Past Ecosystems

In short: Past ecosystems are studied using fossils, pollen, charcoal, ice cores, tree rings and Aboriginal knowledge. Australia was once part of Gondwana and covered in rainforest, and as it drifted north and dried out, fire-tolerant eucalypts, acacias and grasses spread.

Starting point: what you already know

• Fossils and rock layers show life changing over time (see Evidence for Evolution).
• Continents move slowly over millions of years (plate tectonics, Year 10 Earth science).
• Ecosystems contain biotic and abiotic factors (see Ecosystems, Food Webs and Energy Flow).

How we know about past ecosystems

Australia's story

1. Gondwana (about 180 million years ago). Australia was part of a southern supercontinent with Antarctica, South America, Africa, India and New Zealand. It was wet and covered in cool rainforest with conifers and southern beeches. Dinosaurs lived here.
2. Breaking away (from about 85 to 45 million years ago). Australia separated from Antarctica and drifted north, alone. Its plants and animals evolved in isolation.
3. Drying out (last 15 million years). As Australia moved into warmer, drier latitudes and the world's climate cooled, rainforest shrank to small patches. Hard-leaved, fire-tolerant plants like eucalypts, acacias and grasses spread across the continent.
4. Ice ages (last 2.5 million years). Sea levels rose and fell. When sea levels were low, people could walk from New Guinea to Tasmania.
5. People arrive (at least 65 000 years ago). Aboriginal peoples arrived and became part of these ecosystems, managing land with fire.
6. Megafauna disappear (about 45 000 years ago). Most giant animals became extinct.

Megafauna

Megafauna are animals over about 45 kg. Australia's included Diprotodon (a wombat relative as big as a rhinoceros), Procoptodon (a 2 metre short-faced kangaroo), Thylacoleo (the marsupial lion) and Genyornis (a huge flightless bird).

Why did they disappear? Scientists still debate it:

• Climate change: drying climate reduced food and water.
• Human hunting: people arrived not long before many species vanished.
• Changed fire patterns: altered vegetation.
• Most likely, a mix of these, with different causes in different places.

Quick summary

• Fossils, pollen, charcoal, ice cores, rocks, tree rings and Aboriginal knowledge all tell us about past ecosystems.
• Australia was part of Gondwana and was once covered in rainforest.
• After it drifted north and dried, fire-tolerant eucalypts, acacias and grasses spread.
• Megafauna went extinct about 45 000 years ago, probably from a mix of climate change and human impact.
• Ice cores show past $\mathrm{C}{\mathrm{O}}_2$ and temperature.

Quiz