Winter Mortality of Spruce-Living Spiders: Effect of Spider Interactions and Bird Predation

Abstract

Field experiments were performed to test the hypothesis that winter survival of spiders in spruce Picea abies is influenced by spider interactions and bird predation. By enclosing spruce-branches with fine-meshed nets spider migration and bird predation were prevented. Three types of branches were used: (1) untreated controls, (2) net-enclosed controls and (3) net-enclosed branches with autumn spider densities experimentally raised to between five and twenty times the natural densities. The experiment lasted for 19 wk. Natural mortality for the untreated control branches was on average 88%. When bird predation was excluded mortality fell significantly to 68%. Raised autumn density of spiders raised mortality to 93%, compared to 68% for the net-enclosed controls. Mortality thus seemed to be density-dependent on the net-enclosed branches. It is concluded that both interactions between spiders and bird predation have impact on spider survival during winter, but small (length < 2.5 mm) and large (2.5 mm) spiders are possibly affected differently by those selective forces. Large spiders seem to be more vulnerable to bird predation than small. At high spider densities large specimens survive better than small.