Evolution has great fun on islands. Their isolation means it has the luxury of working its magic for long periods away from mainland gene pools.
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The Galápagos, Hawaii, Madagascar, and Mauritius all teem with species that are found nowhere else. And sitting all on its own in the south-west Pacific, 2,000km from the nearest landmass, New Zealand is no exception. More than half of its native species are endemic, including ancient and unique lineages of reptiles and plants and a spectacular variety of strange flightless birds. And yet something is missing.
Why didn't New Zealand have any terrestrial mammals?
It’s not that New Zealand has no mammals at all, or that it hasn’t had them in the past. It’s just that there have always been very few of them. The only native mammals on the islands now are three species of bat and various seals, sealions and cetaceans. The paucity of terrestrial mammals is explained by New Zealand’s quirky geological history.
How was New Zealand formed?
Until about 80m years ago, it was part of the supercontinent of Gondwanaland. This was still the age of the dinosaurs but fossil evidence shows that at least one species, known as the Saint Bathans mammal, was present when New Zealand split from the mainland.
As New Zealand drifted eastwards, it started sinking under its own weight, to the point that only a few small islands remained above water. That New Zealand exists at all today is due to its position on the boundary between two of the great tectonic plates – the Pacific and the Australian. Volcanic activity around this weakness in the Earth’s crust caused new land to rise from the water.
Over millennia, birds, bats and insects were able to colonise the islands. However, no terrestrial mammals were able to make the sea crossing. Ecological niches conventionally occupied elsewhere in the world by mammals were filled by moas, kiwis, kākāpō, weka, takahē and other birds that ditched their aerial capabilities in favour of a life on terra firma.
This unusual community endured in splendid isolation until around 1280 when New Zealand was colonised by Polynesian islanders, the Maori, who brought with them rats. And after their arrival in 1769, Europeans introduced three more species of rodent, as well as weasels, stoats, ferrets, hedgehogs, rabbits, cats and brushtail possums. New Zealand now has more mammals than at any point during its history. But introduced mammalian predators have contributed to the extinction of such endemic birds as the bushwren, huia and North Island snipe.
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Main image: New Zealand sea lion/Credit: Getty