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.