The Death of the Dinosaurs
When New Zealand first began to rift away from Gondwana, dinosaurs still walked amongst the shade of giant conifers, the ocean was ruled by vicious marine reptiles and the skies were dominated by flying pterosaurs. But around 65 million years ago a meteorite roughly 10 kilometres wide crashed into the Yucatan peninsula. The shockwave, the ensuing tsunamis and firestorms, and the toxic dust that clouded out the atmosphere brought the reign of the most successful group of vertebrates on planet earth to an end.
But as is often the case with the natural world, this major disruption event also created opportunities, as the roles previously held by dinosaurs and other reptiles were left open to other enterprising groups of animals. The mammals which till this point had been mostly confined to burrowing into holes and scurrying about at night, rapidly diversified. They became top predators, giant herbivores, some taking to the sea and some even taking to the air.
A Land of Beaks & Feathers
However by the time of this great mammal radiation, New Zealand was already over a thousand kilometres from the nearest continental landmass. This provided an extraordinary opportunity for birds, allowing them to assume roles that in most other places had been claimed by mammals. Giant herbivorous Moas took the places held by creatures such as cows and deer in other countries, selectively grazing on different plants. They were the prey for the largest eagle ever recorded, the extinct haasts eagle which would swoop down from height and puncture their backs with its large talons. The majority of New Zealand animals have analogues with mammals groups around the world: kiwi with their heightened sense of smell, fossick in the undergrowth for insects like badgers; tiny flightless wrens scuttled about in the grass like field mice; and the kōkako scrambles up tree branches and glides around the forest like a flying squirrel.
The Plants Respond
This unique assemblage of animals, unlike anywhere else on the planet, meant a different set of challenges for New Zealand’s plants. What follows is a collection of hypotheses put forward about how plants adapted to these challenges – but bear in mind that these are difficult claims to prove conclusively and the debate rages on.
One interesting feature of New Zealand flora is the curiously large number that show a divaricating growth form, making them compact and tangled. Even some trees such as the tall Matai begin life as a shrubby tangled juvenile, going on to produce a single trunk. It has been suggested that this might be a response to browsing by moa, nurturing precious leaves from prying beaks behind a tangle of twigs and branches. When divaricating and non-divaricating plants are fed to similar modern birds (such as emus and ostriches) they have distinctly more trouble accessing the leaves of the divaricating forms.
Other curious features of New Zealand plant life is the great amount of heteromorphy – where juvenile plants have a distinctly different growth form to those of adults. It has been suggested that this too, might be an adaptation to moa browsing. Lancewood (Pseudopanax crassifolius) for example in its juvenile form has dense, tough, spiked leaves, but once it grows tall enough these transition to much softer green leaves. Perhaps lancewood developed this striking growth form in order to deter moa when it was still within their reach, and having survived past their greatest extent could afford leaves that were more photo-synthetically efficient.
It is possible that bird herbivores would depend more on sight rather than smell, and there are a range of hypotheses that examine how this might have affected plant-life. The mottled appearance of many small seedlings and shrubs (New Zealand Jasmine – Parsonsia capsularis and Horopito – Pseudowintera colorata) might serve to break up the outline of the leaf making it harder to identify. Lancewood has also been put forward as one of the world’s first known examples of plants using camoflague. It’s small mottled seedlings look very similar to decaying leaf litter, possibly disguising them from browsing birds.
No Mammals Whatsoever?
Despite New Zealand being constantly upheld as the quintessential example of a land without mammals, this is not strictly true. A fossil of a small mouse-sized mammal unrelated to modern groups was found in Central Otago and is estimated to be around 16 – 19 million years old. In all likelihood it was the ancestor of an early proto-mammal that was carried away from Gondwana on the New Zealand landmass. Their small size seems to suggest that they did not have the same level of evolutionary success experienced elsewhere, probably remaining as small, nocturnal insectivores in the shadow of the dominant bird species. They went extinct however, most probably before the arrival of humans. Today the only remaining native land mammals are several species of bat, which would have flown here long after New Zealand’s separation from Gondwana.