AN ABSTRACT OF THE THESIS OF Juraj Halaj for the degree of Doctor of Philosophy in Entomology presented on May 6, 1996. Title: Abundance and Community Composition of Arboreal Spiders: The Relative Importance of Habitat Structure. Prey Availability and Com

This work examined the importance of structural complexity of habitat, availability of prey, and competition with ants as factors influencing the abundance and community composition of arboreal spiders in western Oregon. In 1993, I compared the spider communities of several host-tree species which have different branch structure. I also assessed the importance of several habitat variables as predictors of spider abundance and diversity on and among individual tree species. The greatest abundance and species richness of spiders per 1-m-long branch tips were found on structurally more complex tree species, including Douglas-fir, Pseudotsuga menziesii (Mirbel) Franco and noble fir, Abies procera Rehder. Spider densities, species richness and diversity positively correlated with the amount of foliage, branch twigs and prey densities on individual tree species. The amount of branch twigs alone explained almost 70 % of the variation in the total spider abundance across five tree species. In 1994, I experimentally tested the importance of needle density and branching complexity of Douglas-fir branches on the abundance and community structure of spiders and their potential prey organisms. This was accomplished by either removing needles, b