Importance of habitat structure to the arthropod food-web in Douglas-fir canopies

This study tested the hypothesis that habitat structure dictates the distri bution and community composition of arboreal arthropods. A diverse arthropo d assemblage of Douglas-fir canopies, which included Araneae, Psocoptera a. Collembola and Homoptera, was chosen as a model system. Habitat structural diversity, defined as needle density and branching complexity of Douglas-f ir branches, was manipulated in a four-month experiment by needle removal, thinning and tying of branches. Abundance of canopy spiders declined signif icantly following needle density reduction and branch thinning, branch tyin g significantly increased spider abundance. Distinct habitat utilization pa tterns were found among individual spider guilds. Orb weavers (Araneidae) d ominated spider assemblages in structurally simple habitats, whereas tied b ranches were colonized primarily by sheet-web weavers (Linyphiidae) and noc turnal hunting spiders (Anyphaenidae and Clubionidae). Spider species richn ess and average body size of several spider species increased in structural ly more complex habitats. Arboreal spiders appeared to be limited by strong bottom-up effects in the form of habitat quality and, to a lesser degree, prey availability. Habitat manipulations did not affect densities or biomas s of flying arthropod colonists in the branch vicinity. Needle removal and branch thinning led to a significant decline in the abundance of Psocoptera and Collembola. Tying of branches resulted in an eight-fold increase in Co llembola numbers. organisms most sensitive to habitat alterations. Canopy h abitat structure modified vertical dispersal of Collembola from forest litt er, which may have significant implications for arboreal consumers. Our res ults lend strong support to the importance of habitat structural diversity in explaining general patterns of arthropod abundance and diversity on plan ts.