POPULATION-DYNAMICS OF THE EARTHWORM APORRECTODEA-TRAPEZOIDES (ANNELIDA, LUMBRICIDAE) IN A WESTERN-AUSTRALIAN PASTURE SOIL

The changes in size and age-composition of an earthworm population were studied in a Western Australian pasture developed since 1912. The population size in the surface 10 cm was estimated by handsorting during the cool wet season (19 weeks from May to September). Two species, both exotic, were found, Aporrectodea trapezoides (Lumbricidae) and Microscolex dubius (Acanthodrilidae). Of the 615 individual specimens collected, 99.7% were A. trapezoides. The abundance of A. trapezoides increased from 58 m-2 at the time of the opening rains to 170 m-2 (88.6 g live weight m-2) after 10.5 weeks. Near the end of the wet season (in October) the density was 37 m-2. At the time of the opening rains the population consisted of juvenile and semimature individuals. Clitellate earthworms were found 1 month later and predominated from August (10.5 weeks) to the end of the season. Egg capsules were found from August through October. Egg capsules incubated in the laboratory at 16-degrees-C hatched on average after 42 days and produced two juveniles. Juvenile and immature earthworms collected from a quiescent state at the end of summer matured within 1 month when reared in moist soil in the laboratory.