Tuatara: Volume 4, Issue 1, July 1951
The Role of Earthworms in New Zealand Soil
The Role of Earthworms in New Zealand Soil
The study of the role of earthworms in New Zealand soils is a subject which has received little attention from biologists. The great number of earthworms frequently found in the soil is sufficient to indicate that their activities must have a marked influence on the soil in which they live. However, in New Zealand there has been a great lack of exploratory work in many groups of invertebrate animals and earthworms are among these groups. It is essential, before any useful work on the ecology of an animal group can be done, to determine the exact nature of that group; in other words to classify and describe the species present. So a study of earthworms in New Zealand soils falls into two distinct parts:—
Classification of the fauna.
Ecological observations in the field and experiments to investigate the activities of earthworms in the soil and the effect of their activities on the soil.
Classification of Fauna
The Introduced Species form a small group of nine species belonging to four genera of the family Lumbricidae. The group is of European origin and must have entered the country in soil brought in on the roots of plants, and in the soil commonly used as ships' ballast in the early days of New Zealand's colonisation. Although the group is small it is of great importance, since the earthworm fauna of pasture and cultivated lands consists entirely of these few species.
The Endemic Species make up a very large group, including more than 120 species, belonging to 23 genera of the family Megascolecidae.
The origin of the endemic earthworm fauna of New Zealand has been the subject of much discussion in the past and raises some very interesting zoo-geographical problems. The species comprising the endemic fauna fall into three sub-families of the family Megascolecidae; the Acanthodrilinae, which occur also in South America, Australia, South Africa and the subantarctic islands as the dominant element of the fauna, and sporadically in other parts of the world; the Megascolecinae, which appear to have originated in the Indo-Malayan region and occur also in Australia and the Pacific Islands; and the Octochaetinae, which occur only in India, page 23 Madagascar and New Zealand. The dispersal of earthworm groups is complicated by their inability to survive immersion in sea water for more than a few minutes and their consequent dependence upon continuous land bridges as a means of dispersal. The distribution of the sub-family Acanthodrilinae was taken by two biologists, Benham and Beddard, as evidence of the former connection of New Zealand and the Southern continents to an antarctic continent, at some time when the antarctic continent was subject to a temperate climate. Another biologist, Michaelsen, claimed that it was more likely that the Acanthodrilinae had formerly been more widespread in northern regions and had spread southwards into the southern continents. More recently evolved earthworm groups, also evolved in the north and spreading southward, would finally have eliminated the Acanthodrilinae in all but the most southern continents, where they now find refuge. In New Zealand it has been found that the Acanthodrilinae occur throughout the country, as the dominant element of the fauna in the South Island and the southern regions of the North Island, and as a minor element of the fauna in the northern regions of the North Island. The more recently evolved sub-family Megascolecinae forms the dominant element of the fauna in the northern regions of the North Island and is virtually non-existent south of a line running approximately through Opotiki, Taupo and out to the west coast at Awakino. It seems from the present distribution of these two groups that there must have been two distinct invasions of earthworms into the New Zealand region — the first an invasion of Acanthodrilines and the second, probably a very long time later, an invasion of Megascolecines. It seems obvious that the latter group must have come from the north since they occur only in the north of the North Island, and so far as the former group is concerned there is no reasonable evidence that it did not come from the same direction. The fact that the Megascolecinae have already reduced the Acanthodrilinae to a minor element of the fauna in the north of the North Island indicates that, given sufficient time, they could destroy all trace of the latter group, and probably have done so, in the Indo-Malayan region from which they apparently came. I would say that the Megascolecinae have arrived in New Zealand during Tertiary times and that their present area of distribution within the North Island was determined by the following two factors:—
The distribution of land between the two main islands, with the sea at times during during the Tertiary covering much of the central North Island, would have prevented the Megascolecinae from penetrating into the southern regions of New Zealand.
The volcanic activity in the central North Island and Taranaki districts probably destroyed all forms of soil animal life in the area over which the great ash showers fell during the Pliocene. When the North and South Islands had acquired approximately their present form there would still have remained an area, uninhabited by earthworms and unfavourable for their activities, separating the page 24 Megascolecinae in the north from the Acanthodrilinae in the south. The ‘ash shower’ country is still largely uninhabited by endemic earthworms. Those few that are found have penetrated into the the region from the surrounding unaffected country and belong to species known from outside the ‘ash shower’ country.
Ever since Charles Darwin, in 1881, published ‘The formation of Vegetable Mould through the action of Worms with Observations on their Habits’, biologists have taken a renewed interest in the effect of earthworms on the soil. The first recorded observations in New Zealand were made near Auckland by A. T. Urquhart for several years from 1882 onwards. Unfortunately Urquhart was unable to give reliable identifications of the species responsible for the phenomena he observed, but his observations are worthy of mention. He estimated the earthworm population in pastures near Auckland as about 784,080 per acre (18 per sq. ft.). His estimate is very much higher than Darwin's (26,886 per acre in English pastures) and that of the German biologist von Hensen (53,767 per acre in garden soil and about half that number in cornfields), but the species found by Urquhart were much smaller than those found by Darwin and von Hensen. (Urquhart states that in pasture lands there were approximately 612 Ib. of earthworms per acre in comparison with von Hensen's figure of 356 Ib. per acre in garden soil). On a section at Manakau, Urquhart placed stones on the surface of the ground and in one case noted that after eight years a stone 6½ in. x 3½ in. x 3¼ in. was embedded to a depth of an inch by the castings of earthworms and by its weight breaking in the tunnels of earthworms beneath it. For comparison with the figures given by Urquhart, Evans (1948) records that in a field under arable cultivation for more than 100 years, receiving 14 tons per acre per annum of farmyard manure there were 450 Ib. per acre of earthworms, while in a field with the same yield of crops receiving inorganic fertilisers there were 110 Ib. per acre of earthworms.
Consideration of the numbers of earthworms occuring in the soil under varying conditions raises the question of what are the soil conditions most suited to the maintenance of earthworm populations. The essential requirements seem to be as follows:—
(i) Adequate food supplies. The food of earthworms consists mainly of leaves, seeds and other portions of plants, and organic particles, living or dead, in the soil passed through their intestines. Under normal forest or pasture conditions food supplies are constantly being renewed and shortage cannot be a great problem.
(ii) Suitable levels of soil moisture. The physiological processes of earthworms are similar to those of fresh water animals and adequate moisture levels are consequently of the utmost importance. In the soils of New Zealand forests moisture levels seem never to fall low enough to page 25 produce any marked effects on earthworm populations, but in the soils of pasture lands, which are in many cases subject to extreme dessication in the summer, earthworms are forced to retreat deep into the subsoil and remain there in a dormant state until the return of more favourable conditions in the topsoil. A special case of the effect of seasonal dessication is seen in saline soils such as are found in the Ahuriri lagoon, where the salinity of the groundwater prevents earthworms from retreating deep into the subsoil. In such cases all the adult earthworms die out during dry periods leaving cocoons in the topsoil to hatch in the autumn rains. The saturation of the soil by very heavy rains markedly affects the earthworm fauna of pasture and croplands. After heavy rains large numbers of dead earthworms may be seen on the soil surface, often in puddles. They are forced to come to the surface by the rapid exhaustion of oxygen in the water filling their burrows, and once at the surface they are rapidly killed by ultra-violet light from the sun's rays.
(iii) Suitable soil temperatures. Earthworms are unable to survive sudden extreme variations in soil temperature. In this respect, high temperatures are unlikely to cause any great mortality, since marked increases in soil temperature are seasonal rather than diurnal and, as previously stated, earthworms in many parts of New Zealand retreat into the subsoil and go into a state of dormancy during the summer months. However, sudden extremes of cold are very common in the soils of inland districts subject to frosts and many earthworms die in the first frosts of winter. Those that survive the first frosts seem to develop a greater resistance to low temperatures and will survive the winter. The common introduced species of New Zealand seem to possess varying degrees of toleration for low temperatures. This fact probably accounts for the varying proportions of the various species found in some districts of the North Island.
(iv) Suitable soil texture. This is a soil factor which rarely affects the earthworm fauna but in certain cases it has been noted as important. Frobably the most striking case is demonstrated in soils derived from Taupo ash. In these soils earthworms are usually rare and often absent, except where the land has been in pasture for some years, probably because of the coarse nature of the pumice particles, the low clay fraction of soils derived from pumice and the low content of organic matter in the soil. Where the Taupo ash is well supplied with organic matter and has been covered by even a thin layer of Rotomahana mud earthworms are more abundant. Other cases of low earthworm populations associated with unsuitable physical soil conditions have been noted in Te Kopuru sand and in Taumata clay loam (a very compact soil) in North Auckland.
(v) Suitable pH conditions. In some cases where the soil pH is very low no earthworms are found, e.g. Otonga peaty loam (deep phase), a soil with a pH of 3.6. The common introduced species seem to be able to page 26 thrive in soils of pH ranging from 4 to 8 and have rarely to deal with soils of a pH outside that range in New Zealand.
Within very wide limits the influence of soil conditions on earthworms is relatively slight. Earthworms seem to possess to a very high degree the ability to adapt themselves to their immediate environment. In contrast to this the effect of the activities of earthworms on soil conditions is of great importance. The burrows of earthworms admit of soil aeration, materially aid drainage, facilitate the downward movement of roots and produce a certain amount of melanisation (‘worm mottling’) through topsoil and humus being washed downward into deserted burrows. The phenomenal improvement in hill country pastures claimed by two farmers in Taranaki to have been due to the castings of a ‘unique’ species of ‘casting worm’ is probably due almost entirely to the effect of common introduced earthworm species on the structure of soils in which they were not previously present.
The value of the castings of earthworms, especially in pasture soils, has been the subject of much work and discussion. Casting, in combination with other earthworm activities, plays an important part in the melanisation of the soil and the build-up of humified layers. An experiment carried out by von Hensen many years ago will serve to illustrate this point. Von Hensen took a vessel 18 in. in diameter, filled it with sand and put a layer of leaves on the top. He then introduced two earthworms into the vessel. Leaves were soon dragged down into the sand to a depth of up to six inches and after six weeks there was 0.4 in. of humus formed in an even layer over the surface of the sand. The amount of soil cast by earthworms at the surface in pasture lands has been estimated many times. Darwin (1881) gives a figure of 7.56-18.12 tons per acre per year, giving a total depth of one inch in 10 years. Evans (1948) states that in a 300-year-old pasture 25 tons per acre per year is cast by earthworms at the soil surface. In forest soils the presence of earthworms accelerates the formation of mull, since by dragging leaves from the surface into their burrows they deepen the zone of humification.
The general suitability to earthworms of overall environmental conditions in New Zealand has permitted specialisation of endemic species so as to take advantage of a great variety of soil conditions. They are found in the soils of forests, scrubland, mountain tops and plains and in many other ecological niches including swamps, rotten logs and under the bark of trees at heights of up to 100 feet above the ground. Certain species are confined entirely to the topsoil while others are confined to the subsoil. The great specialisation of most species to suit well defined environmental conditions makes their destruction inevitable as soon as their environment is changed by the conversion of virgin lands into pasture. The only species which survive such changes are those that live in the subsoil and are only slightly affected by a change to pasture conditions and a few, apparently little specialised species, that seem able to adapt themselves to the change page 27 in their environment and then to maintain themselves in the face of strong competition from introduced species. When native vegetation is removed and pasture is planted the first introduced earthworm species to appear in the soil is usually Octolasium cyaneum, a large sluggish species which spreads fairly rapidly and becomes dominant, sometimes with a few survivors of the endemic fauna as minor elements of the fauna. O. cyaneum is followed fairly rapidly by smaller and more active species of the genera Lumbricus and Allolobophora. The common introduced species appear to have a digestive system capable of coping with a wide variety of plant residues, but find their optimum conditions in pasture soils. By their great rate of reproduction, immense activity and consequent heavy drain on the food supplies the common species of Lumbricus and Allolobophora soon gain undisputed possession of the soils of pasture lands. It is an interesting fact that when pasture land reverts into scrub or forest the whole process is reversed and Octolasium cyaneum, or sometimes an endemic species, again becomes dominant.
Darwin, C. R. (1881) —‘The formation of Vegetable Mould through the Action of Worms with Observations on their Habits’. London.
Evans, A. C. (1948) —‘The Importance of Earthworms’. Farming 2, 1948: 59-60.
Urquhart, A. T. (1883) —‘On the Habits of Earthworms in New Zealand.’ Trans. N.Z. Inst., xvi: 266-275.