New Zealand has at least 171 species of native earthworms and 23 non-native species. Scientific knowledge is continually improving; the current known species are:
- 123 species of Acanthodrilidae, two introduced (non-native)
- 53 species of Megascolecidae (three introduced)
- 17 species of Lumbricidae (all introduced)
- one species of Glossoscolecidae (introduced).
Several native species grow to 30 centimetres or more. The longest, Spenceriella gigantea, grows to 1.3 metres.
The native earthworms apparently arrived in New Zealand in two waves.
- The acanthodrilids probably arrived in the Cretaceous period (65–145 million years ago).
- The megascolecids came in the Tertiary period (1.8 million–65 million years ago).
While many people have contributed to the knowledge of earthworms in New Zealand, the two main experts have been Sir William Benham of the University of Otago, who published many papers between 1891 and 1950, and Ken Lee at the Soil Bureau, Department of Scientific and Industrial Research, during the 1950s.
Although once widespread, today New Zealand’s native earthworms are mostly confined to areas where the soil is disturbed less often – forests, old gardens, hills and mountains. There are 36 known native species that live in forest litter. Seven species have been recorded from Stewart Island.
Lumbricids: introduced earthworms
Earthworms known as lumbricids arrived with European settlers in the 19th century. They were generally brought by accident, among plants, or in the soil used as ships’ ballast. This was offloaded at the ports, and the worms gradually spread outwards. Some farmers, after seeing the benefits, introduced earthworms to their land.
Aporrectodea caliginosa, probably New Zealand’s most common lumbricid, also dwells in topsoil. During summer it aestivates (the equivalent of hibernating in winter) – often about 30 centimetres below ground.
As native forest has been replaced by exotic plants and farmland, introduced species have come to dominate. Some are widespread, probably through deliberate introduction from overseas or locally, or from being carried on machinery or produce.
Gone to ground
The Taupō volcanic eruption (around 232 AD) produced coarse, sandy soils that led to the death of most native species, including earthworms. As the forest grew back, some Rhododrilus earthworms returned to the area. But they disappeared again when settlers felled the forest to plant pasture. These species are now found only in patchy forest remnants.
Food for wildlife
New Zealand’s native kiwi, kingfishers, robins, brown teal and paradise shelducks eat earthworms. On farms, red-billed and black-billed gulls and mynas ‘follow the plough’, pecking worms from the turned soil.
Thrushes, blackbirds and starlings eat earthworms in gardens and on farms. Rooks, magpies and little owls are also partial to them.
The endangered land snail Paryphanta busbyi busbyi, from Northland's Aupōuri Peninsula, is nocturnal, and eats mainly earthworms. This is probably true of other Paryphanta and Powelliphanta snails. Earthworms are also eaten by other small animals, and may have some parasites within their bodies.