Monsters We Met - Other causes

Some scientists believe there is evidence that humanity hunted the megafauna to extinction. The 'overkill' hypothesis holds that the megafauna vanished only a few centuries after the arrival of man and that hunting was the primary cause of the extinctions

Other causes

Overhunting by humans is not the only factor involved in extinctions. Many other theories have been proposed to account for vanishing species. The main ones are:

• climate change
• environmental changes precipitated by people
• hyper-diseases - did these kill off the American megafauna?
• horrible hitchhikers - deliberately or accidentally introduced animals
• lies, damned lies and statistics

Europe

Climate change

The tundra-steppe habitat, a unique mixture of tundra and steppe plants that existed all across Europe and Asia, vanished at the end of the ice age. Herbivores such as the woolly rhino, which had thrived here, found themselves homeless.

	Did humanity hunt the megafauna to extinction?

Some creatures were able to migrate north as the ice melted, following the tundra into the Arctic. However, Arctic regions have a shorter summer and less intense sunlight than temperate latitudes, resulting in a shorter growing season for plants. The number of animals that the present day Arctic can support is therefore far less than ice-age central southern Europe could. As the herds of woolly mammoth and musk oxen went north, fewer and fewer would have been able to find enough to eat. Eventually they would either have been too few for a viable population or out-competed by better adapted rivals such as reindeer.

	Pollen enables scientists to piece together the region's vegetation in past times.

Americas

Climate change

Many of the extinctions occurred as the climate was changing and plant communities were forced to adapt. At the end of the ice age, the North American environment changed from a fairly homogeneous mix of plants and animals to a patchwork of more specialised habitats. Different geographical areas developed more environmentally stable but very specific habitats. Particular plants and animals often became restricted to particular regions, and animals that could not make the transition to these new conditions became extinct.

	The last period of glaciation, took place between 70,000 and 10,000 years ago.

Disease

Humans, brown bear and saiga antelopes all migrated into the Americas at about the same time. Could one of these, or the parasites and insects that accompanied them, have carried a hyper-disease?

This theory doesn't have many adherents. To wipe out creatures as diverse as elephants, ground sloths and sabre-tooths it would have to have a scattershot effect. Yet, it would also need to be specific to kill camels but not llamas. Why were no diseases introduced when earlier waves of animals such as caribou, woolly mammoths and moose migrated in? And why were animals migrating out of the Americas, for example mink and the ancestors of European bison, immune?

Lies, damned lies and statistics?

In 1984, PD Gingerich pointed out that in North America a higher than normal rate of new species arose in the early Pleistocene. The high rate of species loss at the end of the Pleistocene may therefore be a statistical artefact as, over time, speciation and extinction tend to balance.

Of course, there still has to be a reason for a species to vanish, but each species extinction might have a separate cause - the result of competition with other animals, loss of habitat or pressure of hunting by humans.

Next - Other causes in Australia and New Zealand