stability of the web. 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 webs 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. Stability of the system is reduced by the use of nitrogen-containing inorganic and organic fertilizers, which cause soil acidification. Interactions not included in food webs 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