The action of an animal ecosystem engineer: Identification of the main mechanisms of earthworm impacts on soil microarthropods

Abstract

Non-trophic interactions are shaping soil food web structure and functions. Particularly, the action of ecosystem engineers, such as earthworms, are likely to fundamentally impact the abiotic and biotic properties of their environment. The present study aimed to identify the main mechanisms through which earthworms belonging to varying ecological groups – epigeic, endogeic and anecic species – affect soil microarthropods by reviewing the literature on this topic and by performing meta-analyses. Earthworm ecological groups differed considerably in their impacts on microarthropods, whereas effects did not vary significantly between microarthropod taxa at the habitat scale. Inconsistent impacts of epigeic species on soil microarthropods are most likely due to differences in earthworm densities. Effects can thus be positive in the case of moderate densities or negative in the case of high densities and associated distinct changes in the physical structure of the upper soil organic layers. By contrast, impacts of endogeic earthworms appeared to be mainly negative and were primarily due to competition with microarthropods for food resources. Consequently, negative impacts on soil microarthropods intensified with increasing earthworm density and biomass. This interaction between endogeic earthworms and microarthropods is better referred to as amensalism due to the competitive predominance of earthworms. Impacts of anecic earthworm species differed significantly from that of endogeic ones; they were neutral at the habitat scale and positive on the microhabitat scale. Moreover, impacts were independent of earthworm densities due to the quasi-territorial behaviour of anecic earthworms. Positive effects were mainly attributed to the formation of stable microhabitats by anecic species; namely burrows/middens, rich in nutrients and microorganisms. The present study points to the relevance of the non-trophic biotic interactions that drive the composition of belowground food webs by identifying the most essential mechanisms underlying the impacts of animal ecosystem engineers on soil microarthropods. Moreover, as earthworms emerge as important biological invaders, the results of the present study may help to fully appreciate, estimate and model the consequences of this momentous global change phenomenon. Particularly, the spread of exotic epigeic and endogeic earthworm species likely threatens soil microarthropod density, diversity and functions.