<P> 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 . </P> <P> Stability of the system is reduced by the use of nitrogen - containing inorganic and organic fertilizers, which cause soil acidification . </P> <P> 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 . </P> <P> 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 . </P>

Which of the following organisms is located only in the 3rd trophic level of the soil food web