HABITAT STRUCTURE AND PREY AVAILABILITY AS PREDICTORS OF THE ABUNDANCE AND COMMUNITY ORGANIZATION OF SPIDERS IN WESTERN OREGON FOREST CANOPIES

The significance of habitat structure and prey availability in spider biology has been well investigated in a number of communities, but onl y briefly in forest canopies. This study gathered indirect evidence fo r the importance of these two factors as determinants of spider abunda nce and diversity in arboreal communities of western Oregon. Arthropod s were collected by harvesting and bagging tips (1 m long) of lower cr own branches from red alder (Alnus rubra), western redcedar (Thuja pli cata), western hemlock (Tsuga heterophylla), noble fir (Abies procera) and Douglas-fir (Pseudotsuga menziesii). Several characteristics of a rthropod habitats were measured: tree diameter at breast height, maxim um horizontal and vertical branch spread, number of branching angles a nd leaves, and total biomass of twigs and foliage. The highest numbers of spiders per branch were collected from structurally more complex t ree species including Douglas-fir and noble fir. These tree species al so had the highest spider species richness. The greatest similarity in spider community structure was found among tree species with shared b ranch characteristics such as needles. The biomass of foliage and prey availability were the best predictors of spider abundance on individu al tree species. Biomass of twigs alone accounted for almost 70% and 6 0% of the variation in total spider abundance and species richness, re spectively, across a wide range of arboreal. habitats. hey availabilit y accounted for less of the variation. Selected habitat variables also predicted the abundance of several prey groups including Aphidoidea, Psocoptera, Diptera and Collembola. Our results suggest that habitat s tructure and prey availability in combination may play significant rol es in structuring the spider community of western Oregon forest canopi es.