Biogeography deals with the geographical distribution of plants and animals and the processes that result in such distributions. The process of continental drift has played a major role in determining the global distribution of plants and animals.
The most ancient animals in Tasmania have an ancestry that extend back to a time when the Earth's continents were all joined as single landmass known as Pangea which existed beyond 200 million years ago. Pangea split from east to west into Laurasia (North America and Eurasia) and Gondwana, the two remaining connected at Gibraltar with the Tethys Sea separating them. The presence of closely related organisms in both the northern and southern hemispheres cannot be accounted for by migration.
Some species which have distributions indicative of an ancient origin dating back to the existence of Pangea include:
- Peripatus, a member of the phylum Onycophora, which comprises only about 100 species. Tasmania is the world centre of diversity for this ancient phylum.
- Trogloneta sp. (an undescribed spider from Kutikina Cave) is from the same genus as a species in France and two species in North America.
- The hairy cicada, a soundless and ancient species of cicada. The hairy cicadas belong to the primitive cicada family Tettigarctidae, which consists of only two species.
The breakup of Gondwana
Africa and India where the first to break away from Gondwana some 125 million years ago. Consequently, the present flora of Africa and India have fewer remnants of the Gondwanan flora than those continents that remained connected for a longer period - South America and Australia. Nonetheless, there are some similarities among the older plant families such as the Proteacae.
As Australia separated from Antarctica about 45 million years ago, the continent began to drift northwards. For some 30 million years the continent remained isolated, allowing the Australian flora and fauna to continue to evolve free from the impact of other species entering from adjoining continents.
The climate of Australia grew progressively drier, allowing the dry and fire-adapted eucalypts, casuarinas and acacias to become a dominant feature of Australia's flora. The vast expanses of temperate rainforests species which once covered much of Australia, and the other continents that comprised Gondwana - South America and Antarctica - began to contract to the wetter south-east margins of mainland Australia and Tasmania. Today, Tasmania acts as the Australian stronghold of these ancient forests.
About 15 million years ago, the Australian plate began colliding with South East Asia. The collision also pushed up the islands of Wallacea, (the central islands of Indonesia east of Java, Bali, and Borneo, and west of the province of New Guinea, including the whole of Timor, Sulawesi, the Moluccas, and the Indonesia region of Nusa Tenggara) which served as island stepping-stones that allowed the movement of plants and animals from south-east Asia to reach Australia. The straits between the islands were narrow enough to allow plant dispersal, but served as an effective barrier to the movement of land mammals. However, various species of bats and rodents were able to move into Australia. Today, these two groups are the only native placental mammals to have colonised Australia. All other placental mammals now found in Australia have been introduced by humans.
Tasmania’s Gondwanan species
Plants: Plants that reveal a common history with the Gondwanan supercontinent - often called the Antarctic flora - include conifers in the families Podocarpaceae, Araucariaceae and the subfamily Callitroideae of Cupressaceae, and angiosperms such as the families Proteaceae, Griseliniaceae, Cunoniaceae and Winteraceae, and genera like southern beech (Nothofagus) and fuchsia (Fuchsia). Many other families of flowering plants and ferns, including the tree fern Dicksonia, are characteristic of the Antarctic flora.
Fossil evidence suggests that temperate rainforest was widespread in Australia, Antarctica, South America and New Zealand around 45 million years ago. Such fossils and the surviving species in Tasmania provide evidence of the ancient link to Gondwana.
Intriguing evidence of Australian species, such as the dominant rainforest tree of Tasmania, Nothofagus, has also been found in Antarctica, suggesting that cool temperate rainforests once covered the Antarctic continent before the region became covered by ice.
There are a number of examples of fossils with a Gondwanan connection, including the recent discovery of fossilised foliage of the giant conifer Fitzroya tasmanensis near Cradle Mountain. Presently this tree only grows in Chile, but this discovery provides excellent evidence for the similarities in forest types across Gondwana. These Gondwanan forests most likely included King Billy pine, pencil pine and other Tasmanian gymnosperms (conifers).
Flowering plants arrived in Gondwana about 100 million years ago and expanded rapidly across the supercontinent. The most successful was the myrtle or southern beech (Nothofagus) which provides arguably the strongest evidence for the Gondwanan landmass. There are about 40 living species left today distributed across a number of land masses - three species in Australia, nine in Chile and Patagonia, five in New Caledonia, 14 in New Guinea and four in New Zealand.
Animals: There are numerous examples of animals in Tasmania that have their closest relatives living today in land masses that were once part of the Gondwanan supercontinent.
The Mountain Shrimp, Anaspides spp.
The invertebrates provide abundant evidence of Gondwanan ancestry. Possibly the best known example is the mountain shrimp (Anaspides tasmaniae) which is very similar to Triassic (230 million year old) fossils. Currently, its closest relatives are found in New Zealand and South America. The Tasmanian cave spider is considered to be one of the most primitive spiders in the world, and is the only member of its family outside Chile.
Another fascinating discovery in recent time was the vertebrate and invertebrate marine fauna of the Port Davey/Bathurst Harbour area in southwest Tasmania, which are also considered Gondwanan relics. One of these animals, the Port Davey skate, is unique, as it is the worlds only brackish water skate. Its closest relatives are found in New Zealand and Patagonia.
Platypus and echidnas evolved from ancient ancestors which inhabited Gondwana. Close relatives of marsupials thrive in South America and fossil platypus have also been discovered in South America.
Other vertebrates with strong Gondwanan affinities include the parrots, the two families of frogs within Tasmania (Myobatrachidae and Hylidae) and the major family of freshwater fish which occurs in Tasmania, the Galaxiidae.
The diagram below shows the modern day distribution of various groups of animals and plants which have Gondwanan roots.
The world-wide distribution of just a few of the Tasmanian plant and animal groups that have their origins in the supercontinent of Gondwana. Place your cursor over each to see its current global distribution. Requires Shockwave - see help pages for details of this browser plugin.
The Movement of Species from South-East Asia
Other factors in addition to continental drift have played a role in determining the distribution of Australia's plants and animals, and specifically, the flora and fauna of Tasmania. The repeated lowering of sea levels during the ice ages of the Pleistocene facilitated the movement of many species into Tasmania, including the five species of rodents and eight species of bat that are found in Tasmania. Dingoes, which were brought in to Australia from south-east Asia by humans some 5,000 years ago were unable to disperse into Tasmania because the end of the last Ice Age resulted in rising sea levels re-flooding Bass Strait.