Is there intraguild predation between winter-active spiders (Araneae) on apple tree bark?

Abstract

We studied whether there si intraguild predation among overwintering invertebrates in bark traps installed on apple trees. We found that 86% of the overwintering invertebrates in traps were predators, which were dominated by spiders (72%) of different guilds and different body sizes. Spiders had spatially segregated in the traps so that smaller specimens hid in an interstitial space that was inaccessible to larger specimens. Spiders of the bark-dwelling guild, Anyphaena and Philodromus, are winter-active and can prey on other smaller spiders. Analysis of their abundance in traps indicated a negative relationship between Anyphaena and Philodromus or other spiders. Inspection of traps during winter revealed an influx of spider specimens confirming their winter-activity. In a semi-field experiment, where overwintering in cardboard bands was studied under controlled conditions, we observed mortality of both winter-active Philodromus and Anyphaena spiders due to their interspecific predation. The mortality was affected by their size ratio – larger specimens consumed smaller ones. Presence of alternative prey, theridiid spiders, did not affect their mortalities, but the presence of shelters (interstitial spaces) did. In boards with interstitial spaces smaller spiders suffered lower mortality than in boards without the interstitial spaces. Obtained results suggest that intraguild predation between overwintering spiders can decrease their population density during winter.