What happened to Australia’s megafauna? Diprotodons, giant kangaroos, marsupial lions, and other huge mammals roamed this continent until the middle of the last ice age. They lived alongside some spectacular birds and reptiles, including a goose bigger than an emu, giant horned land-turtles, and a goanna weighing 100kg. Now, Australian landscapes that have plenty of space and resources for big animals are strangely empty.
Why these animals went extinct has been debated for over 150 years. In The Future Eaters Tim Flannery argued that hunting by people was the cause, but the idea was resisted because there was too little evidence that the extinctions followed closely on human arrival, and it seemed unbelievable that a sparse human population could have inflicted enough hunting to wipe out more than 50 species of large animals.
Recent research, which I summarise in a new book, strongly supports hunting as the sole cause of the extinctions. We now know that the megafauna disappeared very soon after people came to Australia 50-45,000 years ago. The climate at the time was benign and stable and there was no general increase in fire, ruling out climate change or increased burning as contributing factors.
We also have the key to understanding how it was that hunting caused so many extinctions so quickly. The common feature of the extinct beasts was that they had much lower reproductive rates than the survivors. Slow reproduction meant that even very low rates of killing amounted to unsustainable harvest. People might have killed big animals only rarely, spending most of their time hunting smaller game or gathering plant food, but this could still have driven the big things extinct.
There is very little archaeological evidence of this hunting, but that is to be expected given the low rates that were probably involved. Anyway, most archaeological sites older than 40,000 years in Australia contain little or no bone because of poor conditions for its preservation, so there is not much chance of finding megafauna bones with marks showing they were killed by people.
The argument is probably not over yet. Some palaeontologists don’t accept that there was a human-caused mass extinction because for some extinct species we don’t have fossils of the right age to show that they were still alive immediately before people arrived. They argue that these species went extinct before people arrived, as part of some long-term natural process.
This is a weak argument because it stands on gaps in the fossil record. The fossil record is inevitably full of gaps, meaning that even if all megafauna went extinct within the same short period, fossil evidence of this would be missing for some. More careful analysis of the data shows no decline in megafauna diversity before human arrival, and complete extinction by a few thousand years after.
Objectors also point to a few fossil sites suggesting stable coexistence of megafauna and people after 45,000 years ago. Most of these sites were excavated decades ago, when dating technology was poor. More rigorous study has pushed the dates on megafauna bones back to 45,000 years or earlier, with no more than brief human-megafauna overlap. The major remaining battleground in this part of the debate is Cuddie Springs in New South Wales, a controversial fossil and archaeological site that still awaits definitive dating.
But perhaps the most interesting issue is whether it should matter to us now that human hunting, and not something else, caused the megafauna extinctions. It does matter. Here are two reasons why:
First, the fact that megafauna were removed by people, and not because of an underlying environmental change, means that our ecosystems are incomplete without them. Big animals have big impacts on vegetation. Browsing controls the growth of shrubs and small trees and prevents aggressive species from dominating. Fruit-eating megafauna swallow seeds and deposit them, enveloped in fertiliser, far from the parent tree. Plants evolve in response to these impacts, adopting growth forms or defensive structures to protect themselves against over-browsing, and producing large fruit that are attractive to big animals and have durable seeds.
These interactions can be seen in action in Africa, the only continent that still has its prehistoric megafauna. In Australia, their ghostly imprint remains in the form of plants that grow defensive spines even though they have no browsers to defend themselves against, and produce large fruits that fall to rot in piles beneath the parent tree because no animal moves them.
The general effect of plant-megafauna interactions is to promote vegetation diversity, both in structure and species. Removing megafauna should result in simplification of vegetation. There is some evidence that this happened about 45,000 years ago across large areas of inland Australia, when it seems that complex and patchy tree/shrub/grassland communities were replaced by uniform shrublands.