Human Impacts on Island Ecosystems (australia)

Human Impacts on island ecosystems. (Australia)

There are few paleovegetation records in Australia before 18,000 years ago. To find out the history of the Australian fauna scientists used stable carbon isotopes from the emu eggshell (a flightless bird native to Australia). The samples were largely taken from Lake Eyre. The carbon composition of the eggshell tells us the composition of the bird's diet (over 3-5 days). The emus are mixed feeders herbivores, eating leaves, shoots, fruits, flowers, shrubs and grasses. The variation of the carbon values reflect the changes in the birds diet and hence the composition of the flora.

There are 2 different types of plants in Australia, defined by their different photosynthetic pathways. One uses the C4 pathway and the other C3. The photosynthetic pathways depend firstly on the season of rainfall and on the geographical position i.e. north to south variations. C3 grasses grow best in areas affected by the winter monsoons (southern Australia). Whereas C4 grasses dominate areas that are affected by the summer monsoon (central and northern Australia). The majority of trees and shrubs across Australia are C3 plants. Emu normally lay their eggs in the winter so their preferred diet is C3 plants. The relative abundance of C4 grasses has varied over the past 65,000 years.

The fauna found in Australia prior to human arrival was very different to the animals living there now. Over 85% of all animals that exceeded body mass of 44kilos are now extinct. They became extinct in the Late Pleistocene period. These very large animals were mostly marsupials but also included a flightless bird Genyornis newtoni and 3 large reptiles. Two other birds, the emu (Dromaius novaehollandiae) and cassowart (Casuarius casuarius) survived.

These large, extinct species are called the Australian megafauna.

The extinction of these species was once thought to be the effect of climate change and human predators. It is now believed to be from indirect consequences of human activity particularly burning practices. To find out which of the theories are correct dates needed to be found out to tell us the exact dates for extinctions, the arrival of the humans and the changes in climate.

To find out these dates the eggshells of the large flightless birds (Genyornis and Dromaius) were used again. Bird populations are primarily controlled by climate, nutrients and predation, including humans. Eggshell can tell us the composition of the birds diet and hence the nutrient availability. Eggshells can be collected from deposits formed in the drier and wetter seasons and therefore can tell us the environmental/climate change. The youngest eggshells can provide a date on the bird's extinction. Dating eggshells has another advantage over bone dating in that their structure make s them resistant to diagenesis (turning into rocks).

The eggshells of both birds were found close to each other in all areas. Only Genyornis became extinct.

Dating the arrival of the first humans has proved to be very difficult. It is generally believed that

Humans arrived in Australia around 50,000 years ago. There are evidence of Genyornis presence in Australia from more that 100,000 years ago until their sudden disappearance 50,000 years ago (same time as the humans arrived in Australia). The simultaneous extinction of Genyornis from all the sites, during an era of modest climate change, suggests that human impact and not the climate change was responsible. There were some significant climate changes during the Pleistocene period. There was a long period of aridity, which was terminated around 60,000years ago and was followed by a wetter period. . The eggshells of Genyornis were still present in the soil composition at these times. The climate than began to get dryer again and there was no more evidence for the presence of Genyornis.