Effects of Intra- and Interspecific Interactions in Earthworm Assemblages: A Comparative Study

Among the factors modifying the structure of associations and activity of earthworms, the influence of intra- and interspecific interactions has been studied insufficiently. In field experiments the relative importance of these interactions has been compared in lumbricid species that are trophic competitors: in the soil detritophages Allolobophora chlorotica and Aporrectodea caliginosa and in the leaf litter consumers Lumbricusrubellus and L. terrestris. For each species the relative significance of the intra- and interspecific interactions were evaluated by comparison of the state of lumbricid individuals in conditions of low (control) and high population densities in one- or two-species populations. The increase in population density (due to either conspecifics or species-competitor) usually negatively affected the important parameters of the current earthworm generation (growth, mortality, and fecundity), and also of the subsequent generation (numbers and size of the cocoons produced). For three species (except for L. terrestris), trends were revealed indicating the greater strength of intraspecific than interspecific interactions. The species reactions were determined by different factors: by the trophic competition for the litter-dwelling L. rubellus, whereas for the soil dwellers Al. chlorotica and A. caliginosa (in conditions of a surplus of food resources) they were presumably determined by competition for the space.