Causes: ice ages
Within the human era (around the last 200,000 years) an extinction wave, often referred to as the sixth extinction, has been sweeping around the world. On each landmass the first group of animals affected has been the megafauna – the largest mammals and birds of each landmass.
The extinction of the large mammals of North and South America was recognised in the early 19th century. For many years it was thought that the inhospitable ice-age climate caused their extinction – even though it happened at a time when the climate was improving, at the end of the last ice age in the late Pleistocene period (around 14,000 years ago).
… or hunting?
In the 1960s American paleontologist Paul Martin suggested that human hunting was a primary cause for the major extinctions 10,000–12,000 years ago in America. Since then, this ‘Pleistocene overkill’ hypothesis has remained controversial, but it is supported by an increasing amount of evidence, including data from improved radiocarbon dating methods. This has shown that a wave of regional extinctions took less than 1,000 years to cross North America, with megafauna disappearing from a region soon after humans first arrived there.
Among the species lost from North America were mammoths, mastodons and giant sloths, along with the American horse, camel, lion, cheetah and sabre-cat.
Human impact in Australia
The first humans to settle beyond Africa and Asia were those who reached Australia (via Indonesia) between 50,000 and 40,000 years ago. The following few centuries or millennia saw the end of Australia’s giant mammals and birds. Huge cow-like marsupials and a 500-kilogram relative of the goose, nicknamed ‘the demon duck of doom’, vanished as hunting and fire transformed the large continent.
Most of the world’s remaining large wild land animals – such as elephants, tigers and giraffes – are on the continents of Africa and Asia, where they co-evolved with hominids (members of the human family). These animals evolved defence strategies to keep pace with early humans’ methods and tools for hunting. This meant that the contest between predator and prey was relatively even. In lands colonised after humans reached their modern form, such as Australia, the Americas, Oceania (including New Zealand), hunters encountered animals that had evolved free of human predation. This prey could be hunted more quickly than it could replace its numbers by reproduction.
Animals arriving with humans
As they migrated around the globe, people took animals with them to new land masses. On oceanic islands where there were few mammal predators or browsers, the new species have wiped out native species, or made radical changes to the environment by altering the vegetation.
New Zealand: the last wilderness
New Zealand was the last large habitable land mass to be colonised by humans, and is the most recent to experience an extinction event. It was the last primeval wilderness on the planet. Extinctions of species such as moa and Haast’s eagle began with the arrival of humans and kiore (Pacific rats), and are still happening. Most evidence points to a first human arrival date between 1250 and 1300 AD. It has been suggested that kiore might have reached New Zealand much earlier, around 2,000 years ago – with transitory human visitors.