Stability of food webs
Mathematical modeling in food webs has raised the question of whether complex or simple food webs are more stable. Until the last decade, it was believed that soil food webs were relatively simple, with low degrees of connectance and omnivory. These ideas stemmed from the mathematical models of May which predicted that complexity destabilized food webs. May used community matrices in which species were randomly linked with random interaction strength to show that local stability decreases with complexity (measured as connectance), diversity, and average interaction strength among species.
The use of such random community matrices attracted much criticism. In other areas of ecology, it was realized that the food webs used to make these models were grossly oversimplified and did not represent the complexity of real ecosystems. It also became clear that soil food webs did not conform to these predictions. Soil ecologists discovered that omnivory in food webs was common, and that food chains could be long and complex and still remain resistant to disturbance by drying, freezing, and fumigation.
But why are complex food web more stable? Many of the barriers to top-down trophic cascades also promote stability. Complex food webs may be more stable if the interaction strengths are weak and soil food webs appear to consist of many weak interactions and a few strong ones. Donor controlled food webs may be inherently more stable, because it is difficult for primary consumers to overtax their resources. The structure of the soil also acts as a buffer, separating organisms and preventing strong interactions. Many soil organisms, for example bacteria, can remain dormant through difficult times and reproduce quickly once conditions improve, making them resilient to disturbance.
Despite their complexity, some interactions between species in the soil are not easily classified by food webs. Litter transformers, mutualists, and ecosystem engineers all have strong impacts on their communities that cannot be characterized as either top-down or bottom-up.
Litter transformers, such as isopods, consume dead plants and excrete fecal pellets. While on the surface this may not seem impressive, the fecal pellets are moister and higher in nutrients than the surrounding soil, which favors colonization by bacteria and fungi. Decomposition of the fecal pellet by the microbes increases 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. 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.