Inter-And Intraspecies Relationship Relationships in Earthworms

Alexei V. Uvarov¹, Alexei V. Tiunov¹, Stefan Scheu²

¹Institute of Ecology and Evolution; Leninsky prospect 33, 119071 Moscow, Russia;  ²Institute of Zoology, Technische Universität Darmstadt, Schnittspahnstrasse 3, 64287 Darmstadt, Germany

The role of earthworms as keystone animal group regulating activities of soil microorganisms, plant growth and interactions within soil communities has been extensively investigated. Less is known about the interrelationships among earthworms themselves. Earthworm associations are ecologically diverse and comprise the major part of invertebrate biomass in soil; hence, interactions within the associations must have a profound significance for functioning of soil systems. Summarising a very restricted number of studies all belonging to the latest decade (by Butt, Lowe, Capowiez, Baker, Uvarov, Scheu and some others), several conclusions can be made on variation of interactions between earthworms: (a) both intra- and interspecies relationships were documented; (b) both types of relationships were shown for representatives of any ecological group (epigeic, anecique, endogeic); (c) interrelationships between the species belonging to different ecological groups were documented;  (d) various types of interactions are common, i.e. positive, negative, visibly neutral, one-sided or mutual; within species populations, (e) density-dependent effects on ecophysiological processes and activities of earthworms (respiration, reproduction and growth, feeding and burrowing activity, mortality) were found; (f) interactions between adult and juvenile earthworms were revealed.

However, little is known about mechanisms regulating the interrelationships in earthworms. E.g., negative relationships were often explained in terms of competition for space and/or resources (or even by cocoon predation) though it was rarely shown directly, and the data are contradictory. Lowe & Butt (2002) related the intensity of interactions to the degree of species niche overlap but also stated that the outcome of interactions could not be predetermined simply by reference to ecological groups of the earthworms. In general, interspecies interactions often seem to be situation-specific. In some cases, their character is affected by the environmental factors, such as soil compaction, temperature regime or winter conditions. Within species populations, at higher densities reproduction and growth are generally reduced and mortality increased.

As an example, complex relationships between two epigeic species regularly occurring together in two- or multispecies earthworm natural associations (litter-dwelling Dendrobaena octaedra [Savigny] and litter/topsoil dweller Lumbricus rubellus Hoffmeister), are analysed based on the authors' data. Under long-term joint maintenance L.r. tended to outcompete D.o.: offspring production, density and biomass in D.o. populations were significantly decreased. In contrast, L.r. rather gained in the presence of D.o.; however, no predation of L.r. on cocoons of D.o. was documented. In summer reproduction rates of D.o. were higher at constant than diurnally fluctuating temperature regime, at lower population density and higher litter supply. In winter populations of L.r. deadly suffered from occasional frosts tolerated by D.o., which also had higher recovery potential due to parthenogenesis. These characteristics contribute to the undestanding of mechanisms regulating coexistence in these earthworm species.