Interactions between earthworms and soil protozoa: A trophic component in the soil food web

Earthworms and protozoa are, in terms of biomass, the most important groups of soil fauna in beech forests on limestone in southern Lower Saxonia (Germany). To investigate the effect of high protozoan numbers on earthworm distribution, a multiple choice feeding experiment was set up in fumigated soil, reinoculated with different numbers of naked amoebae, protozoa commonly found in that soil. Distribution of Aporrectodea caliginosa (Savigny) after wk was correlated with numbers of amoebae in soil. Other experiments confirmed digestion of protozoa by earthworms. The weight gain of young A. caliginosa in soil with amoebae was twice that in soil without protozoa. Direct observations of fresh cast material confirmed that active protozoa were digested by A. caliginosa. Lower protozoan numbers in faeces of Octolasion lacteum (Orley) than in the surrounding soil were found by dilution series, indicating grazing on the active part of soil protozoa by earthworms. The experiments provide evidence that A. caliginosa is actively searching for places with high protozoan densities and that protozoa may play a significant role in earthworm nutrition. (C) 1997 Elsevier Science Ltd.