Document: DAV-3-52-25

Impact of spider predation on densities of forest-floor Collembola.

CHEN, B.*, K.L.LAWRENCE and D.H.WISE

University of Kentucky, Lexington, KY 40546 USA ¹

Abstract:
As consumers of detritus and fungi, and as prey of spiders, Collembola form a major link in the detrital food web of forest-floor leaf litter. We performed two field experiments to determine the extent to which spider predation limits Collembola densities. In Experiment 1, we reduced densities of cursorial (non-web building) spiders for 1.3 yrs by pitfall trapping in 2x2-m fenced plots. Activity-densities of several cursorial spider families were lowered, but the reduction was greatest for wolf spiders (> 90%). Reducing spiders had no effect on total Collembola densities. However, after 1 yr densities of tomocerid Collembola, which accounted for 1/3 of the total Collembola biomass, were 2x higher in spider-removal plots, compared to fenced and open controls. This difference persisted from May to September (p < 0.001). In Experiment 2, conducted 2 yrs later, both cursorial and web-building spiders were reduced by pitfall trapping and litter sifting for 1.3 yrs in 1-m² fenced plots. Fenced controls (sifted and non-sifted) and non-sifted open control plots were also established. At the end of the experiment, total Collembola densities were marginally higher in the removal plots compared to pooled controls (p = 0.080). Tomocerid numbers were higher in removal plots, but the apparent response was not significant (p = 0.36). Reducing spider numbers had no indirect effect on the rate of disappearance of individually tethered leaves or leaves in litterbags. Results of Expts. 1 and 2 reveal that spider predation impacts Collembola populations, but effects are variable. Differences in Collembola responses have at least two possible explanations: 1) a 100-yr drought during Expt. 2 may have altered Collembola behavior, lowering their exposure to spider predation; and 2) different combinations of spider guilds were manipulated in the two experiments, differentially affecting patterns of intraguild predation and predation on Collembola.

Keywords: field experiment, trophic cascade, detrital food web, spiders, Collembola, intraguild predation, decomposition, leaf litter

This abstract is being presented at: 11:15 AM in session:
Oral Session #44: Terrestrial Invertebrates: Foodwebs and Plant Responses.