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Abundance and community composition of arboreal spiders : the relative importance of habitat structure, prey availability and competition

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, by
thinning branches or by tying branches. Tying branches resulted in a significant increase in
the abundance of spiders and their prey. Densities of spiders and their prey were reduced
by removal of needles and thinning. The spider community of needle-sparse branches was
dominated by orb weavers (Araneidae), whereas tied branches were preferably colonized
by sheet-web weavers (Linyphiidae and Micryphantidae), and nocturnal hunting spiders
(Anyphaeilidae and Clubionidae). Spider species richness and diversity increased in
structurally more complex habitats.
In 1994 and 1995, I excluded foraging Camponotus spp. ants from canopies of sapling Douglas-fir. Biomass of potential prey organisms, dominated by Psocoptera, increased significantly by 1.9 to 2.4-fold on the foliage following ant exclusion. Hunting spiders, dominated by the Salticidae, increased significantly by 1.5 to 1.8-fold in trees without ants in the late summer. The exclusion of ants did not affect the abundance of web-building spiders. Documented aggressive behavior of aphid-tending ants suggests interference competition between hunting spiders and ants. / Graduation date: 1996
Date06 May 1996
CreatorsHalaj, Juraj
ContributorsLattin, John D., Ross, Darrell W.
Source SetsOregon State University
Detected LanguageEnglish

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