Extinction Event: Australian Megafauna (HSC SSCE Biology): Revision Notes
Extinction Event: Australian Megafauna
Understanding extinction
Extinction occurs when all members of a species die out completely. Sometimes, multiple species disappear around the same time in what scientists call a major extinction event. The Australian megafauna extinction is an important example of such an event that scientists continue to study today.
Species that lived in the past may no longer exist today. The types and numbers of species on Earth are constantly changing, and studying past extinctions helps scientists understand population dynamics and the factors that influence species survival.
What were the Australian megafauna?
During the Pleistocene epoch (1.6 million to 10,000 years ago), Australia was home to giant animals collectively known as the Australian megafauna. These were enormous versions of animals we might recognise today, including:
- Diprotodon: A giant wombat-like marsupial
- Megalania: A massive goanna (monitor lizard)
- Procoptodon: A giant, flat-faced kangaroo
Megafauna existed on other continents too. Europe and North America had woolly mammoths and sabre-toothed cats, while Africa had (and still has) giraffes, hippopotamuses, and elephants. Interestingly, Africa is the only continent that retained most of its megafauna species.
The ice age context
Earth has experienced many ice ages throughout its 4.5 billion year history. The last ice age was unique because humans were present for it. During this period, the continents were in their current positions, and the climate was very cold and dry. While humans and many other mammals survived this epoch, numerous species went extinct.
The current epoch, called the Holocene, began 10,000 years ago. It represents an interglacial period, which is a warmer phase between ice ages.
Theory 1: Changes in climate
Many scientists initially believed that climate change during the last ice age caused the megafauna extinction. Here's how this theory suggests the process occurred:
How climate change affected Australia
The continent gradually dried out as the ice age progressed. Rainforests, which had covered much of Australia, began to contract due to the increasingly dry conditions. This was significant because rainforests stored moisture and returned enormous amounts of water to the atmosphere through transpiration. The monsoon rains that once penetrated south, keeping rivers and lakes full, began to fail as rainforests diminished.
Eventually, eucalypt forests replaced the rainforests, but these were less efficient at retaining and returning water to the atmosphere. This created a drier overall climate.
As conditions became hotter and drier, fires became more common. Initially sparked by lightning strikes, these fires spread easily through the drier vegetation. Only those plants and animals that could survive drought and fire conditions reproduced successfully, leading to significant changes in Australian flora and fauna.
Arguments supporting the climate change hypothesis
- Large animals like megafauna needed ample water supplies. When water became scarce, they would have struggled to survive.
- Megafauna may not have been able to cope with sudden temperature changes.
- Breeding seasons could have been disrupted by climate shifts.
- The plants they fed on may have become less available or less nutritious.
Arguments against the climate change hypothesis
Critical Questions About the Climate Theory:
- The last ice age was probably similar to previous ice ages, yet there's no evidence that earlier ice ages caused similar mass extinctions. Why would this one be different?
- Evidence suggests some extinctions occurred before the peak of the last ice age, which doesn't fit the climate change timeline.
- Today's climate change doesn't seem to specifically target large, slow-moving species in the same way.
Theory 2: The arrival of humans
Aboriginal people arrived in Australia approximately 65,000 years ago, probably by 'island-hopping' from the north. They were highly effective hunters and used sophisticated land management techniques.
Fire-stick farming
Indigenous Australians used fire strategically to manage the landscape, a practice known as fire-stick farming. They would burn back vegetation to regenerate grasses, which would attract animals for both the animals themselves and for human hunting. By increasing plant growth, they increased animal populations, making more prey available.
Hunting and natural selection
Evidence from the Madjedbebe site (300 km east of Darwin) suggests humans hunted megafauna. Because larger animals were generally slower, they were easier targets and more likely to be killed. The smaller, faster animals that escaped survived to reproduce, passing on their genes. Over time, this selection pressure led populations to evolve smaller body sizes.
Indigenous people appear to have hunted Australian land animals that were larger than themselves. The introduction of dingoes from Asia about 4,000 years ago may have further impacted native fauna, potentially driving the thylacine and Tasmanian devil to extinction on the mainland.
Arguments supporting the human hypothesis
The main evidence linking humans to increased fires is that carbon deposits in fossils date to approximately 40,000 years ago, which aligns with the oldest archaeological sites found beyond northern Australia.
Additionally, the smaller megafauna species that went extinct had short limbs, making them slow-moving. In contrast, the largest surviving Australian species today (such as red and grey kangaroos) are among the fastest, suggesting that speed was a survival advantage.
Arguments against the human hypothesis
Challenges to the Human Theory:
- There is limited fossil evidence of kill sites where humans actively hunted megafauna.
- Very little evidence exists of humans and megafauna coexisting at the same locations.
- When comparing animal sizes, there is overlap between the smallest extinct species and the largest present-day species, which doesn't clearly support a size-based selection pattern.
Theory 3: Level of nutrients
A third theory suggests that Australia's exceptionally low soil nutrient levels may have caused nutrient depletion throughout the food web. This could have favoured smaller animals, which require fewer nutrients to survive.
Evidence for this theory includes the observation that Australian mammals are generally smaller than their counterparts on other continents, even today. This size difference might reflect the long-term nutritional limitations of Australian ecosystems.
Evidence of humans and megafauna coexisting
When scientists find bones from two different species close together at a fossil site, they can infer that these species lived at the same time and possibly interacted. This type of evidence is crucial for understanding whether humans and megafauna overlapped in time and space.
Cuddie Springs fossil site
Cuddie Springs, located in central New South Wales, is a significant fossil site where megafauna bones and human-made stone tools have been found in close proximity. The site contains several distinct layers, each providing insights into different time periods.
Archaeological Evidence at Cuddie Springs:
Important discoveries at this site include:
- A kangaroo leg bone showing signs of butchering by tools (30,000 years old)
- Stone tools (approximately 18,000 years old)
- Mixtures of megafauna bones
- Charcoal from camp fires (approximately 28,000 years old)
- A sandstone grinding stone (approximately 30,000 years old)
These findings suggest that humans and megafauna coexisted for thousands of years in the same regions.
Additional evidence has been reported from sites like Lancefield Swamp, north of Melbourne, where Aboriginal artefacts and megafauna remains have been uncovered together.
The ongoing debate
There is ongoing debate about what caused the extinction of Australian megafauna. Until recently, many scientists supported the climate change theory as the primary cause. Others argued that humans alone were responsible for most extinctions worldwide.
Current researchers increasingly believe that a combination of factors was responsible. The extinction may have been initiated by climate change, with human hunting and fire-stick farming delivering the final impact that pushed megafauna species over the edge into extinction.
This combination theory acknowledges that ecosystems are complex, and multiple stressors can interact to cause dramatic changes. It also recognises that neither climate nor human activity alone fully explains all the available evidence.
Key Points to Remember:
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Extinction means all members of a species have died out. The Australian megafauna were giant animals that lived during the Pleistocene epoch (1.6 million to 10,000 years ago).
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Three main theories explain megafauna extinction: climate change (drying and fire), human arrival and hunting (65,000 years ago), and low nutrient levels in Australian soils.
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Evidence supports and contradicts each theory. The climate theory struggles to explain why this ice age was different from earlier ones. The human theory lacks extensive fossil evidence of kill sites.
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Fossil sites like Cuddie Springs show humans and megafauna coexisted, with tools, bones, and charcoal found together dating to 30,000-18,000 years ago.
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Most scientists now believe a combination of factors caused the extinction, with climate change weakening populations and human activity providing the final pressure that led to their disappearance.