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.