There have been times in the earth’s history when the rate of extinctions was particularly high. This is called an extinction event – a period which sees the wholesale extinction of species of animals or plants over large areas, in the sea, or on land, or both. It can happen on a regional or global scale.
Some extinction events have involved almost all organisms living at the time. Five major worldwide events over the last 550 million years have been recognised.
Permian–Triassic extinction: the third extinction
One of the largest extinction events, called the third extinction, happened at the end of the Permian period, about 250 million years ago. Over 94% of the species recognised from fossils as being alive before the event became extinct at this time.
Cretaceous–Tertiary extinction: the fifth extinction
The extinction of the large dinosaurs worldwide (including those in New Zealand) occurred at the end of the Cretaceous period, 65 million years ago. This is the most widely known extinction event, although it was far less catastrophic than the one at the end of the Permian period. The Cretaceous–Tertiary extinction is also called the fifth extinction.
Loss of diversity in New Zealand
In New Zealand, plant and animal diversity was probably at its greatest during the Miocene period, around 15 million years ago. As in Australia today, eucalypts, she-oaks, and Wollemi pines grew. There were crocodiles, large bats that resembled fruit bats, and a bigger range of ducks and geese than in later times.
A major cooling phase around 2.4 million years ago led to changes in the fauna and flora worldwide. On continents, plants and animals could gradually move closer to the equator as the higher latitudes cooled. But New Zealand did not have such a latitudinal range, so it lost some of its subtropical species around that time.
Climate change is always accompanied by changes in the distribution of plants. In New Zealand, changes to the forests undoubtedly affected the distribution and abundance of bird species such as moa and the giant Haast’s eagle (Aquila moorei). For example, the eagle and two large moa (the stout-legged moa Euryapteryx gravis, and the heavy-footed moa Pachyornis elephantopus) were common on the West Coast and in north-west Nelson during the most recent (Ōtira) glaciation, 20,000 years ago. Fifteen thousand years ago, as the climate warmed and multi-storeyed forest returned, these species vanished west of the main divide (the line of highest peaks in the Southern Alps). They were still plentiful in their preferred habitat of forest–shrubland mosaic to the east, right up to the time of Polynesian settlement around 1250–1300 AD.