Predator population size structure alters consumption of prey from epigeic and grazing food webs

Numerous studies have found that predators can suppress prey densities and
thereby impact important ecosystem processes such as plant productivity
and decomposition. However, prey suppression by spiders can be highly
variable. Unlike predators that feed on prey within a single energy
channel, spiders often consume prey from asynchronous energy channels,
such as grazing (live plant) and epigeic (soil surface) channels. Spiders
undergo few life cycle changes and thus appear to be ideally suited to
link energy channels, but ontogenetic diet shifts in spiders have received
little attention. For example, spider use of different food channels may
be highly specialized in different life stages and thus a species may be a
multichannel omnivore only when we consider all life stages. Using stable
isotopes, we investigated whether wolf spider (Pardosa littoralis,
henceforth Pardosa) prey consumption is driven by changes in
spider size. Small spiders obtained > 80% of their prey from the
epigeic channel, whereas larger spiders used grazing and epigeic prey
almost equally. Changes in prey consumption were not driven by changes in
prey density, but by changes in prey use by different spider size classes.
Thus, because the population size structure
of Pardosa changes dramatically over the growing season,
changes in spider size may have important implications for the strength of
trophic cascades. Our research demonstrates that life history can be an
important component of predator diet, which may in turn affect community-
and ecosystem-level processes.