TERRESTRIAL AND FRESHWATER OLIGOCHAETES
The oligochaetes are an ecologically diverse group. Approximately 50% of oligochaete species are “earthworms” in the sense that they live in the soil but the term earthworm describes a number of species from differing families. Earthworms are found in all but desert soils and are ecologically and economically important as conditioners of soils and processors of organic material. It is accepted that the native British earthworm fauna would have been wiped out by glaciation and perma-frost during the Pleistocene period. It is interesting to consider whether some of that fauna would have survived in less affected corners of Cornwall and the Isles of Scilly whilst the ground froze over much of Britain. The present British earthworm fauna is believed to result largely from natural recolonisations from the continent after the glaciation and also from agricultural introductions. Aquatic species occur in freshwater, brackish and marine habitats and are generally smaller than terrestrial species often being found in enormous densities. It is likely that difficulties in identifying these small species have contributed to the low level of recording seen for the group. Consequently an accurate assessment of their status cannot be made but recent additions to the identification guides may remedy the situation by encouraging more recording work. Meantime no native species are considered scarce or rare. The distributions of most freshwater species are believed to be cosmopolitan (Brinkhurst, 1971) and several are known to be common but relatively little is known of the distribution and ecology of the less common species. Aquatic species include sludge worms, members of the family Tubificidae, which may be the only macroinvertebrates surviving in high levels of organic pollution. Several exotic earthworm species have been imported into Britain to botanic gardens - the following species is one of this group.