Spiders in decomposition food webs of agroecosystems: Theory and evidence

The involvement of spiders in decomposition food webs has the potential to affect agricultural productivity through two quite different types of inter actions: (1) cascading, top-down effects of spider predation on rates of nu trient mineralization-spider-initiated trophic cascades in the detrital foo d web that could alter rates of decomposition and release of nutrients to p lants; and (2) a bottom-up linkage, through spiders, between decomposition and grazing food webs-energy from the detrital web contributing to elevated spider densities, which in turn might reduce pests and enhance net primary production. Scant experimental evidence exists to refute or support either hypothesis. The first set of interactions is most likely to be of signific ance in no-till and conservation tillage fanning. In theory, spiders have t he potential to enhance productivity by increasing rates of mineralization, but theory also predicts that spiders, by preying on important detritivore s and fungivores, depress rates of litter decomposition. Field experiments by Kajak and her colleagues have uncovered such negative effects of spiders in mown pastures. Although this negative effect could reduce plant growth, the expected time lags in most types of crops suggest that the overall imp act of spiders on plant production will be determined more by the interacti ons comprising the second hypothesis. However, the later hypothesis, that b ottom-up control processes in the decomposition web affect crop productivit y via energy subsidies to spiders and other generalist predators in the gra zing web, remains conjecture without clear experimental confirmation. This hypothesis should be tested in agroecosystems in which detritus-based food webs can feasibly be manipulated.