Predation on spiders: Ecological mechanisms and evolutionary consequences

Birds are common predators of arthropods in many ecosystems but their impact on spiders has not been assessed. Therefore, the experimental evidence for bird predation effects on spider populations was examined. In particular, the present review focuses on the questions: what are the ecological mechanisms and what are the evolutionary consequences? Data from 17 field experiments, mainly in forest ecosystems, showed that spider communities were often significantly affected by bird predation. Comparisons of experimental effects were based on the ratio of mean density on experimentally enclosed vegetation and on controls. In 27 tests, a significant effect was detected (mean ratio 3.03) but in 9 tests the effect was nonsignificant (mean ratio 1.03). Furthermore, field experimental studies on bird predation effects on certain spider species or certain genera were reviewed. In three investigations, significant predation effects were found on agelenid, linyphiid and theridiid spiders but there were no significant effects on lycosids. Selective bird predation on large individuals has been shown in studies on spider communities and single species. Data on bird predation effects on species richness were lacking although impact on large species was expected to be important. Three field experiments showed that different spider families may experience differences in bird predation pressure. An aviary experiment showed that frequently moving spiders had a higher risk of predation than sedentary individuals, but the evidence from field experiments supporting the hypothesis of high predation pressure on moving spiders was limited. This included sex-specific differences in size and movement, although at least one experiment showed that males had higher winter mortality than females. One experiment showed that bird predation can affect anti-predator behavior. In conclusion, the present evidence showed that bird predation on spiders in several contrasting forest ecosystems is strong. However, there are many hypotheses regarding bird predation on spider populations that should be examined in future field experiments.