Macroarthropod communities on vine maple, red alder and sitka alder along riparian zones in the central western Cascade Range, Oregon

This study addresses the structure and distribution of the arthropod communities on vine maple, red alder and sitka alder of riparian zones in the Western Cascade mountain range. Samples were collected from the riparian zones of ten montane streams in and around the H.J. Andrews Experimental Forest, Linn/Lane County, Oregon from May to July of 1992 and May to November of 1993. Arthropod taxa were sorted and tabulated for calculations of intensity (# individuals per biomass of plant sampled) and diversity. The study was designed to test if host tree species and riparian morphology (summarized as stream order) correlate with the organization of arthropod communities.
The arthropod communities were dominated by Homoptera (particularly Aphididae), spiders (Aranea) and a leaf mining weevil (Curculionidae). Less abundant, but consistently present were Lepidoptera, Hymenoptera (Symphyta) and Diptera (Mycetophilidae).
Multivariate (ordination and classification) and two-sample statistical analyses revealed that arthropod species inhabiting these riparian areas were segregated by host tree and stream order. Of the 24 dominant taxa, 12 taxa (10 herbivores and 1 fungivore) showed significant affinities to particular host tree species while 7 taxa (3 predators, 2 fungivores, 1 scavenger and 1 herbivore) significantly differed in intensity with stream order.
Macroarthropod communities collected from red alder represented the greatest diversity, a result of both high species richness and evenness. Vine maple showed the least diversity, but highest intensity as a result of high aphid abundance. Guild structure revealed that the proportions of sap-sucking to chewing herbivores differed among the host tree species. The high abundance of aphid species resulted in a ratio of nearly 80:1 of sap-suckers to chewing herbivores on vine maple. High abundance of the weevil Isochnus goniophallus resulted in a ratio of nearly 3:5 of sap-suckers to chewing herbivores on sitka alder. The same ratio on red alder was nearly 4:1.
Spiders were the most abundant of the predator trophic level. Webspinning spiders were sianificantly more abundant at the more shaded and humid low order sites, while hunting spiders were more abundant at the more open and sunny mid order sites. This segregation of taxa implies that diversity within the compound community of the riparian zone is a function of the different component communities existing therein.