The ecological significance of the central North Island ash showers: The distribution of animals

[First paragraph(s)...]
In the preparation of this paper an attempt has been made to gather together the opinions of as many zoologists as possible. Knowledge of most of the animals of the ash-shower country is, however, confined to observations on discontinuous areas, and the paper will serve mainly to show us how little is known.
Soil animals are greatly affected by the environmental conditions encountered in ash-shower country. This may be illustrated by studies of earthworm distribution. In the north of the North Island the earthworm fauna consists very largely of species of the sub-family Megascolecinae. In the South Island and the southern part of the North Island most of the species belong to the sub-family Acanthodrilinae. The southern boundary of the range of predominantly megascolecine species coincides almost exactly with the northern boundary of the ash-shower country
The boundary of the range of the predominantly acanthodriline species also lies within the ash. shower country. Among the reasons for this pattern of distribution of earthworms are the following:
(1) Ash-shower soils are not generally a favourable environment for earthworms, due to their seasonal aridity and frequently to their coarse physical structure.
(2) A few species of earthworms have entered the ash-shower country from surrounding unaffected areas, and have become established in ash-shower soils.
(3) There has been insufficient time for local specialisation or subspeciation since the last great Taupo shower.