Populations of Arboreal Spiders (Araneae) on Douglas-Firs and True Firs in the Interior Pacific Northwest

Abstract

Population densities and relative abundances of spider families were estimated on Douglas-fir, Pseudotsugata menziessii var. glauca (Beissn.) Franco and true firs, Abies spp., in three geographical provinces of the interior Pacific Northwest: the Southern Cascades, the Northern Cascades, and the Blue Mountains. Mean densities, estimated by sampling tree foliage on lower branches in June 1981, were 9.96, 5.21, and 6.03 spiders per square meter of branch area for the three areas, respectively. Eleven families were represented in the sample. Over half of all individuals were hunting spiders of the families Salticidae and Philodromidae, of which several species are believed to be important predators of defoliating insects. The rest were web-spinners, mostly of the families Dictynidae, Araneidae, Linyphiidae, and Theridiidae. The relative abundance of families had a consistent statistical pattern in which their frequencies were apportioned according to a log series distribution. This suggests that arboreal spider communities of interior Pacific Northwest fir stands have a similar familial structure, probably determined by the branch and foliage characteristics of their habitat.