Plant and animal groups on islands often undergo radiations – where one species rapidly evolves into many species that may live in different environments. On islands there are usually few plant and animal types, which reduces competition. Also, mountainous island archipelagos offer diverse habitats where new species can evolve.
New Zealand lost most of its Gondwanan plants and animals progressively as it sank into the ocean. Around 30 million years ago, the land area was probably less than 20% of its current size. Many groups of plants and animals became extinct or were greatly reduced. This was an advantage to species that persisted or were new to arrive.
For example, from a single moa ancestor, which lived around 30 million years ago, at least 10 moa species evolved. They differed in form and size (20–250 kilograms), and were present in all habitats from alpine zones to the coast.
Insect groups have also radiated into many specialised forms. Examples include cicadas (40 species) and wētā (70 species), which have species in all habitats from the seashore to mountain tops.
Reasons for radiation
For many plant and animal groups the opportunity to radiate was provided by three factors:
- New Zealand's fragmentation into islands (about 30 million years ago)
- the cooling climate (from about 10 million years ago)
- the increasingly mountainous landscapes (from about 5 million years ago).
Examples of radiation and adaptation
Parrots and parakeets have adapted to New Zealand’s alpine and subantarctic environments. The world’s most southerly parrots and parakeets live on the subantarctic islands, and the kea is an alpine parrot.
New Zealand lizards are unusual in their tolerance of cool climates, having specific adaptations such as giving birth to live young (rather than laying eggs). Insects such as cicadas have invaded the alpine zone, which they do not do in other countries.
Many new, species-rich groups appear to have originated from long-distance dispersal. The herb Epilobium, originally from North America, arrived recently – probably in the last 3 million years. More than 40 species have radiated into numerous open and mountainous habitats. Ranunculus (buttercups), a diverse group found in mountains and subantarctic islands, arrived only 5 million years ago, and now there are more than 40 species with highly variable leaves.
Few of New Zealand’s plants and animals appear to be ancient remnants from Gondwana. Arrivals during the past 85 million years have rapidly evolved and radiated into different habitats and form many of the native land species.