17. “Nursery Australia” – an ecosystem
– sculpted by: isolation; K/T extinction; & climate change.
Isolation : When Australia finally separated from Antarctica in the later stages of the breakup of supercontinent Gondwana about 70 million years ago, it became isolated by land with its dinosaur (& bird) rich ecosystem which also contained recently evolved marsupials & flowering plants. (Placental mammals seem to have evolved later than the start of Australia’s isolation & never got to board Australia until very recent times).
K/T event: Just 5 million years after Australia’s isolation (at 65 Ma), there was a mass extinction event (K/T event) which killed off all dinosaurs around the world. It is generally accepted that this extinction event was directly related to the impact of a large asteroid in the Gulf of Mexico. It caused the extinction of many other species of land & sea, animals & plants. Some estimates suggest that upto 75% of species on Earth became extinct. In the far southern hemisphere the effects many have been less severe & although much biomass would have been destroyed, most local plant species were still able to recover. Species that survived tended to be smaller, and live in sheltered habitats with burrows or aquatic refuges. In the subsequent recovery, adaptive evolution often took advantage of vacated lifestyle niches. The absence of the previously dominant dinosaurs made a big difference.
Logarithmic Time Scale – providing world time context/relationships. The red box represents this part of story
The Survivors: With the dinosaurs gone, and being isolated on the Australian continent, the uniquely Australian fauna & flora was primed for development. The key ingredients were the local survivors of the K/T extinction event and the prevailing climatic regime.
Representatives of the most genetically advanced groups of plants & animals had survived in Australia. With the previously dominant predators gone, genetically modified experimental prototypes probably had a better chance of development & survival, than in more overtly competitive situations elsewhere on Earth. Perhaps this contributed to how kangaroos & emu’s got started and flourished in Australia.
Climate change during this period was another very important control on ecosystem development. For the 1st 15 million years of this (65-40 Ma), the Australian continent was located near the south pole during a period global greenhouse conditions. Climatic conditions over Australia were relatively humid and forests would have been widespread. Then at about 40 Ma the Australian continent started to move rapidly (7 cm/year) northwards and for the 1st time a circum-Antarctic current started to flow strongly. The local & global climate started to change with an ice-cap staring to form over Antarctica at the south pole, and the climate in Australia becoming dryer & more arid. Forests became open woodland, and woodland became grassland or desert.
Adaptation to prevailing conditions: Kangaroos & Emus were well suited to this change in climate being well adapted to life in grasslands. Varieties of marsupial megafauna evolved but were mainly extinct again by 16,000 yrs ago. Many Australian native plants have developed adaptations for tolerance of, or even dependence on bush fires . Some species like Banksia & Grass-trees struggle to propagate without bushfires. This suggests that periodic fire was a normal aspect of life to some evolved early Australian ecosystems. Another key adaptation was tolerance of multi-year drought conditions. Over extensive parts of the Australian continent a range of different Eucalypt & Acacia woodland, and grassland ecosystems became established.
Remnants: In isolated protected pockets remnants of the earlier humid forest (Gondwana type) ecosystems survive. Examples include the Tarkine forests of NW Tasmania, numerous patches along the eastern coast of Australia, and similar forests in New Zealand & New Caledonia. Closer to Wattle Flat in the northern Blue Mountains are tiny sheltered remnant ecosystems with Wollemi Pines which were part of Gondwanan forest ecosystem.
End of Isolation: About 15 million years ago the northern parts of the Australian continent came into contact with foreign lands belonging to a northern continent. This facilitated transfer of some plant & animal species and brought an end to Australia’s virtual isolation.