Pattern of seasonal variation in rates of predation between spider families is temporally stable in a food web with widespread intraguild predation

Intraguild predation (IGP) – predation between generalist predators
(IGPredator and IGPrey) that potentially compete for a shared prey
resource – is a common interaction module in terrestrial food webs.
Understanding temporal variation in webs with widespread IGP is relevant
to testing food web theory. We investigated temporal constancy in the
structure of such a system: the spider-focused food web of the forest
floor. Multiplex PCR was used to detect prey DNA in 3,300 adult spiders
collected from the floor of a deciduous forest during spring, summer, and
fall over four years. Because only spiders were defined as consumers, the
web was tripartite, with 11 consumer nodes (spider families) and 22
resource nodes: 11 non-spider arthropod taxa (order- or family-level) and
the 11 spider families. Most (99%) spider-spider predation was on spider
IGPrey, and ~90% of these interactions were restricted to spider families
within the same broadly defined foraging mode (cursorial or web-spinning
spiders). Bootstrapped-derived confidence intervals (BCI’s) for two
indices of web structure, restricted connectance and interaction evenness,
overlapped broadly across years and seasons. A third index, % IGPrey (%
IGPrey among all prey of spiders), was similar across years (~50%) but
varied seasonally, with a summer rate (65%) ~1.8x higher than spring and
fall. This seasonal pattern was consistent across years. Our results
suggest that extensive spider predation on spider IGPrey that exhibits
consistent seasonal variation in frequency, and that occurs primarily
within two broadly defined spider-spider interaction pathways, must be
incorporated into models of the dynamics of forest-floor food
webs.