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

2. National elements of significance – an overview

As a result of the evolutionary process, the Australian flora and fauna is distinct in many ways from that found elsewhere. Long isolation has led to the evolution of many animals and plants found only in Australia (i.e. endemic species) and its stability has allowed the survival of many ancient and relict species. Also Australia's climate and landscape variability have fostered the development of a biota with many adaptations to local conditions – and these species have interacted in unique ways, to form diverse communities. These conditions have allowed the evolution of several highly speciose groups, such as eucalypts and ants.

Endemism

Although many endemic genera and species are to be expected on a large island continent, levels of endemism in Australia are notably high. Compared with other countries, Australia not only has a large number of endemic species, but these also form a high percentage of the total 181 (see Figure 2). From what is presently known, 85 per cent of flowering plant species 50, about 82 per cent of mammal species excluding whales 149 48 212 215 and 89 per cent of reptile species are endemic to Australia 25.

While endemic species and genera are remarkable from a biological point of view, endemic families are even more important. It is of great significance, therefore, that endemism occurs at the taxonomic level of family in Australia, with six mammal, about four bird, eight fish, and 14 families of flowering plants endemic to Australia as well as numerous families of invertebrates 215 214 114 11 190.

Figure 2: On a scale, Australia has a very high proportion of endemic species.

Figure 2: On a scale, Australia has a very high proportion of endemic species.

Source: WCMC 1992

Species richness

Australia's plants and animals are among the most diverse in the world and some areas have globally outstanding species richness, such as the Great Barrier Reef, the rainforests of north Queensland and the South West Botanical Province of Western Australia.

Also, it is found that when Australian groups of plants and animals are compared with their counterparts in the rest of the world, several are notable for the numbers of species they include. For instance, Australia has many vascular plant species, and is particularly well known for its orchids. In addition, it is one of the most lichen-rich countries in the world 137 181. Marsupials are diverse and numerically abundant and Australia's reptile fauna is also significantly large. Several invertebrate groups also contain many species. For example the ant and cockroach faunas comprise 15 per cent and 12 per cent of the global faunas respectively 109 181.

Ancient and relict species and communities

The geological stability of the Australian landmass has resulted in the ability to trace the origins of some organisms living in Australia today back to extremely ancient times. For example, Australia has outstanding examples of stromatolites, structures formed by microbial mats of cyanobacteria (blue-green algae) and other microorganisms. The oldest known stromatolites are about 3500 million-year-old fossils and were discovered near the mining centre of North Pole in the Pilbara, Western Australia. Actively growing stromatolites can be found at Shark Bay, Western Australia, where one type of cyanobacterium, widespread in the microbial mats, is thought to be a direct descendant of forms that flourished 1900 million years ago. It represents one of the longest continual biological lineages known 19.

Australia has many examples of lineages that have survived since Pangaean and Gondwanan times. These often provide a living history of the groups to which they belong, and give clues to the evolution of many other extant (that is, still existing) species and groups. Such animals and plants, which can be seen as 'living fossils', are evidence of our past biotic links with other continents and countries and give vital information on past climates. By increasing our understanding of evolutionary processes, they also help us to identify groups and regions important for contemporary evolutionary change.

The ancient lineages of many groups are reflected in their current distributions. Organisms that are of Pangaean origin typically possess primitive features and have a globally disjunct distribution as noted in Section 1. Many extant plants and animals with Gondwanan origins have distributions reflecting their centres of origin and the state of fragmentation of the Gondwanan landmass. The ratite birds, which include the emu and ostrich, are believed to have Gondwanan origins as they are found on most southern continents, including Africa, Australia and South America 140. Acacias and the Proteaceae are also probably of Gondwanan origin, with a wide distribution on southern continents occurring (mainly) in India, Africa, Madagascar, Australasia and South America 171. Species of Nothofagus (southern beeches) however, are absent from Africa 171, and fossils of this genus have never been found there 201. The less cosmopolitan distribution of Nothofagus indicates that it may have evolved later, as Gondwana was fragmenting.

A further distinction can be made between extant organisms with links to ancient ancestors and those that have remained relatively unchanged from a previous age, often in biogeographically isolated areas (i.e. relict species). Examples include the blind trapdoor spider Troglodiplura lowryi from caves in the Nullarbor Plain and the conifer Microstrobos fitzgeraldii which is found at a few sites in the Blue Mountains (NSW). The only other member of the Microstrobos genus is found in Tasmania. The palm Livistona mariae, another relict species, occurs in gorges in the Macdonnell Ranges in the Northern Territory, and is a survivor of times when moist conditions prevailed more generally 52.

Distinctive Australian adaptations

Adaptations to Australian environments have often occurred over long periods and adaptive radiations have resulted in surviving species often being distinctive. The platypus (Ornithorhynchus anatinus) is perhaps the most well known example, but is joined by other species such as the marsupial moles (Notoryctes typhlops and N. caurinus), the blind gudgeon (Milyeringa veritas), the possibly extinct Lord Howe Island stick insect (Dryococelus australis) and the Queensland bottle tree (Brachychiton rupestris).

While many species possess unique combinations of adaptive features, some adaptations and ecological features can be regarded as typical of the Australian environment. These include sclerophylly and adaptations to frequent fire, aridity, an often unpredictable climate, high levels of salinity, low and patchy levels of soil nutrients, and variable hydrology.

Sclerophylly

Many Australian plants have leathery, hard, spiny or reduced leaves and relatively short internodes. These characteristics are known as sclerophylly and are believed to be adaptations to impoverished soils. Sclerophyllous characteristics are strongly represented in Australian Myrtaceae (including tea-trees and eucalypts), Proteaceae (grevilleas, hakeas, banksias, waratahs and relatives), Epacridaceae (heaths), and Mimosaceae (mainly acacias) 9.

Sclerophylly, while primarily an evolutionary response to poor soils, may have preadapted many Australian plants to drier conditions, such as occur in Mediterranean climates. It is thought that because of the generally weathered and infertile nature of Australian soils, sclerophylly has become progressively more widespread to an extent not matched on other continents 9.

Fire

Fire is an integral component of most Australian ecosystems and habitats. It is a natural environmental variable and fires occur with different intensities, times of year and frequencies across Australia. It is also a major management tool.

Many plants have adapted to particular fire regimes and a number of species rely primarily on fire to assist development of offspring (i.e. recruitment) such as Eucalyptus regnans 55. In many cases fire can crack resistant seed coats, open fruit held on the parent plant and provide a rich medium for seedling growth. For some species, alteration of such fire regimes can threaten their viability. The reduced occurrence of fire has contributed to two species being considered endangered: Rutidosis leptorhynchoides and Swainsona recta 92.

Salinity

Salt lakes are a prominent part of the Australian environment and in contrast to other continents, expanses of saline waters in Australia far exceed those of freshwater 102. Some salt lakes such as Lake Corangamite in Victoria are permanent, whereas the majority, including Lakes Eyre, Frome and Torrens in South Australia are temporary and only fill occasionally. While species diversity is generally reduced with increased salt concentrations, the fauna and flora of salt lake communities possess major adaptations that aid survival in such extreme conditions, including rapid life cycles and mechanisms for salt excretion.

Variable hydrology

In addition to the variation in salt concentrations in saline lakes, the level and flow rate of water in Australian freshwater systems is also highly variable 104 (See figure 4). Many inland streams and rivers only flow after unpredictable rains. After rain both the temporary and permanent water courses can flood vast areas and leave transient pools and lakes as they evaporate. Some adaptations, such as fish spawning only after a rise in river levels, are a response to this variability.

Ecological interactions

Australian plants and animals often interact in ways that are not only considered ecologically significant but in many cases are unique to Australia. These ecological interactions often result in co-evolution – that is different taxa evolving together. Many features of flowering plants, for example, can only be understood by observing the way they interact with animals; conversely many vertebrates and invertebrates have features that reflect their close association with plants. Such interactions between plants and animals include: pollination by insects and vertebrates; dispersal of seeds and fruits; the interactions between plants and the animals that feed on them; and plant defences against animal attack.

Refuge value

Australia's wetlands, coasts and islands have great significance as breeding and feeding refuges for migratory and wide-ranging species, some of which, such as Abbott's booby (Papasula abbotti; found on Christmas Island), are rare and endangered. Marine turtles and marine mammals, as well as seabirds, find feeding and breeding grounds along Australia's coastline and there are many Australian wetlands internationally recognised as valuable habitats for migratory birds. The Australian Antarctic Territory and Australia's subantarctic islands are also important for their refuge value and provide nesting sites for millions of seabirds.