Dispersal and Distribution
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.