NEGATIVE EFFECTS OF ANT FORAGING ON SPIDERS IN DOUGLAS-FIR CANOPIES

Spiders and ants are potential competitors and mutual predators. Indir ect evidence from previous research has suggested that ant foraging ma y significantly lower the abundance of arboreal spiders in young Dougl as-fir plantations in western Oregon. This study tested the effect of foraging by ants, dominated by Camponotus spp., on spider assemblages in Douglas-fir canopies in a 5-month ant-exclusion experiment. The bio mass of potential prey organisms on foliage, dominated by Psocoptera, increased significantly by 1.9- to 2.4-fold following ant exclusion. T he removal of ants did not affect the abundance of flying arthropods i n the vicinity of tree canopies as indicated by sticky trap catches. T he abundance of hunting spiders, the majority being Salticidae, increa sed significantly by 1.5- to 1.8-fold in trees without ants in the lat e summer; neither the abundance of web-building spiders nor the averag e body size of hunting and web-building spiders were significantly aff ected by ant removal. Spider diversity and community structure did not differ significantly between control and ant-removal trees. The major ity of prey captured by ants were aphidoidea (48.1%) and Psocoptera (1 2.5%); spiders represented only 1.4% of the ants' diet. About 40% of o bserved ants were tending Cinara spp. aphids. Our observations suggest that the lower abundance of hunting spiders in control canopies with ants may be due to interference competition with ants resulting from a nt foraging and aphid-tending activities. Direct predation of spiders by ants appeared to be of minor importance in this study system. This study did not provide sufficient evidence for exploitative competition for prey between ants and spiders.