The ancestors of many of today’s native plants and animals arrived after New Zealand separated from Gondwana, around 85 million years ago. But how? There are many hundreds of kilometres of open ocean around New Zealand.
Plants have spread more successfully than animals. Most plants can survive for a long time as small seeds. They may be damaged and still survive, and they only need basic climate and soil requirements to start growing.
Species that are isolated in different environments for any length of time evolve into distinct forms. So, New Zealand species identical to those elsewhere must have arrived recently – or spread recently from New Zealand to other countries.
Native plants also found elsewhere are usually those that release microscopic spores (for example, mosses and ferns) and flowering plants with small seeds (such as orchids). Their spores and seeds are easily carried by wind. Wetland herbs are often dispersed by migratory birds carrying seeds and spores in mud on their feet or feathers. Forest trees with heavy seeds are less likely to spread from one land mass to another.
Animals often have more specific requirements than plants. While the vast majority of plants are hermaphrodites (they have both male and female reproductive organs), most animals are unisexual – which means they need both a male individual and a female individual to reproduce. Most animals need certain plants or prey for food, so these must be present for them to survive. Some, such as social insects, have complex social structures which make it unlikely that a few individuals could establish a new population. Only birds, or microscopic or asexual animals, disperse as easily as plants.
Animal species found both in New Zealand and other places can usually fly. These include birds (harrier, pūkeko), butterflies (monarch butterfly) and dragonflies. Strangely, few insects or spiders have arrived recently, even though many can fly and some are small enough to be carried by the wind.
Aussies on Māui
Before the development of the Māui gas field off Taranaki, entomologists put insect traps on the drilling platforms. An amazing number of exhausted Australian moth and butterfly species, borne on trans-Tasman gales, were captured. Of the species caught, only a handful had successfully become established on mainland New Zealand.
Many plants and animals have arrived since New Zealand split from Gondwana, despite not having features that would normally make the journey possible. Skinks, for example, may have arrived about 30 million years ago, and buttercups only 5 million years ago. Chance transport by wind and bird movements is unlikely – but these species have arrived over tens of millions of years. Given enough time, even the improbable will happen.
Some species flew or blew to New Zealand. For animals that do not fly, only very rare and unusual events could have brought them to the country. They may have come on floating masses of earth, wood or plants, possibly washed out to sea in floods or tsunamis. Unusually strong and persistent winds or storms could have brought them to New Zealand.