The influence of Hormogaster elisae (Oligochaeta, Hormogastridae) on the colonisation of defaunated soil by microarthropods in laboratory cultures

Summary Controversy surrounds the effect of earthworms on soil microarthropod abundance and distribution. To shed further light on the topic, the present work investigated the capacity of soil microarthropods to colonise defaunated soil in the presence and absence of earthworms. Microcosms composed of plastic boxes were prepared with a central cage containing two endogeic earthworms ( Hormogaster elisae ). In one experiment, the cage contained defaunated natural soil while that outside was non-defaunated. In a second experiment, the soil outside the cage was defaunated while that inside the cage was non-defaunated. All microcosms were kept at 13 °C with 20% soil–water content for 21 days before extracting microarthropods for enumeration by standard methods. In the first experiment, the majority of microarthropod groups were not able to colonise the defaunated soil containing earthworms, but did colonise it when earthworms were absent. In the second experiment, nearly all the microarthropod groups left the central cages containing earthworms, while in the controls without earthworms the majority stayed inside. The results indicate that different microarthropod taxa are affected unequally by the presence of the earthworms. Due to their greater mobility Tarsonemidae and sometimes Gamasida were able to colonise the defaunated soil even when earthworms were present. In contrast, Oribatida, members of which disperse slowly and are very sensitive to soil perturbations, were generally unable to colonise the soil whether earthworms were present or not. However, the presence of H. elisae had a negative effect on the numbers of most groups of microarthropods and on their ability to colonise new environments. Physico-chemical perturbations produced by endogeic earthworms acting as ecosystem engineers, competition for organic matter and passive predation are discussed as possible causes of this negative effect.