Biodiversity publications archive

Australia's biodiversity: an overview of selected significant components

Biodiversity series, Paper no. 2
Biodiversity Unit
Department of the Environment, Sport and Territories
, 1994

Executive summary

Origins

Australia has a rich and distinctive flora and fauna. The major reason for this is related to the history of the Australian landmass. The world's continents were once all joined in a single landmass called Pangaea. In the Jurassic period (about 160 million years ago) a northern continent, Laurasia, and a southern continent, Gondwana, split apart. Gondwana fragmented gradually over geological time, with India and then New Zealand moving away from the Australia-Antarctica-South America group during the Cretaceous period (140 million years ago). The latter group of continents separated from each other during the Tertiary period (from about 70 million years ago). By about 40 million years ago, Australia was fully separated from Antarctica and commencing a northward drift as an isolated continent.

At the time of Australia's break from Antarctica, there were both marsupial and placental mammals on the continent but, apart from bats, placentals subsequently died out. Marsupials radiated widely and became adapted to the increasing aridity caused by climatic changes during Australia's northward movement. The characteristic Australian flora, including eucalypts, acacias, casuarinas, spinifex, grasstrees and banksias, also diversified during this time. As Australia moved closer to the Asian landmasses, invasions and colonisations from its northern waters were common, including many mangroves, corals, lizards, snakes, placental mammals, birds, scorpions and insects.

Despite this long history, the last 200 years have seen the most dramatic changes to Australia's biological diversity. Following European settlement many ecosystems were radically simplified and fragmented, and a suite of exotic plants and animals was introduced. Associated large-scale disruption of indigenous societies has meant that in many cases the traditional knowledge of Australian flora and fauna has been permanently lost.

Significant features

Australia's long isolation has resulted in a flora and fauna that is both highly endemic and has great species richness compared to many other parts of the world. Endemic groups are those that are unique to a region and Australia has both a large number of endemic species, and a high proportion of endemic species in particular groups. For example 85 per cent of vascular plant species and 82 per cent of mammal species, excluding whales, are endemic to Australia. Species richness refers to the number of species in a particular area and is very high in Australia, especially in some notable areas such as the Queensland rainforests, the Great Barrier Reef and the South West Botanical Province of Western Australia.

Much of Australia's biota has an exceptionally long history. Stromatolites, such as those at Shark Bay, Western Australia, are formed by microbial mats of cyanobacteria (blue-green algae) and other microorganisms. Stromatolites date back 3500 million years in the Australian fossil record, and are thought to have been made by organisms which were among the first on earth. Many other organisms found in Australia today can be linked through the fossil record to ancient ancestors in existence during the continental shifts of the Pangaean and Gondwanan times. Indeed some modern species, surviving in isolated or specialised habitats, have changed little over time (relict species) and represent links to various stages in the history of Australia's biodiversity.

Because of the particular climatic and environmental features of the Australian continent some unique combinations of adaptive features have evolved. These include adaptations to arid conditions, variable water levels, fire and high salinity.

Australia's biodiversity

Australia's large span of latitude, with a variety of climatic zones from the tropical north to the cool temperate south, has allowed the development of a rich and diverse flora and fauna on land, in wetlands, and in the surrounding seas. Australia's external territories also contain endemic and unusual plants and animals.

Australia has a vast array of vascular plants most of which are unique, and more endemic families of flowering plants than any other country. Acacias and eucalypts have adapted to almost every habitat and dominate the landscape while the family Proteaceae is more diverse in Australia than elsewhere with 42 genera from a world total of 72. Of these, 35 are endemic to Australia (including banksias, grevilleas, hakeas, macadamias and waratahs). The more primitive seed-bearing plants are represented by the cycads (palm-like cone bearing trees) and a number of conifers, for example the huon pine and kauri pines.

Much of the Australian continent is dry, and many native Australian plants are uniquely adapted to survive harsh conditions. Sclerophylly is an adaptation to arid conditions in which the leaves are rigid and have a thick waxy cuticle. It is found in many plant groups, such as some acacias, eucalypts and banksias. Adaptation to fire is also common and some Australian plants require fire for regeneration.

On land there is an enormous diversity of mammals, birds, reptiles, frogs and invertebrates. All three major groups of modern mammals (monotremes, marsupials and placentals) co-exist in Australia and of the three species of egg-laying mammals (monotremes) known worldwide, two occur here (the platypus and echidna). The marsupials are the most common mammals in Australia with about 141 species, the majority of which are endemic.

Nearly half of Australia's birds, and most reptiles and amphibians are endemic and many groups of these animals are highly diverse compared to other continents. For example parrots are more varied in Australia than elsewhere in the southern hemisphere, and about 93 per cent of Australian frogs and 89 per cent of reptiles are found only here. The arid zones have an exceptionally large number of lizard species compared to similar habitats on other continents. Australia also has a huge invertebrate fauna with an estimated 140 000 species, many of which await discovery. The insects are extremely diverse and some families and most species occur only in Australia. With some 4000 species, ants are particularly diverse compared to the rest of the world, and form a vital part of many arid ecosystems.

Numerous interesting and highly specialised interactions have 'co-evolved' between organisms in Australia as part of their adaptation to the environment and to maximise use of resources. Seed dispersal, pollination and protection from predators are common themes of such interactions and these arrangements are often mutually beneficial to the organisms involved. Ants, which disperse seeds for numerous plants, are involved in a complex web of feeding and predation interactions, especially in arid areas.

The fauna of inland waterways and wetlands is varied and includes the well-known Australian inhabitant, the platypus, and two species of crocodiles: the endemic freshwater crocodile; and the more widely distributed saltwater crocodile. The waterways support a diverse invertebrate fauna much of which is endemic, including freshwater sponges, snails, shrimps, crayfish, isopods and many species of insects. Some faunal groups, such as freshwater mussels, crayfish and southern frogs, are particularly diverse in Australian waterways compared to elsewhere. Other groups that are common elsewhere, however, are either absent or have relatively few representatives here, including some insect, frog and bird families. Freshwater fish species are relatively few but include some endemic families. However, some fish, such as southern trouts, which occur all over the southern hemisphere, are most abundant and diverse in Australia. The inland aquatic flora is not as distinctive and, although some endemic species are found, most species are widely distributed.

Only a limited number of organisms in the world have adapted to life in salt lakes and Australia has many notable examples, including a number of endemic shrimps and gastropods. Many organisms have also evolved adaptations to cope with periods of drought, for example drought-resistant eggs or larvae; burrowing; and life-cycle adaptations including opportunist breeding after flooding.

Australia's seas support one of the richest fish faunas in the world. This is strongly regional, with a northern tropical zone; eastern and western warm temperate zones; and a southern cool temperate zone. The northern tropical zone has most species diversity but the majority of species are also distributed widely in the Indian and Western Pacific Oceans. The temperate zones have less species diversity but a majority of the species are endemic. Some fish families are uniquely diverse in Australian waters, for example pipefishes and seahorses, leatherjackets and anglerfishes, and there is an extremely rich fauna of sharks and rays. A number of marine species that are not endemic to Australia find security in Australian waters and are found here in greater numbers than in other parts of their range, for example a number of turtle species and the dugong.

Australia's external territories, including the Australian Antarctic Territory, Christmas Island, Norfolk Island and other offshore islands, have highly diverse and endemic marine communities. The islands also provide some important breeding sites for seabirds and seals. Christmas Island is the only known nesting site in the world for the rare but far-ranging Abbott's booby.

'Living fossils'

Australia's flora and fauna are well represented in the fossil record. Fossils of kauri pines and cycads have been found in 175 million-year-old fossil fish beds in New South Wales, indicative of their ancestral origins when all the continents were joined before 160 million years ago. Other plant groups (for example the family Proteaceae, southern beeches, acacias and many rainforest species) can be traced to a time when the southern landmass of Gondwana had split from Pangaea.

Some existing invertebrate species, such as the velvet worms, are relict species, having remained relatively unchanged for hundreds of millions of years. Several other invertebrate groups also have Pangaean origins while other faunal groups have fossil records extending back to Gondwanan times. These include the emu and cassowary, the parrots, and some turtles, geckos, frogs and invertebrates.

The Australian aquatic fauna provides several examples of animals with ancient origins. Crocodiles have a fossil ancestors record dating back over 200 million years and the Queensland lungfish is among the most ancient of living bony fishes with a fossil ancestry dating back over 400 million years. A number of fish families including the barramundi and salamanderfish have Gondwanan or Pangaean ancestry as do numerous aquatic invertebrate groups.

Many ancient marine animals live on the southern marine platform of Australia, which has been relatively stable for some 40 million years and the direct ancestral lineage of many invertebrate species found there today can be traced in the deposits to ancient times.

Special areas

The overall biodiversity of a region is related to the variety of habitats and biological communities that have developed there. In this context Australia has a number of areas and specialised habitats that warrant special mention. The rainforests of north Queensland, the South West Botanical Province of Western Australia and the alps of southeast Australia form diverse assemblages of terrestrial habitats that are important for the extremely rich and endemic biotic communities that they support. The rainforests contain one of the highest concentrations of primitive plants in the world and harbour important links in the history of plant evolution.

The flora and fauna of Australia's inland waters have had to adapt to unpredictable rainfall, seasonal variation in evaporation rates, and high levels of salinity. Overall there is a scarcity of aquatic habitats, particularly in the arid and semi-arid areas. Despite this, Australia has a variety of wetland habitats provided by areas such as the Kakadu wetlands area in northern Australia and the lower Cooper wetlands, including the Coongie Lakes, in the centre. These wetlands include diverse habitats that vary in the wet-dry cycle, and the flora and fauna inhabiting them are notable for both their diversity and the large fluctuations of abundance that can occur.

The gorges and caves of central Australia, the coastal mangroves, the granite outcrops of the southwest of the continent and the mound springs of the Great Artesian Basin all represent specialised habitats where plants and animals have developed unique characteristics and adaptations.

Australia has a number of special marine habitats that are worthy of particular mention. The most well-known and spectacular of these is the Great Barrier Reef, which covers 350 000 square kilometres off the tropical northeastern coastline and has an incredibly rich biodiversity associated with coral reefs and islands.

The giant kelp forests of Tasmania and Victoria and the seagrass meadows found in many of our coastal waters provide important habitats for many marine species. Western Australian seagrass meadows, which collectively cover as much area as all the rainforests of Australia, are the most diverse in the world. The 'forests' and 'meadows' are home to many invertebrates and fish, and provide nurseries for many of their young.

Australia – a biological 'ark'

Overall, Australia is an island continent that, due to its long geological isolation and special climatic features, is home to an enormous variety of uniquely evolved plants and animals. An exceptionally high proportion of these are only found in Australia, although a common ancestry with 'counterparts' in other regions can often be traced back to ancient times. For some groups of plants and animals the number of species (species richness) is outstanding and for many others the numbers are relatively high compared to most other parts of the world. These factors combine to make Australia's biological 'ark' globally significant in any terms.

Summaries

This executive summary can be used with summaries of each major section to provide a non-technical overview of the paper.