Distribution of Heteromurus nitidus (Hexapoda, Collembola) according to soil acidity: interactions with earthworms and predator pressure

Culture (8 weeks, in sieved fresh humus) and choice (16 weeks in compartmented boxes containing fresh or defaunated humus, or 5 days on compacted humus) experiments at varying pH values demonstrated that the soil-dwelling Collembolan Heteromurus nitidus (Entomobryomorpha) can live and even prefer humus with pH<5.0, contrary to results of field studies. Choice experiments on moder (pH 7.8) and calcic mull (pH 3.9) showed that H. nitidus was significantly attracted by the earthworms Allolobophora chlorotica and Aporrectodea giardi whatever the humus form, except when moder was present on both sides. This attraction by earthworms may partly explain the field distribution of H. nitidus. A strong predator pressure was detected in some of the replicates, which seemed to have an effect on densities and distribution of H. nitidus, as well. Causes of the attraction by earthworms (food resources, pore size, moisture) are discussed. A trophic cause is particularly suspected.