# Past Ecosystems

> How we learn about ecosystems of the past from fossils, rocks, ice cores, pollen and Aboriginal knowledge, and how Australia changed from Gondwana to today.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/past-ecosystems/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The cave of bones
At Naracoorte in South Australia, caves have acted as natural pit traps for half a million years. Animals fell through holes in the roof and could not climb out. Their bones piled up layer after layer.

Scientists digging there have found giant kangaroos that walked instead of hopped, a marsupial "lion" with teeth like bolt cutters, and wombat relatives the size of a car. Pollen and charcoal in the same layers tell them which plants grew and how often fires burned. The cave is a library of ancient ecosystems, one layer per page.

**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

| Evidence | What it tells us |
|---|---|
| **Fossils** of plants and animals | What lived there and when |
| **Pollen** in lake mud and peat | Which plants grew. Pollen survives for thousands of years and each plant's pollen has a unique shape |
| **Charcoal** layers | How often fires burned |
| **Ice cores** from Antarctica | Ancient air bubbles show past $\mathrm{CO_2}$ levels. Oxygen isotopes show past temperatures |
| **Rock types** | Coal means swamp forest. Salt beds mean evaporating seas. Tillite means glaciers |
| **Tree rings** and coral growth bands | Wet and dry years, warm and cool years |
| **Aboriginal rock art** and oral histories | Animals and landscapes that people saw, some now gone |

## 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.

### Key idea: Fossils are the evidence for change
Rainforest fossils (leaves, pollen) found in places that are now dry desert show that the Australian environment has changed. Plants that suit drier, fire-prone conditions evolved and spread as the climate changed.

**Mind map**

```text
Past ecosystems
  Evidence
    Fossils and pollen
    Charcoal
    Ice cores
    Rock types
    Aboriginal knowledge and rock art
  Australia's story
    Gondwana rainforest
    Drifted north alone
    Dried out
    Eucalypts and acacias spread
  Megafauna
    Diprotodon
    Marsupial lion
    Climate, hunting or both
```

### Year 11: Ice cores and climate
In Antarctica, snow builds up year after year and turns to ice, trapping tiny bubbles of air. A core drilled 3 km deep holds about 800 000 years of air. Scientists measure:

- $\mathrm{CO_2}$ in the bubbles: it has gone up and down between about 180 and 300 parts per million in natural cycles. Today it is over 420 ppm.
- The ratio of the oxygen isotopes oxygen-18 and oxygen-16 in the ice, which shows the temperature when the snow fell.

The two records rise and fall together. This evidence is used to understand how today's rising $\mathrm{CO_2}$ may change future ecosystems.

### Year 12: Past ecosystems and extinction risk
Past extinctions show that species with **small ranges**, **slow reproduction** and **large body size** are most at risk when the environment changes quickly. These lessons guide conservation of today's species, and they link to biodiversity in Module 6 and the 2025 syllabus focus on biodiversity.

## 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 $\mathrm{CO_2}$ and temperature.

## Quiz

Try these questions from the chapter quiz.

1. Which evidence shows which plants grew in an area thousands of years ago?
   - A. Ice cores
   - B. Pollen in lake mud
   - C. Tree height today
   - D. Weather records
   - Answer: B. Pollen in lake mud. Pollen survives and can be identified.

2. Australia was once part of a supercontinent called
   - A. Pangaea only
   - B. Gondwana
   - C. Laurasia
   - D. Atlantis
   - Answer: B. Gondwana. Australia was in the southern supercontinent Gondwana.

3. What covered much of Australia when it was part of Gondwana?
   - A. Desert
   - B. Rainforest
   - C. Grassland
   - D. Ice
   - Answer: B. Rainforest. It was wet with cool rainforest.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/past-ecosystems/#quiz