Story: Evolution of plants and animals
Page 5 – Ecological influences
Large, flightless animals generally cannot cross oceans. This explains why New Zealand has no land mammals, iguanas, land turtles and snakes. A number of advanced groups are missing among the ants, wasps, hornets and termites, including the most aggressive and destructive species.
Advanced groups of predators and competitors evolved elsewhere over the past 85 million years. But they could not cross the ocean and did not become established in New Zealand.
Birds had no predators on the ground; they were preyed upon by a range of small to very large predatory birds. So for most birds, flight was unnecessary – even risky. Before humans arrived, a quarter of New Zealand’s land and freshwater birds were flightless. Many more were poor fliers. As they did not need to be light enough to fly, most became larger than their flying relatives. Adaptations for self-defence were all geared towards predatory birds. They included limited flight, camouflage, ground nesting and freezing when disturbed.
Some animal groups proliferated in the absence of predators. New Zealand is home to more than 80 species of skinks and geckos, an extraordinary number for a coolish, temperate climate. The forest understorey, moss and litter habitat has over 1,400 species of snail. There are more than 1,800 moth species, and over 2,000 species of fly. This may be due to the lack of small mammal predators such as rats and mice, aggressive colonial ants, wasps and hornets, and competition from termites. Specialised invertebrates that feed on or parasitise snails are also largely absent.
In place of mammals
New Zealand’s birds, bats and insects expanded into areas and ecological roles normally occupied by mammals. The kiwi has hair-like feathers, a keen sense of smell, and powerful digging feet. Nesting in a burrow, it is much like a badger.
Moa fed in forests and grasslands like goats, deer and cattle. They became very large, in order to better deal with a coarse vegetable diet. In place of mice, flightless wrens, ground-walking bats, large carnivorous Paryphanta land snails, and giant wētā (up to 70 grams in weight) fed on the insects and plants of the forest floor. The most likely explanation for gigantism is that greater size increased the range of foods animals could eat.
Flowers in other continents often have intricate shapes and bright colours. Most are bisexual, relying on animals to pollinate them. New Zealand lacks the main specialised flower pollinators – long-tongued bees, hawkmoths and hummingbirds. There are very few butterflies.
Plants with complex flowers that need specialised pollinators could not become established. They could not set seed or, if they did, the seed produced by self-pollination was inferior (as genetic material was not being exchanged between different plants). The failure of plants with complex flowers to grow, in turn, hindered the establishment of specialised pollinators.
Typically, native flowers are small (sometimes only a few millimetres in diameter), simple, and white or dull coloured. These blossoms are suited to New Zealand's main pollinators – small, short-tongued native bees, night-flying moths, flies and beetles. The flowers often have a strong scent to attract a wide range of pollinators. Lack of complex floral structures increases the risk of self-pollination. To help ensure that genetic material is exchanged between plants, many (23%) of New Zealand's flowering plant genera are unisexual, more than anywhere else.