Biodiversity series, Paper no. 2
Department of the Environment, Sport and Territories, 1994
By any criteria, Australia's biodiversity differs from that of the rest of the world. Australia has many odd and different creatures, and to understand how they came to be here, and nowhere else, it is necessary to look into the past. The major reason for Australia's distinctive biodiversity is that for most of its history Australia has either been at the end of a chain of linked continents or widely separated from others. During that time of wide separation, Australia became a natural laboratory isolated from the rest of the world. As a result, while all continents except Australia (and Antarctica) share long records of, for example, primates, cats and bovids (cattle, sheep and goats), only Australia has the platypus, the koala and wombats.
Another feature of the Australian continent that distinguishes it from most other lands is its geological stability. Today, the Australian landmass lacks some of the features that characterise large portions of other continents, such as active mountains (including volcanoes), great rift systems, and great permanent lakes. During the last 300 million years, all these features have been active or present at some time, but a significant part of the continent has been geologically stable. Accordingly, and in contrast to other continents, vast areas have been subject over a long period to relatively low rates of topographical change.
Geological evidence indicates that, prior to 160 million years ago, all the continents were aggregated as the single landmass of Pangaea. While the distribution patterns for many biotic groups have been erased by extinction or subsequent radiation, scientists have been able to identify a number of components of the Australian biota that have remained relatively unchanged since Pangaean times. Surviving examples of these, which typically possess primitive features and are found throughout the world - but with a disjunct distribution - include several invertebrate, microbial and plant taxa.
(millions of years ago)
Source: adapted from Kukalová-Peck 1991
Figure 1: About 200 million years ago, the world's continents were aggregated in one landmass, Pandgaea. Over a period of many millions of years, Pangaea fragmented into two supercontinents, northern Laurasia, and southern Gondwana. These in turn became the continents as we know them today.
Note: The black dots indicate the approximate positions of the South Pole.
By the middle of the Jurassic period, 160 million years ago, Pangaea was partly divided into the northern supercontinent Laurasia, and its southern counterpart, Gondwana. Gondwana remained intact until the early Cretaceous, some 140 million years ago. At that time Australia was broadly in contact with Antarctica, India and New Zealand and indirectly with South America and Africa, and biotic affinities between these landmasses are clearly evident. Gondwana fragmented over an extended geological period with India, Africa and then New Zealand moving away from the Australia-Antarctica-South America group during the Cretaceous period and the latter group of continents separating from each other within the Tertiary period (see Figure 1).
Australia started to separate from Antarctica about 53 million years ago and remained connected via Tasmania and the South Tasman Rise until about 38 million years ago. For more than 20 million years, Australia moved northwards in relative isolation through 30 degrees of latitude. This movement, with the progressive development of the present intense atmospheric circulation pattern, brought about massive climatic changes in the southern hemisphere. Rainforests became geographically more variable with an increase in drier communities, and were progressively replaced from the centre towards the margins of the continent by more open-canopied vegetation types. Sclerophylly, which originally developed in response to the low nutrient status of soils within the early rainforest vegetation, pre-adapted plants to survival and diversification under increasingly arid conditions, leading to the characteristically 'Australian' flora that includes eucalypts, acacias, casuarinas, spinifex, grasstrees and banksias. While aridity generally increased, there were several climatic fluctuations, with warm and moist conditions alternating with drier and cooler phases, and a progressive trend towards a more arid environment.
As well as the diversification of Australia's flora during this period, there was also considerable faunal adaptive radiation. Of particular significance is the fact that at the time of Australia's break from Antarctica, there were both marsupial and placental mammals on the continent 57 but apart from bats, placentals subsequently died out. 201 As Australia underwent the climatic changes outlined above, its marsupial fauna underwent a major radiation into a variety of habitats and many became adapted to drier regions by, for example, the production of highly concentrated urine and the ability to live without access to free water 149 215. Some invertebrate groups, such as the ant genus Melophorus, the termites Drepanotermes, and the collembolans Folsomides and Corynephoria, have adapted particularly well to the arid zone and may have radiated there. 194.
As Australia's top quarter drifted north of the tropic of Capricorn into the tropics, new lifeforms colonised its northern waters. Many tropical species of mangrove and coral joined Australia's biota as the northern waters warmed. Contact with the Sunda Arc occurred about ten million years ago but over a period of millenia, and opportunities for invasion increased as the landmasses drew closer. Eventually, when the Australian tectonic plate made contact with that of Asia, Australia was more easily invaded by terrestrial fauna. Examples of the Asian Tertiary invaders include many families of lizards and snakes, placental mammals such as the conilurine rodents, many birds, scorpions of the families Scorpionidae and Buthidae and many insects 64.
While invasions and colonisations are often chance events and relatively rare, many of the taxa which entered Australia during this time nevertheless invaded several times. Dung beetles of the cosmopolitan genus Onthophagus may have invaded as many as 34 times. 100 Some of the taxa spread throughout the continent and underwent extensive adaptive radiation.
In most cases these recent invaders have scarcely diverged from their Asian relatives and where there is divergence it is usually at the species or subspecies level. Examples include the wood frog, Rana daemeli, native rats and the rainbow bee eater, Merops ornatus 64.
Plant genera that most probably entered Australia by this route include those of the primitive families Annonaceae, Hernandiaceae and Myristicaceae and the two species of Ericaceae, Agapetes meiniana and Rhododendron lochae 11. However, accumulated fossil evidence has demonstrated that there was no massive invasion of rainforest plants from Southeast Asia into Australia as had been traditionally accepted 158.
The invasions outlined above form the most recent component of a continuum of invasions from Asia and elsewhere and may be taken as post-Tertiary, that is, from about 1.6 million years ago. Since the late Tertiary, Australia has been subject to the dramatic climatic fluctuations associated with global glacial-interglacial cycles. Fire, which has always been a component of the Australian sclerophyll vegetation environment, also became more prevalent. The combined impact of these factors resulted in a marked expansion of open sclerophyll forest and woodlands and, during cool dry glacial periods, the development of extensive steppe grassland and herbfields in southern Australia. Rainforest and other fire sensitive communities became largely restricted to isolated wetter east coast areas and to fire-free stream valleys and rocky outcrops 79. These trends accelerated because of the burning activities of Aboriginal inhabitants after their arrival at least 50 000 years ago 135. The character of the woodland changed particularly; it became more open and grasslands spread further.
It is also probable that Aboriginal people were involved in the massive phase of megafaunal extinction within the late Pleistocene. There had been a trend towards increased body size of Australian marsupials through the Tertiary and Quaternary periods, culminating in the late Pleistocene, which witnessed the extinction of almost all the large marsupials – some 40 species 99. This loss of a substantial grazing and browsing component would in itself have resulted in major changes to the entire Australian environment.
In the last 200 years, however, the most dramatic change to Australia's biological diversity has occurred in the shortest period of time. After European settlement, many ecosystems were radically simplified and fragmented, a suite of exotic species was introduced, and many native species were displaced or became extinct. The associated large-scale disruption of indigenous societies has meant that in many cases traditional knowledge of, and interactions with, Australian flora and fauna, such as burning regimes, have been permanently lost or altered.