The term “earthworm” cannot be satisfactorily defined in the scientific sense but it serves to describe a large number of species of the order Oligochaeta which inhabit soils and accumulations of decaying plant materials, and are occasionally found in shore and aquatic habitats. The earthworms of New Zealand consist of two groups of species. A large group of 173 native and five introduced species (27 genera) belong to the family Megascolecidae and a smaller group of 14 introduced species (seven genera) belong to the family Lumbricidae.
The external features of the native earthworms are illustrated in the following diagram. Their size is extremely varied; the largest, Spenceriella gigantea from North Auckland, attains a length of 4 ft 6 in. and a diameter approaching ½ in., while a number of the smaller species are from ½ in. to 1 in. long and less than 1/10 in. in diameter. Most of the native species are red or brown. The red colour is sometimes due entirely to the haemoglobin of the blood showing through the epidermis, but red, purple, or brown pigments are frequently present on the dorsal surface, the ventral surface being usually unpigmented or very lightly so. Certain species, especially those that inhabit forest leaf mould, are deeply pigmented, some with striking patterns of contrasting colours, and these species are usually pigmented both dorsally and ventrally.
The body of the earthworm is divided into segments which are separated externally by intersegmental furrows that correspond to internal septae. Minute chaetae, or bristles, used in locomotion, occur on each segment except the first, each segment having 8, 10, 12, or more. Commencing on or in front of the fifteenth segment is the clitellum, a thick glandular portion of the epidermis, developed over a number of segments on sexually mature worms in the region of the male and female pores. Earthworms are hermaphroditic, both male and female organs being present, but they are not self-fertilising and, when mating takes place, sperm cells are exchanged. During oviposition the clitellum secretes a gelatinous sleeve-like structure, worked forward over the body, which receives ova as it passes the female pores and sperm cells as it passes the spermathecal pores. As the “sleeve” passes off the anterior end of the body, its ends are sealed and it becomes a cocoon.
Native earthworms feed almost entirely on dead and decaying remains of plants and, because of their limited capacity to move about, they are obliged to live very close to their sources of food. The presence of free water is essential for they have virtually no mechanism for conserving moisture. Respiration takes place by diffusion of gases through the moist body wall; hence both moisture and dissolved oxygen are essential. Earthworms are injured and may die by exposure to daylight, except when the intensity is very low, the more pigmented species being more resistant to light damage than the less pigmented. They are killed by temperatures in excess of 85°F–100F, but in most New Zealand habitats they escape the effects of extreme high or low temperatures by retreating to lower layers.
The pH tolerance varies from species to species but no native earthworms have been found in soils lower than pH4. Most earthworms are able to tolerate submersion in water and there are a few species that prefer an aquatic life. During heavy rains, however, earthworms are commonly driven to the surface, but this is most probably due to the shortage of oxygen in the water in their burrows.
Despite the limitations imposed by their environmental requirements, earthworms occupy a wide range of habitat but, as their name implies, have achieved their greatest success as soil-dwelling animals. The prolonged geographical isolation of New Zealand and the predominance of forest vegetation have resulted in the development of a forest-earthworm-fauna with species highly specialised to occupy particular ecological niches in particular types of forest, and there is an obvious stratification into leaf-mould, topsoil, and subsoil groups. The leaf-mould species are small, active, and heavily pigmented. The smallest is 15mm long and the largest 180 mm but most are between 20 mm and 50 mm. They do not make permanent burrows but move around freely in the loose material just as arthropods and other animals do; hence they are more prone to capture by predatory birds and are more frequently exposed to ultraviolet light than those species inhabiting topsoil or subsoil. Their ability to move quickly gives some protection from predators and their heavy and varicoloured pigmentation affords some protection from ultraviolet light and provides a degree of camouflage. Thirty-six species are found almost exclusively in leaf mould; most are confined to relatively small areas but at least four species (Diporochaeta obtusa, D. punctata, Neochaeta forsteri, Plagiochaeta sylvestris) are widely distributed.
Both the topsoil and the subsoil dwellers have two distinct methods of making burrows in which to live. In the first method soil is swallowed and subsequently cast either at the soil surface or in natural cracks and cavities in the soil and in deserted burrows. In the second method the anterior end of the body is extended and inserted in spaces between the soil particles and then, by contracting the longitudinal muscles, the body is expanded laterally, compressing the soil to form a burrow. Usually burrowing consists of a combination of these two methods, the former predominating in more-compact soils and the latter in less-compact soils. As a burrow is formed it is lined with slime and thus has smooth walls firmly compacted by the lateral pressure applied during its construction.
The topsoil species are generally larger than leafmould species, the smallest is about 25 mm and the largest 300 mm but most are between 75 mm and 200 mm. They are not as heavily pigmented, nor do they move as rapidly as the leaf-mould species. In most species the body is circular in cross-section but in some (especially in Maoridrilus spp. and Neodrilus spp.) the body is almost square in cross-section with pairs of chaetae on the four corners and with thick body-wall muscle layers. Such species are generally the most active. Topsoil earthworms generally make shallow permanent burrows which they leave either at mating time or in order to forage for the food which they take back into their burrows. Most species appear to continue burrowing outside the confines of their living space and apparently live, to some extent at least, by ingesting soil and digesting the organic matter contained in it. Forty-eight native species are found almost exclusively in topsoil. A number of them found in tussock grassland areas (species of Rhodo-drilus in central North Island, and Maoridrilus in eastern South Island) are widely distributed, but most other topsoil species are confined to small areas.
Subsoil earthworms are usually large, sluggish, and unpigmented. The smallest is 32·5 mm and the largest 1,400 mm, but most are between 100 mm and 400 mm in length. The majority are circular in cross-section and have weakly developed body-wall muscles. They occasionally come to the surface or near to the surface for food, but otherwise are found only in the subsoil. They make very extensive burrows extending both laterally and vertically in the subsoil and occasionally going up into the topsoil. (Burrows of Spenceriella gigantea have been found about 20 mm in diameter and still continuing downwards at a depth of 11 ft 6 in.) They appear to make these burrows to obtain food by ingestion of soil and not primarily for shelter, like the burrows of the topsoil species. As they move forward they may deposit castings in the section of burrow left behind and it is not uncommon to find burrows partly filled with subsoil castings.
Earthworms are frequently found under logs and stones, under the bark of dead trees, in rotting logs, and in the litter of epiphytes in the axils of branches of trees. Most of these are species normally found in leaf-mould or topsoil and, more rarely, in subsoils, but one, Megascolides suteri, is found almost exclusively in rotting logs and is able to digest decaying fragments of wood. Ten species recorded from the Auckland Islands have all been found under logs and stones. Most of the ground is covered with acid peat (pH3·6–4·6) which is an undesirable habitat for these particular species.
The aquatic “earthworms” have apparently been derived from terrestrial ancestors and have secondarily adopted aquatic habits. They should not be confused with a number of families of small Oligochaeta collectively known as the “microdrili” and which are primarily aquatic animals.
Two native species, Diporochaeta aquatica and Pontodrilus lacustrus, have been collected from deep water, the former in Lake Manapouri and the latter in Lake Wakatipu. They apparently swim freely in the lake waters and feed on organic matter suspended or sinking through the water. The apparent absence of similar aquatic worms from other New Zealand lakes may be due only to lack of suitable sampling.
Four species, Decachaetus violaceus, Diporochaeta chatamensis, Eodrilus paludosus, and Perionyx helophilus, have been collected from swamps and bogs. The first two have been recorded only once, whilst the remaining two have been collected from various habitats including swamps. It cannot therefore be said that any native species are undoubtedly confined to swamps. The anaerobic conditions typical of subsurface horizons of swamps and bogs would prevent earthworms from living anywhere but very close to the surface.
Much work on the distribution and manner of dispersal of earthworms has been based on the assumption that they are unable to survive immersion in salt water, even for a short period, and consequently cannot have been transported across the sea by rafting. A number of native earthworms, however, live in the intertidal zone of seashores under stones and debris or in brackish water. Of seven species collected from such habitats, five (Microscolex aucklandicus, M. campbellianus, M. macquariensis, Rhododrilus cockaynii, and R. leptomerus) are found under stones and logs and in the soil under tussocks, forest, or other vegetation, as well as in the intertidal zone. They are therefore well fitted for the colonisation of the isolated sub-Antarctic islands on which they are found as the dominant section of the earth-worm fauna. Pontodrilus matsushimensis var. chathamianus, a Chatham Island seashore worm, is a local variant of a species widely distributed on the shores of Pacific islands.
The ability of earthworms to spread over large continuous areas is restricted by the specialised habitat preferences of species. Slow spread by their own efforts must have been mainly responsible for their dispersal from the original points of entry into the New Zealand mainland. In the process of spreading, closely defined habitat preferences have become established and consequent specialisation has been responsible for the evolving of new species. Litter-dwelling earthworms and topsoil earthworms that emerge from their burrows at night and move freely about the surface, spread rapidly throughout areas with suitable environmental conditions. The larger subsoil worms would necessarily spread more slowly, yet one such species, Octochaetus multiporus, which can tolerate a wide range of environmental conditions, is found from Manawatu Gorge south-ward and is common in all South Island districts east of the Main Divide and in Stewart Island. Much of this area would have been unsuitable for earthworms in comparatively recent glacial times. Transport by birds and floating organic debris are means whereby dispersal has been assisted and, since most earth-worms can live for a time in fresh water, dispersal is expedited within a river system.
Two subfamilies of the family Megascolecidae are recognised in the native earthworm fauna, the Acanthodrilinae and the Megascolecinae, and these have dissimilar distribution patterns. The Acantho-drilinae are distributed throughout New Zealand while the Megascolecinae are confined almost entirely to northern and western districts. The most prominent break in the distribution pattern is south of Auckland where a northern fauna dominated by megascolecine genera (in particular the genera Megascolides) gives way to a fauna dominated by acanthodriline genera. During early Pleistocene times the ignimbrite eruptions which covered the central North Island presumably destroyed the vegetation and associated soil fauna, and successive volcanic outbursts have helped to maintain a barrier between the Auckland area and those further south. In the central North Island area there has developed a fauna dominated by a few species, each of which is widely distributed within the area and is to be found in one or more adjacent areas.
From the evidence of known distribution it is most likely that the native earthworms came originally from the Indo-Malayan or Australian region and entered New Zealand across a land-bridge connection from the north. They probably arrived in at least two waves, the first (subfamily Acanthodrilinae) in Cretaceous times and the second (subfamily Megascolecinae) in Tertiary times.
The most common introduced earthworms belong to the family Lumbricidae and, since such a large part of New Zealand has been cleared of the original vegetation and sown down to pasture, the lumbricid earthworms which feed on dead root and leaf material from pasture have become the dominant earthworm fauna both in pasture and in cultivated lands. They have been introduced at so many places and are so well acclimatised that there is no discernible relationship between their present distribution and geographical barriers as is the case with the native earthworms. The composition and population density of the lumbricid fauna is related directly to the level of fertility of the soil as modified by top-dressing and the composition of the pasture.
After the clearing of the land, native earthworms decline rapidly. The leaf-mould fauna is eliminated since there is no supply of leaf-mould; the topsoil fauna is usually eliminated but occasionally persists in a much reduced form; the subsoil fauna may be relatively unaffected but, if the soil is continually cultivated, this, too, fails to survive. There is an interval during which there is no further change in the condition of the depleted native fauna or during which earthworms are completely absent. The duration of this interval is related to the level of fertility of the soil and the availability of a population of lumbricid earthworms adjacent to the cleared land.
In the pumice land south of Rotorua, samples of the earthworm fauna were taken at various stages of development. Clearing of theland caused the extinction of the native earthworm Rhododrilus similis, and in pastures up to three years old no worms were found. As the pasture developed further and the “humus build up” at the surface became deeper, lumbricid species appeared. The first to appear was Octolasium cyaneum, a large sluggish species commonly found in the topsoil of low-fertility pastures on many soil types, the population being of the order of 100,000 per acre. After four or five years in pasture, Lumbricus rubellis then appeared and increased in numbers and, as the fertility of the pasture was built up, Allobophora calignosa and A. terrestris became established and dominated the earthworm fauna. In highly fertile pastures, numbers were of the order of 1–2 million per acre.
Earthworms affect soil fertility in various ways. Their burrows provide drainage channels through the soil, improve its aeration, and assist deep root penetration. The lumbricid species of New Zealand pasture lands are all topsoil dwellers but in summer, if the surface soil becomes too dry, they retreat into the subsoil and go into diapause – a state of suspended animation. The vertical channels so made remain for a short time as subsoil drainage channels but, as their thin walls are not usually firmly compacted, they soon collapse.
Earthworms increase the efficiency of the organic cycle by hastening the decomposition of forest litter, and in pastures by speeding the release of plant nutrients from dead roots and pasture litter. Their casts are generally less acid and higher in nitrogen and available phosphorus and in exchangeable calcium, magnesium, and potash than is the case in undisturbed soil. The quantity of soil deposited at the surface in the form of worm casts was found (Evans 1948), on eight fields with different management histories, to range between 1 and 25 tons per acre per year. Calculations, based on total populations and taking account of species that cast beneath the surface, showed that from 4 to 36 tons of soil per acre per year passed through the alimentary systems of earthworms and were cast at or near the surface.
Three common lumbricids of New Zealand pasture, Allolobophora terrestris, A. calliginosa, and Lumbricus rubellus, are surface-casting species and together would soon cover a field with a thick spongy layer of casts were it not for such factors as the impact of raindrops, movement of surface water, and trampling by stock. Measurements on highly fertile farms south of Auckland have shown that, under grazing conditions, the build-up of compacted cast material amounts to slightly more than 1/10 in. per year.
These topsoil earthworms play an important part in our grassland farming. They stimulate pasture growth by removing dead root material, loosening up the sod, and providing an enriched layer of cast soil in which perennial grasses and clovers are able to re-root year by year. But all is not to the advantage of the farmer. In performing these beneficial tasks, earthworms inevitably play their part in reducing the bearing strength of the topsoil by weakening the sod and casting at the surface. Moreover, there follows the resulting increased poaching of the surface soil by stock during winter months.
by Norman Hargrave Taylor, O.B.E., formerly Director, Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt.