Australia is an island continent with a distinctive and unique flora unlike that found on any other land mass. The evolution of its plants was a result of Australia's long isolation from the other continents.
The story starts in the cretaceous period (136 Ma) when the flora of the world began localising, particularly in the southern continents (Australia, Antarctica, India, South America and Africa) due to the splitting of Gondwana. (See the section on continental drift).
In Australia, there was a great flood during the Cretaceous period which broke the continent into four distinct islands. Between these islands were shallow seas, which supported extensive marine life. The existence of that marine life created a lot of calcium carbonate (CaCO3) and when this broke down, mixed with the soils (seabeds at the time) to give them an alkaline property. The 'islands' from this era (the part of Australia that was never flooded) maintained their acid soils.
Direct evidence for this island theory comes from the fact that many molluscs and other fossilised sea creatures have been found in outback Australia, as well as the alkaline nature of the soils.
During the massive floods, the flora on each island diversified and evolved with each island developing its own unique flora. This was partially due to the long periods of stability that arose.
During the floods of Australia, were three islands of key importance:
- South Western Australia
- Adelaide Region (South Australia)
- South-Eastern Australia (Victoria & New South Wales)
This forced the process of evolution to speed up so that new plants could colonise the new land areas.
Between the flooding and the most recent drought in Australia, there have been four Ice Ages, the first being 500 000 years ago, and the last being 10 000 years ago. In some parts of the Northern Hemisphere, there was ice up to 3000 metres (3 kilometres) thick, however Australia only had ice to a maximum depth of 30 metres in Tasmania and Mount Kosciusko (Australia's largest mountain).
In Europe many species were wiped out, as they had nowhere to 'travel'; in Asia and North America, the distribution of plants gradually moved south and then re-entered the north once the climate became warmer, reducing the number of extinctions. (Of course, there were plants which for varied reasons could not shift their distributions, and became extinct). Because Australia was largely unaffected by these Ice Ages, there was minimal impact on the flora.
However, roughly 5000 years ago there was a long-lasting major drought across the Australian continent which lasted for several centuries. During this time there were extinctions, as well as the creation of dunes. Soils became mobile.
There were a few wetter pockets that were unaffected by these climatic changes, these being:
- The Grampians
- Mount Buffalo
- Flinder's Ranges
It is thought that it was in these dry periods that the genus Eucalyptus (Gum Trees) which are perhaps one of the most widely recognised Australian plants, changed from being a comparatively minor component of the landscape to being a major component.
Though the aforementioned drought would have helped in the spread and diversification of Eucalyptus, its major dominance started considerably earlier than a mere 5000 years ago!
Of course, there are many more genera unique to Australia than Eucalyptus:
- Australian flora is relatively unique.
- 80% of Australian native species are found only in Australia
- 30% of Australian native genera are found only in Australia.
Because palaeobotanists are only human - affected by the same prejudices as everyone else, they decided that the Australian flora could be classified into three groups:
- Indo-Malaysian - the northern rainforests
- Antarctic - southern rainforests including Nothofagus spp.
- Australian - Eucalyptus & Acacias
Australian plants are now classified as either being:
- Relictual - rainforest remnants of Gondwana (now closed forests)
- Australian - localised flora
There are similarities between the Asian rainforest (that is India - not China, Japan etc) and Australian rainforest that suggest a common ancestry. A survey conducted by Dr. L. Webb found that Australian and Indian rainforests shared 47 genera, but only 41 with Papua New Guinea (which was attached to Australia until relatively late).
So there is a foreign influence - a shared ancestry. And of course there are some migratory species such as some orchids which were introduced from wind-borne seed.
In the Tertiary period (65 Ma) the climate was moist and mild. The entire Australian continent was probably covered by a subtropical rainforest-type vegetation. This is sometimes referred to the pan-Australian flora.
As mentioned earlier, this forest was evolved from the Gondwanan rainforests. These forests were also the predecessors of the forests of India, Madagascar, Africa and South America.
Evidence for the theory that most of Australia was covered in rainforests comes from fossils of Australia's only native holly - Ilex arnhemensis. I. arnhemensis today only grows in the lowland forests of northern Australia, however fossils of this species have been found all over Australia.
As the climate changed, so to did the vegetation. While in some areas the sub-tropical vegetation (Gondwanan rainforests) persisted, in other areas a cool temperate rainforest evolved. And of course in many areas where the soil was nutrient-poor, the open forests containing Eucalyptus and Acacia species developed.
Suggestions have been made that Australia was the origin of the Gondwanan rainforests, owing to the high number of primitive flowering and other plants which still live in these forests (particularly in Queensland). In fact, Australia has the highest concentration of primitive plant families in the world. Of the 19 plant families that are thought to be the most primitive, 13 are found in Queensland.
Psilotum nudum is a native of Queensland, Australia and is an ancient Rhynia-type (Psilophyta) plant which is still growing there today. It is one of the oldest vascular plants living on earth.
It is now accepted that the sclerophyllous (dry-country) vegetation that dominates most of the Australian continent evolved from these Gondwanan forests.
It was about 16 Ma in the late Tertiary period that the arid-zone Australian flora evolved, including the sclerophyllous flora.
Most of the Australian sclerophyllous flora evolved from the Gondwanan forests including Eucalyptus and Acacia, however the Proteaceae evolved from the Antarctic portion. (Proteaceae includes the common Australian genera Banksia, Grevillea, Hakea, Isopogon, Buckinghamia and Dryandra amongst others. Proteaceae evolved on Gondwana and grew in what are now known as Africa, South America, Australia, Antarctica and Madagascar.
Once the Eucalypts and acacias began to dominate, the rainforests declined. This was partially due to the increasing incidence of fire in the Australian landscape. Since the acacias and eucalypts adapted well to these conditions, they thrived. And since there was differences in the soils and environments across the Australian continent, they diversified readily. There are now 800 recognised species of Eucalyptus and 900 species of Acacia in Australia.
Of course, there was human influence also. There is a widely-accepted theory which states the Australian aborigines contributed to the change in the Australian landscape.
Aborigines are thought to have colonised Australia about 38 000 years ago. Incidentally, the charcoal deposits in the fossil record increase at around this time. It is known that the aboriginals had a régime of burning, which led to the renewing of the Australian bush.
While fire had already restricted the rainforests of Australia, it is thought that the aboriginal fire program in combination with the dry climate may account for the unusually high levels of sclerophyllous vegetation in Australia. It is also though that the high frequency of fires eliminated the Araucarian forests of Australia, and restricted Dacrydium to Tasmania.
In summary, it is thought that the aboriginal program of burning gave rise to the following changes in the Australian landscape:
- The elimination of forests in many areas
- The prevalence of fire-resistant and fire-dependant species, and species which became tolerant of the fire regime
- The dominance of sclerophyllous vegetation
- The common occurrence of grassy understoreys.
- Soil erosion
- Soil compaction
- Waterway pollution - blue-green algae
Knowing and understanding the evolution of the Australian flora can help everyone appreciate the need to ensure its protection. Protecting small pockets of forest is no good as these are prone to attack from foreign species. Large passages of land are what's needed.
Thankfully there are large passages still in existence to protect.