Earthworm Role in Soil Food Webs

Within the food web, earthworms are typically regarded as organisms in a mid-level trophic category. However, earthworms’ trophic interaction with other organisms is much more complex. As geophagous animals they consume a whole range of organisms in soil aggregates and thus they are sometimes called ecosystemvores. Not all earthworms are the same and their nutrition depends much on their ecological group. Earthworms that live deep in the soil (endogeics) consume older carbon than those that live at the surface (epigeic) earthworms and the organisms that they ingest are also different. In addition, their indirect effects on the food web are also substantial. It is well known that the drilosphere, soil modified by earthworms, consists of microsites where microbivores graze on bacteria and fungi. The microsites are high in organic matter that was either translocated there by the earthworms or is based in earthworm frass. Most earthworms are infected by gregarine parasites that feed on sperm morulae and thus create an “internal” part of the food web. There are many earthworm predators that range from invertebrates, such as flatworms, to snakes and birds. The earthworm’s place in the food web is complex and both its indirect effects as an ecosystem engineer and its direct trophic interactions need to be considered when appreciating the ecology of earthworms.