How soil food webs make plants grow

Abstract

Although trophic cascades (in which feeding by top predators induces a chain of effects along the food chain that affects lower trophic levels) have been well documented in several aquatic systems15, the evidence for cascades in terrestrial food webs remains much more equivocal. If trophic cascades are important in soil food webs, the question emerges as to what effect this has on the ecosystem-level functions performed by the organisms in those lower trophic levels affected by the cascade. The study of Laakso and Setälä6provides clear evidence that lower trophic levels in soil food webs are much more important than higher ones in regulating a key ecosystem function (plant productivity), which would suggest that trophic cascades do not occur or, if they do, that the effects are not detectable at the ecosystem level of resolution. There have been very few studies that have looked at the ecosystem-level consequences of manipulation of the higher trophic levels of soil food webs, although two earlier studies provide relevant data. In the first of these, Santos and colleagues16 found that reducing populations of predatory tyleid mites produced the following chain of events: •Reduce tyleid mites → increase bacterial-feeding nematodes → reduce bacteria → reduce decomposition rate