Earthworms are the largest and most numerous burrowing animals in New Zealand soils. But unless we disturb the soil, it is unusual to see them. We may see birds searching for them on the grass, or among garden litter and mulch. After rain we may see dead earthworms on a path: having emerged from their water-filled burrows they die of exposure to ultraviolet light.
A bird will hunt worms at dawn, when they are active near the surface (before the risk of the sun drying them out). Blackbirds watch for movement, often standing with their heads cocked. Seagulls ‘charm’ worms out by stamping on the ground. The worm may perceive the vibrations as raindrops, and come to the surface for moisture. It was once said that kiwi could smell worms, but it is now thought that they may pick up vibrations through sensors in their bill.
Yet earthworms are an essential part of our soils. Some inhabit litter and dung on the surface while others form almost vertical burrows and may live at more than a metre below the surface.
Worldwide there are about 6,000 species of earthworm, 1,000 of them native to Australia. The ‘Gippsland giant’ (Megascolides australis) is listed in the Guinness book of records at 3 metres long. A species in north-eastern New South Wales often grows to more than 1.5 metres, and is as thick as a garden hose. New Zealand’s longest worm grows to 1.3 metres.
Earthworms belong to a class known as Oligochaeta (meaning ‘few hairs’). This class in turn belongs to the phylum known as annelids (collectively called Annelida, from the Latin for ‘little ring’).
Other annelids are the marine polychaetes (‘many hairs’), and predacious or bloodsucking leeches (Hirudinea).
Earthworms are invertebrates – they lack a rigid skeleton. Their skin is permeable, allowing water to pass through. They must live in damp habitats or they will dry out and die.
The body cavity (coelom) between the muscular body wall and intestine is divided into segments, with a membrane (septum) between each.
Each segment is like a sack, and contains various organs, depending on its position in the body. There are two sets of muscles in each segment: a ring of circular muscles that can constrict or narrow the segment, and longitudinal muscles that can shorten the body. Using these stretching and contracting muscles, they push themselves forward.
The fluid in each segment cannot be compressed, and contraction of the muscles causes the segments, and thus the body, to change shape. Swelling enables the body to open up gaps in the soil, while a limp body can move through tight spaces.
The number of segments in mature earthworms ranges from about 50 to over 500.
Most earthworms have amazing powers of repairing damage to their body. But is it true that if you cut a worm in half, you will end up with two worms? In fact, only one part may survive, and even then, regenerating the missing part depends on where it has been cut.
On all but their first segment, earthworms have hairs, or chaetae, which feel like bristles and aid movement by gripping surfaces. The number on each segment can be important in identifying different types. Earthworms in one New Zealand genus (Hoplochaetina) have at least 12 individual chaetae per segment (H. polycystis has about 70), while the lumbricids (which are non-native) always have eight per segment.
Many body organs are repeated in adjacent segments – for example nephridia, which function like kidneys, occur on most segments. Often there are paired hearts in two to five adjacent segments, and paired testes in two adjacent segments.
The worm’s anterior (front section) has the segments with the most specialised organs. The pharynx, behind the mouth, and the grinding gizzard together occupy about 10 segments. There is a thin oesophagus (the tube linking the throat to the stomach), and then the simple intestine begins at about the 15th to 20th segment, running to the terminal anus.
Earthworms are hermaphrodite: each individual has both male and female organs. But they are not self-fertile, and structures for copulation lie on the rear part of the body. The positions and degree of development of the clitellum (a glandular swelling that produces a cocoon for the eggs), and various pores and papillae (bumps), are very important in identifying species.
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:
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.
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.
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.
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.
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.
Life in the soil is largely dependent on organic material from plants – from above ground (leaves, branches, stems) and below (roots). The soil contains a teeming mass of life – not only roots but bacteria, fungi, protozoa, and a range of multicellular animals of which earthworms are the largest, most abundant and widespread.
In his 1881 book The formation of vegetable mould through the action of worms, with observation on their habits, the English naturalist Charles Darwin described the role of earthworms in soils. He noted, for instance, that lumbricid earthworms drag leaves into their burrows. In the garden of his home in Kent is the ‘worm stone’, which he said gradually ‘sank’ into the earth through the burrowing and casting of earthworms, with an annual casting depth of 2–5 millimetres. He documented the important role of earthworms in mixing mineral and organic matter in the upper layer of soil, improving its fertility and ability to hold water and support better plant production. His book triggered the study of soil biology.
In 1881 Charles Darwin wrote of earthworms, ‘It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures.’ 1 By watching worms he recorded how, among other activities, they mix mineral and organic matter in the upper layer of soil, improving the fertility.
Earthworms have an important role in breaking down soil and complex chemicals in plant litter and other organic matter. Their burrowing improves soil drainage, conditions for roots, aeration and soil structure.
Worm casts on the soil surface are a mix of soil and plant material, rich in microbes and many nutrients. They aid the formation of small clumps of soil (aggregates), which allow the passage of air and moisture through the soil. The number of casts varies according to the seasons, peaking in winter, but they can amount to 25–100 tonnes per hectare a year.
Earthworms help form topsoil by depositing their casts on the soil surface. In one study, sticks and stones left on the surface were buried by worm casts, at a depth of almost 1 centimetre per year.
In New Zealand it is only introduced earthworms such as Aporrectodea caliginosa that leave casts on the soil surface. In native forests, where there are only native earthworms, there is no surface casting.
In native forests the weight of earthworms has been found to be greater than that of all other animal groups in the area, including birds and mammals.
In grazed, fertile pastures the earthworm weight may also exceed that of the grazing animals. The mean weight of earthworms in some of New Zealand’s best agricultural soils ranges from about 640 to 3,000 kilograms per hectare. Compare this with 15 sheep per hectare, weighing about 800 kilograms in total.
There may be up to 1,000 earthworms per square metre, or 10 million per hectare. Numbers are at a maximum in spring, and are usually lowest after summer droughts.
In less intensively farmed and less productive hill country, there may be only a few hundred worms per square metre, including a few native species such as the large Octochaetus multiporus. Of course, the number and weight of grazing animals on hill-country pasture is also less.
When farmland is cultivated, earthworm numbers decline through both physical harm and change in the types and amount of plant material they feed on.
Studies in Canterbury showed that worm numbers dropped from about 900 per square metre (9 million per hectare) under pasture, to less than 200 per square metre (2 million per hectare) under long-term arable cropping. There was also a decline in species diversity, with up to five species (Aporrectodea caliginosa, A. trapezoides, A. rosea, Lumbricus rubellus, Octolasion cyaneum) under pasture, but only two (A. caliginosa, A. trapezoides) under arable cropping.
The weight of earthworms fell from about 9,000 kilograms per hectare under pasture to some 200 kilograms per hectare after more than nine years’ arable cropping. The potential benefits of breaking down organic matter and improving soil structure are also severely reduced.
In the early 1950s, at the Ruakura Animal Research Station in Hamilton, pasture growth was measured on several soil types with and without lumbricid earthworms. The study showed increases of 30–110% in grass growth in response to earthworm activity.
Similarly, in the Otago upland soils, introduced Aporrectodea caliginosa earthworms improved pasture production by 29% after 16 years.
While earthworms are generally seen as beneficial, people playing golf or rugby may not appreciate their casts – mounds of nutrient-rich soil. Worms in sports turf are sometimes killed, the soil is scarified, and plant debris is removed to improve drainage and discourage worms.
As earthworms feed on organic material it is not surprising that they thrive in compost heaps – especially introduced species such as Eisenia fetida and E. andrei. Compost worms, like those in litter and dung, are very efficient at breaking down organic matter. But they are not burrowers, and do not directly contribute to improving soil structure.
The typically damp, organic-rich conditions in compost heaps contrast strongly with those in soil, and different earthworms occupy the two habitats. But as compost normally sits on soil, both types of earthworm may occupy this zone. The larger, soil-dwelling worms are not efficient at breaking down compost, so they should not be put into compost heaps.
Lee, K. E. The earthworm fauna of New Zealand. Department of Scientific and Industrial Research, Bulletin 130. Wellington: R. E. Owen, Government Printer, 1959.
Glasby, C. J., and others. 'Phylum Annelida: bristleworms, earthworms, leeches'. In The New Zealand inventory of biodiversity. Volume 1: Kingdom Animalia: Radiata, Lophotrochozoa, and Deuterostomia, edited by D. P. Gordon. Christchurch: Canterbury University Press, 2007.