Climatic and soil factors determine the limits of the smaller plant areas, but the broad zones of vegetation in Australia -- known as the formation types -- show a close correlation with the mean annual rainfall in the zone. The outstanding feature of Australian climate is the arid center surrounded by concentric belts with successively increasing rainfall. Consequently there is a wide range of vegetation types.
1) Annual Rainfall Less than 5 Inches (125 mm). Desert, chiefly sand dune or sand plain, occurs in this area. The dominant plants are hard-leaved perennial grasses belonging to the genera Triodia and Spinifex.
2) Annual Rainfall 5-10 Inches (125-250 mm). Semiarid regions of two main kinds occur. a) Shrub-steppe, or treeless regions of small shrubs chiefly of the genera Atriplex (saltbushes) and Kochia (bluebushes), dominates. Here plants are extremely drought-resistant. These areas are used for sheep grazing and resemble the sagebrush of the western United States. b) Arid scrub occurs on sandy soils or rocky hills as dense growths of small trees and shrubs dominated by various species of acacia; the most widespread is A. aneura (mulga). Both types show an abundance of annual plants after infrequent rains.
3) Annual Rainfall 10-20 Inches (250-500 mm). Two main vegetation types occur in this zone. In the south, where the rainy season is restricted to winter months, mallee scrub occurs. This is a dense assemblage of plants dominated by various dwarf species of eucalyptus that produce several stems from an underground rootstock and a restricted canopy of leaves at the ends of the branches. In northern and eastern Australia, with predominantly summer rainfall, grasslands occur. The most extensive and important are species of Astrebla (Mitchell grass) and Iseilema (Flinders grass).
4) Annual Rainfall 20-30 Inches (500-750 mm). In this zone occur savanna grasslands, open, parklike regions dominated by eucalyptus with essentially grassy or herbaceous undergrowth. These communities have been used extensively for pastoral purposes and for wheat growing. Savanna grasslands occasionally occur within the sclerophyll (hard-leaved) forest zone, but on soils of higher nutrient status than are generally found in such zones.
5) Annual Rainfall 30-50 Inches (750-1,250 mm). Sclerophyll forests are characteristic of this climatic zone. These are forest communities dominated by various species of eucalyptus, the trees growing close together and with a dense undergrowth of hard-leaved shrubs; grass is rare. On the drier side these forests merge into savanna woodlands and on the wetter side into rain forest. The drier sclerophyll forests resemble the chaparral of California. The greatest concentration of typical Australian species occurs in the sclerophyll forests, which are important sources of hardwood.
6) Annual Rainfall Greater than 50 Inches (1,250 mm). Rain forests are restricted to zones of higher rainfall and usually also to soils derived from basalt. The rain forests contain a varied tree vegetation, often with no specific dominant, including many creepers and luxuriant undergrowth. In the tropical rain forests the species are predominantly of Indo-Malayan origin. In the southern temperate rain forests an antarctic element is prominent.
Approximately 15,000 species of flowering plants are known from Australia, and about three quarters of these are indigenous. J. D. Hooker, in his Introductory Essay to the Flora of Tasmania (1860), pointed out that three main elements played major roles in the development of Australian flora: an antarctic, an Indo-Melanesian, and a native Australian element.
The term "antarctic element" embraces species or allied species common to southeastern Australia, New Zealand, the subantarctic islands, and the southern Andes of South America. Examples of genera showing this distribution are Nothofagus, Drimys, Lomatia, Araucaria, Gunnera , and Acaena. These genera have also been found as fossils in the early Tertiary of the now icebound Seymour Island and Graham Land. Such plants today have a solely antarctic distribution; it has been suggested that they or their ancestors originated when Australia was part of the "supercontinent," Gondwanaland. When Gondwanaland broke up and the continents "drifted" to their present positions, members of the antarctic element became widely separated. It is clear, however, that this antarctic element was widespread in Australia during early Tertiary times, for Oligocene deposits both in South Australia and in Victoria show Nothofagus and Lomatia admixed with Australian genera such as Eucalyptus, Banksia , and Hakea. Today the antarctic element is prominent in temperate rain forests. The term "antarctic element" is also sometimes used to include larger groups of plants that today are restricted to the southern hemisphere and that are common to South Africa and Australia. Genera common to these two areas are Caesia, Bulbine, Helichrysum , and Restio. Any connection between Australia and South Africa seems to be more remote than that with South America. Present opinion suggests that the plants common to South Africa and Australia were derived from common ancestors that migrated north to these southern continents.
This element is found in those plants that today are common to Australia, the Indo-Malayan region, and Melanesia. Floristic analysis reveals two wellmarked streams, one derived from Indo-Malaya, the other from Melanesia. In Australia this element comprises Paleotropic members of many families, especially a host of tropical Sympetalae and shows close affinities with the flora of the Asiatic mainland, especially India, Malaya, and the Malay Archipelago. However, there has been little migration northward of the purely Australian element; two species of Eucalyptus and one of Melaleuca extend through the Malay Archipelago to Lombok and to the Philippines, but do not cross Wallace's Line. The Melanesian element is shown most clearly in the ferns. The Indo-Melanesian element is a relatively old one in Australia, and it is probable that major migration and expansion occurred during the warm, moist climatic period of the Miocene. In Australia today the Indo-Melanesian element is restricted to moist habitats and usually to basaltic soils. It occurs along the northern Australian littoral, but finds its greatest development in the rain forests of Queensland. These rain forests contain a host of species; Rhododendron and Garcinia (mangosteens) are interesting Asiatic genera. Some of the more important from both a vegetation and an economic point of view are Cedrela, Elaeocarpus, Eugenia, Fiscus, Geissois, Sloanea, Tarrietia , and Albizzia. The flora of the arid and desert regions of Australia also show affinities with the Old World tropics or arid regions rather than with a native element.
The Australian element includes genera and
species restricted to Australia or attaining maximum development there;
endemic families of plants are few and relatively unimportant. The great
Australian genera, with more than 100 species each, are Eucalyptus (more
than 500 species), Acacia (species belonging to the section Phyllodineae,
with a simple, flattened, leaflike axis; about 500 species), Grevillea,
Hakea, Melaleuca, Pultenaea, Leucopogon, Goodenia , and Hibbertia. Genera
with more than 50 species each include Banksia, Verticordia, Dryandra,
Dodonaea, Persoonia, Daviesia, Baeckea, Boronia , and Pimelea. The typical
Australian flora is concentrated in southwestern and in southeastern Australia.
Southwestern Australia is rich in characteristic Australian families, about
six sevenths of which attain maximum development in that region. The remaining
one seventh are in the southeast. It is difficult to determine whether
this element is truly native or was derived from earlier Paleotropic or
antarctic migrants. In any case, certain groups of plants today are entirely
Australian in their distribution.
The center of origin of the Australian element is unknown. The same general botanical features are shown in the Australian paleontological record as elsewhere. The first species of flowering plants are recorded from the late Cretaceous of Queensland, but there is no evidence of the typical Australian flora's being widespread until the Oligocene, when such genera as Eucalyptus, Banksia, Hakea , and Persoonia were widespread throughout southern Australia, admixed, as mentioned earlier, with antarctic species such as Nothofagus. From the concentration of family and generic peculiarities in southwestern Australia, Hooker suggested that this region was the centrum in which the Australian element originated and from which it migrated eastward. The flora of southeastern Australia has strong affinities and is closely allied with that of southwestern Australia. The isolation of these areas accounts for the degree of endemism in them today. The results of such an analysis might suggest a western origin for the flora. The importance of parallel geological factors in its evolution must also be taken into account.
The widespread Oligocene flora, with its characteristic Australian plants, provided a starting point. During the long period between the Cretaceous and the Miocene, Australia apparently enjoyed great stability and was reduced to a peneplain; the fossil record during this period suggests abundant rainfall and moderate temperatures. No physical barriers to plant migration can therefore be envisaged, and recent work suggests the existence of a pan-Australian flora during this period. During the Miocene, seas inundated much of southern Australia, and in addition to destroying much of the vegetation must have proved an effective barrier to migration, for floristically they isolated southeastern and southwestern Australia. Late in the Miocene, earth movements began that continued through to the Pleistocene. The earth movements provided habitat diversity and barriers to migration; in addition, a major barrier was the great limestone deposits at the head of the Great Australian Bight. These isolated the flora of southwestern Australia, which developed and were selected on podzolic and lateritic soils.
During the Pleistocene the general climate pattern was apparently similar to that existing today. However, a major change occurred during the early mid-Recent. It was marked by a sharp decline in rainfall, which formed the sand-dune deserts of the center, left its mark on soil profiles, and in southern Australia caused a sharp contraction of the existing flora and the marked discontinuity of species distribution, which is an outstanding feature in southern Australia today. This did not affect, however, the broad pattern of distribution of the flora. The evidence suggests a widespread pan-Australian flora in southern Australia in the early Tertiary, followed by division into two groups in the Miocene. Since then physiographic and climatic influences have isolated floristically the southeastern and southwestern parts of Australia. The latter has preserved great floristic stability.
In the settled areas of southern and eastern Australia alien plants are found, particularly in open communities such as the savanna woodlands, where foreign grasses and herbs have supplanted native species. More than half of these naturalized plants are natives of the Mediterranean region. Plants from South Africa, which has a similar climate, are also prominent. The remaining third are from Europe and western Asia.
Native plants fit for human consumption are insignificant, although they provided food for the aborigines. Macadamia ternifolia is cultivated for its edible nut, in Australia and elsewhere. The most important grasses are species of Danthonia, Iseilema , and Astrebla. Species of Atriplex (saltbushes) and Kochia (bluebushes) are important in arid areas. Materials for tanning are obtained from the barks of various species of acacia, especially A. pycnantha.
Australia constitutes the major portion of the zoogeographical region of Australasia, which also includes Tasmania, New Zealand, New Guinea, and the adjacent islands of the Melanesian and Malay groups westward to Wallace's Line. This imaginary line, which forms the boundary of the truly typical Australian fauna, passes northward between the islands of Bali to the west and Lombok to the east, thence northward through Makassar Strait between Borneo and Sulawesi, and finally northeastward between the Sarangani Islands of the Philippine group and Miangas Island. Wallace's Line is the eastern boundary of the Asiatic-Oriental fauna.
There are now 230 species of mammals in Australia.
Of these three are monotremes, egglaying mammals; about 120 are marsupials,
mammals that carry their young in pouches; and the rest are placentals,
mammals in which the embryo is developed in the uterus.
The most primitive living order of the mammals, the Monotremata, is found in no other part of the world. The platypus (Ornithorhynchus) lays eggs, has a snout that startlingly resembles a duck's bill, is covered with hair, and suckles its young when they are hatched. It is relatively abundant, thanks to the efforts of Australian conservationists. Its closely related cousin, the spiny anteater or echidna (Tachyglossus), resembles a porcupine, but it too lays eggs. The platypus is confined to Australia and Tasmania, whereas the echidnas (Tachyglossus and Zaglossus) are found in New Guinea as well.
The kangaroo is the most widely recognized symbol of Australia, yet it does not display the full range of Australian marsupials, a subclass characterized by the early, immature birth of the young, who are transferred to the marsupium, or pouch, to be nurtured until old enough to fend for themselves.
In general, the primitive and usually less successfully specialized mammalian groups were pushed slowly into the southern continents as new waves of more aggressive and successful mammals made their appearance. Once the monotremes and marsupials were forced into Australia, the connection between that region and the Asiatic mainland was cut off, and the two groups were free of serious competition with the later, more aggressive placental types.
Isolated from competition, the marsupials developed into a great variety of types, sizes, habits, and adaptations, paralleling in many ways the subsequent history of the placentals on the northern continents. Australian marsupials include superficial counterparts of the carnivores, insectivores, rodents, and herbivores. Apart from the American opossums (Didelphidae) and the peculiar South American Caenolestes, marsupials are confined to Australasia.
The Dasyuridae and the Peramelidae are included in the polyprotodont marsupials, which have two or three lower incisors on each side. The Dasyuridae, or "native cats," include Dasyurus, the Tasmanian devil (Sarcophilus), and the arboreal and insectivorous members of the genus Phascogale. The latter genus is widespread throughout Australasia. Closely related to the dasyures is the Tasmanian wolf (Thylacinus), which inhabits the mainland of Australia as well. The banded anteater, Myrmecobius, was derived from the dasyure-thylacine group, as was the pouched "mole," Notoryctes, of north and central Australia. The bandicoots (Peramelidae), distributed throughout Australasia, occupy a place in the regional fauna analogous to that of the Insectivora in the northern world.
The diprotodont marsupials, characterized by having only one pair of lower incisors, are more widely known than the polyprotodonts. Their distribution is confined to Australasia. The most primitive of the diprotodont marsupials is the family Phalangeridae, which includes Petaurus, paralleling the flying squirrels; Acrobates, the pygmy flying phalanger; and Trichosurus, the opossum of Australia. The koala (Phascolarctus), quite similar to a toy teddy bear, is a member of the phalanger family. The koala, with its sleepy and slow reactions, has more than a superficial resemblance to the sloth of South America. The wombat (Phascolomys), confined to Australia proper, is a relatively large, beaverlike derivative of the phalanger stock. The kangaroos and wallabies (Macropodidae) are found throughout the zoogeographic region. Living in the open forest is the most common of the kangaroos, Macropus giganteus, the great gray kangaroo. Other open-country types are Petrogale, Peradorcas , and Palorchestes. Among the interesting genera of this group is the Dendrolagus, the tree-climbing kangaroo, whose feet are specialized for climbing and whose long limbs are good for leaping.
That the marsupials have long held sway in Australia is amply demonstrated by the fossil remains of the giant wombat (Diprotodon) and the flesh-eating marsupial "lion," Thylacoleo.
In pre-European times placental mammals were represented by bats and small rodents believed to have come from the north. The bats include numerous genera of Megachiroptera and Microchiroptera; especially notable is the flying fox, Pteropus. The rodents, which included Anisomys, Conilurus, Crossomys , and Hydromys, are thought to have come on driftwood. Man and the dingo (Canis dingo) were the only two large placental mammals, and it seems certain that the dingo was brought by man about 40,000 years ago.
The ecological balance in Australia has been greatly upset by the introduction of exotic placental mammals since the arrival of Europeans. The rabbit and domesticated herbivores have changed the flora, whereas the fox and domesticated dog and cat have affected other animals.
The bird life of Australia includes many species of great interest and importance. Among the flightless ratite birds are the emu (Dromiceius novaehollandiae) and the cassowary (Casuarius casuarius), a native of north Queensland. The Australian continent is very rich in ducks, such as the shelduck (Casarca) and the musk duck (Biziura). Birds of prey are the wedge-tailed eagle (Uroaetus audax), the whistling eagle (Haliastur sphenurus), the peregrine falcon (Falco peregrinus), and the goshawk (Astur fasciatus). Other outstanding birds are the mound-builder mallee fowl (Leipoa) and brush turkey (Alectura); the bower birds; the birds of paradise; the honey eaters; the lyrebirds; and the great abundance and variety of parrots, pigeons, and ducks with the total absence of vultures and woodpeckers. The Australian region is the center of radiation of the parrots and is populated by all of the subfamilies of the group. The region is also especially notable for the presence of the most primitive of the "songbirds," the lyrebird (Menura) and the scrub birds (Atrichornis).
Australia has a widespread reptile fauna of snakes, crocodiles, lizards, and turtles. Of the approximately 150 species of Australian snakes, the taipan (Oxyuranus scutellatus) is the largest of the venomous group. The Queensland python (Python amethystinus) attains a length of about 20 feet (6 meters). The crocodiles are represented by two species, the estuarine type (Crocodilus porosus) and the smaller river crocodile (Crocodilus johnsoni), both of which are represented in northern Australia and New Guinea. The turtles are represented by about 10 species distributed between the genera Chelodina and Emydura. Noteworthy among the more than 300 species of Australian lizards are the legless lizards (Pygopodidae), found in Australia and New Guinea, and the large monitor lizards (Varanidae), some of which have reached 7 feet (2.1 meters) in length.
The amphibian fauna of Australia is characterized by the absence of Urodeles, or tailed types, but there are numerous genera of toads and frogs. The Australian bufonid toads of the subfamily Criniinae, structurally the most primitive of the true toads, are typified by the genera Crinia, Mixophyes , and Helioporus. There are 16 genera present in the region. Typical of New Guinea are the narrow-mouthed toads Liophyrne, Oxydactyla, Copiula, Callulops , and Xenobatrachus.
Australia has about 180 species of freshwater fish, but there are no carps, killifishes, or salmon and only a few catfishes. Most of the freshwater fauna are descendants of marine types -- the Murray cod (Oligorus), perches (Percalates, Plectroplites, Macquaria), grunters (Therapon), herrings (Potamalosa), garfish (Hemirhamphus), and gudgeons (Gobiomorphus, Carassiops ). There are, however, two noteworthy exceptions -- the Queensland lungfish (Neoceratodus) and the barramundi (Scleropages). In Australia, as in New Zealand, there are a number of species of mountain trout (Galaxias) as well as the blackfish (Gadopsis).
The invertebrate fauna of Australia includes at least 50,000 insects, some of which had remarkable evolutions. A particular curiosity is the giant Gippsland (Victoria) earthworm, which grows to 12 feet (370 cm) or more.