its nutrient value and the isopod is able to re-ingest the pellets. When the isopods consume nutrient-poor litter, the microbes enrich it for them and isopods prevented from eating their own feces can die. This mutualistic relationship has been called an “external rumen”, similar to the mutualistic relationship between bacteria and cows. While the bacterial symbionts of cows live inside the rumen of their stomach, isopods depend on microbes outside their body. Ecosystems engineers, such as earthworms, modify their environment and create habitat for other smaller organisms. Earthworms also stimulate microbial activity by increasing soil aeration and moisture, and transporting litter into the ground where it becomes available to other soil fauna. Fungi create nutritional niche for other organisms by enriching nutritionally extremely scarce food - the dead wood. This allows xylophages to develop and in turn affect dead wood, contributing to wood decomposition and nutrient cycling in the forest floor. In aboveground and aquatic food webs, the literature assumes that the most important interactions are competition and predation. While soil food webs fit these sorts of interactions well, future research needs to include more complex interactions such as mutualisms and habitat modification. While they cannot characterize all interactions, soil food webs remain a useful tool for describing ecosystems. The interactions between species in the soil and their effect on decomposition continue to be well studied. Much remains unknown, however, about soil food webs stability and how food webs change over time. This knowledge is critical to understanding how food webs affect important qualities such as soil fertility. See also Soil biology Soil ecology Soil functions Soil life Trophic cascade Food web Entomopathogenic nematode References External links Soil Foodweb Laboratory in Canada Soil Foodweb Inc international network of soil analysis laboratories and education providers Soil biology