2.3 Identify and describe evidence of changing environments in Australia over millions of years.
Over millions of years, the environment of Australia has been changing. Australia was once part of Pangaea-recall your time line. Antarctica was once a temperate land mass covered in lush forests. Many animals moved between South America and Australia by passing through Antarctica. Fossil evidence shows that plants such as Glossopteris and Gangamopteris were common. Mixed forests, including tree ferns and Antarctic beech (Nothofagus) covered the land. Present day desert areas in Australia were covered with these forests, and flamingos and crocodiles lived there. Giant marsupials that resembled wombats and possums lived in the forests. As Antarctica drifted south the climate changed to colder conditions that exist today, and the flora and fauna that once lived on this continent died out. At the same time Australia was drifting north. 25mya Australia was cool and temperate. 10mya Australia began to warm up. It became progressively drier and rainfall became seasonal. The forests changed and eucalypts (sclerophylls) began to dominate with more open forests and grasslands. 4mya the aridity increased rapidly Rainforests became restricted to coastal pockets and were replaced by grasslands in other areas. The present day vegetation Australia is described as Sclerophyllous. The leaves of sclerophylls are hard to survive arid conditions. The soils in Australia are generally poor as Australia is a very old continent and does not have active volcanoes that bring minerals to the surface. Ice ages then occurred which changed sea levels and may have allowed the formation of land bridges to Asia. The evidence of changing environments comes from - fossil records - dating techniques - ice cores - tree rings - glaciers 2.4 Identify areas within Australia that experience significant variations in temperature and water availability.
Australia is a land of extremes. There are long periods of drought followed by floods. There are deserts, alpine regions and wet tropical rainforests. The desert areas may experience great variations of temperature, being hot during the day and very cold at night. The alpine regions have temperatures that vary from below freezing in winter to hot in summer.
There is an increase in dryness and temperature extremes as you travel inland, and temperatures increase as you go north. The highest annual rainfall occurs in Tasmania, and the northern tropical zones have wet and dry seasons. Some areas in the interior of Australia such as Alice Springs do not have rain for many years and then have floods.
Australia today: Temperature – most of the continent is tropical or subtropical. Inland deserts and grasslands experience great temperature variations
Rainfall – ¾ of the continent receives less than 800 mm annual rainfall. Rainfall is unreliable in inland Australia
Aridity – high temperatures and low rainfall result in much of inland Australia being arid. 1/3 mainland is desert, 1/3 semi arid.
Soils – ancient soils low in minerals
Fire – regular part of environment. Many plants have the ability to survive fire, and some plants need it for seeds to germinate.
Vegetation – forests on the coast, scrub and grasslands further inland.
P259 BIF Species need to possess adaptations for the harsh environmental conditions. For example: Sclerophyll leaves – are thin and small, and have a thick waxy cuticle that resists drying out. These leaves are an adaptation to the low nutrients in the soil and to the arid conditions. Sclerophyll plants include eucalypts, banksias, grevillea, tea trees, bottle brushes, wattles, heath plants
Specific Adaptations to fire o Thick bark with dormant epicormic buds or lignotubers that can regrow after fire eg Eucalyptus, Banksia o Seeds that are protected in woody cones, capsules (eucalypts) or fruits (banksia, hakea) o Seeds that require the heat of a fire to germinate (eg mountain ash) or smoke to stimulate germination (acacia)
Behavioural Adaptations to aridity o Frogs burrow into soil to avoid dry environment o Hopping mouse live in deep burrows to avoid water loss and heat stress, nocturnal, get all water requirements from food o Kangaroos are inactive during the hottest part of the day, and are most active at dawn and dusk 2.5 Identify changes in the distribution of Australian species, as rainforests contracted and sclerophyll communities and grasslands spread, as indicated by fossil evidence.
When Australia became a separate continent 40 mya there were large odd creatures unlike those seen today and temperate rainforests all around. It would have been cold and wet. Nothofagus and Glossopteris species flourished.
As Australia drifted north 15mya, it became warmer so that only small pockets of isolated temperate rainforests survived eg Tasmania and from Lamington National Park (Qld) to the Barrington Tops (NSW).
Consequently many animals became extinct and only small remnants of these rainforests survive. The Antarctic beeches, Nothofagus moorei are living fossils, evidence of this time. With the warmer climate, tropical rainforests began to predominate with fauna such as the tree kangaroo and green ringtail possum (some of which still survive today but only in far north Qld, while fossils are found in central Australia).
About 10-15mya Australia started to dry out and grasslands form, especially inland. Therefore ground dwelling rather than tree kangaroos were prevalent.
Fossils of organisms that lived at this time include emus, lungfish, carnivorous kangaroos, tortoises and diprotodons.(eg Riversleigh). Plants that could survive drier areas started to proliferate so that eventually there were vast tracts of grassland inland. Much later still, fossil evidence (eg pollen fossils) indicates that small trees and shrubs encroached on these drier areas. These plants, the sclerophylls, were adapted to survive where water was scarce.
While the first fossil pollen record of Acacia and Eucalyptus was 25mya, it was many millions of years before they outcompeted Nothofagus and became the predominant species. Since about 2mya there have been many Ice Ages which have caused sea levels to fluctuate so there has been some migration of species between mainland Australia, New Guinea and Tasmania. Fossil evidence indicated it was not until the last Ice Age 10 000 years ago that sclerophylls came to dominate the Australian landscape. The abundance and variety of Australia’s fauna varied in response to the changing vegetation.
Overall the climate is getting warmer and drier. This is supported by fossil evidence which suggests changing patterns of vegetation. Human impact is now considered the most recent cause of vegetation change.
Complete the worksheets:
The Evolution of Australian Plants and
The Evolution of Australian Mammals 2.6 Discuss current theories that provide a model to account for these changes.p266- 7 BIF The changes in the distribution of Australian vegetation can be explained by several theories:
What 2 main theories does your textbook suggest?
Theory 1: Changes in Climate-due to: Continental drift- explains why climate changed Natural selection- explains the change is species due to a change in their environment Adaptive radiation- where remaining species move into new environments and evolve into different species.
Arguments for Arguments against
Theory 2: Human Impact-due to:
Arguments for Arguments against
Complete the worksheet: Models to account for changes in the Distribution of Australian species.