The human factor
The causes of extinctions in Australia and North America are still debated, but there is little doubt that humans are the cause of recent New Zealand extinctions. Research has implicated:
- the mammalian predators introduced by humans
- changes in habitat caused by humans and rats
- habitat destruction by human-caused fires
- direct predation by people.
No link to ice ages
The New Zealand extinctions differ from those in other places because of their lateness. Almost all species made it through the coldest part of the most recent (Ōtira) ice age, which is when the American extinctions took place.
Role of disease
Diseases are commonly suggested as a cause of extinction, but it is rare for any disease to wipe out its host species. The only foreign pathogen to be found in New Zealand birds is bird pox, which has had little impact on the species involved. Diseases usually affect closely related species, but in New Zealand, for example, some wrens and frogs died out, while related species survived nearby.
Bird loss: impact on ecology
The killing off of huge seabird populations by rats on the main islands had the most significant ecological effect on New Zealand. With their loss, the cycling of large quantities of nutrients from the sea to the land has stopped. Extinct land birds included species that pollinated and dispersed the seeds of forest trees and shrubs, and the large plant-eating birds whose browsing may have shaped growth patterns of some New Zealand vegetation.
The habitats of the most endangered species are not those they enjoyed before humans arrived. Former ecosystems may have changed because of the extinction of invertebrates, such as some species of wētā, and vertebrates, such as the seabirds. Indeed, major nutrient sources may no longer be present.
Lessons from the past
Globally, the New Zealand extinction event is the most recent of all the late Quaternary-period extinctions. The rich record of species in fossil deposits makes this one of the best understood extinction events. While extinct species cannot be revived, it is a warning that the present generation act to ensure that no more species become extinct.