the largest trees on Earth. Individual trees can reach as much as 80 meters. (Source: © Jean-Paul Ferrero/Auscape) A total of about 38,000 km2, 11 percent of the land area in Southwest Australia, is under some form of official protection, virtually all of it in IUCN categories I to IV. Many reserves, however, are too small to adequately protect biological resources, and many ecosystem types are not well represented in the protected area system. Typically, the region's reserves represent land that was unsuitable for farming in the early days of settlement. Arable lands are almost exclusively privately owned, and a number of rare species are found only on these private lands.
Southwest Australia represents one of the best opportunities for long-term conservation among the hotspot because of its relatively low population density. However, immediate action is necessary to ensure the survival of the region's unique and highly threatened flora and fauna. In addition to maintaining the integrity of existing protected areas, the current network should be expanded through the creation of new reserves from private and public lands to include species and ecosystems not yet protected.
There are a number of conservation programs and projects currently operating in Southwest Australia. The Western Shield Program, run by the Department of Conservation and Land Management, is working to bring at least 13 native fauna species back from the brink of extinction by controlling introduced predators, such as the fox and feral cat. The main weapon in the fight against these predators is the use of the naturally occurring poison known as compound 1080, found in native plants called 'poison peas' (Gastrolobium spp.). While the native animals have evolved with these plants and have a high tolerance to the poison, introduced animals have only recently been exposed, meaning that they haven't had enough time to develop such a tolerance.
Project Eden is the arid scientific conservation component of Western Shield. This project uses innovative techniques to eradicate feral herbivores and predators and rejuvenate 105,000 hectares of arid zone habitat on Peron Peninsula at Shark Bay for threatened native fauna, and, to promote their reintroduction into the area.
A project focused on the ecology, abundance and predator dynamics of threatened Shark Bay mammals, has been run by the Sustainable Ecosystems program of CSIRO, the Australian national research agency, in partnership with a local salt mining community at Useless Loop for the past 16 years. This work has studied in detail the life cycle of some of Australia's most threatened animals on Bernier and Dorre Islands and has reintroduced the mari, or western barred bandicoot (Perameles bougainville) and burrowing bettong, (Bettongia lesueur), also known as the boodie. These two species had become extinct on the Australian mainland, with their range restricted to a few islands. This extirpation is made worse by the fact that the boodie was once of the most widespread of Australia's kangaroos, its burrows (now abandoned by the original owners) still visible deep in the heart of Australia.
Other conservation projects in Southwest Australia include community-based recovery programs for the threatened Carnaby's black-cockatoo, western ground parrot (Pezoporus wallicus flaviventris), noisy scrub-bird, malleefowl (Leipoa ocellata) and the marsupial 'mouse' known as the dibbler (Parentechinus apicalis). These involve organizations such as Birds Australia, WWF, CSIRO, Department of Conservation and Land Management and local Landcare groups.
Further Reading
- Hopper, S.D., & Gioia, P. 2004. The Southwest Australian Floristic Region: Evolution and Conservation of a Global Hot Spot of Biodiversity. Annu. Rev. Ecol. Evol. Syst. 35: 623-650.
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