The influence of canopy type and seasons on adult aquatic insect emergence and riparian spiders in the Oregon Coast Range

Abstract

The objective of my study was to examine differences in riparian tree and shrub composition associated with adult aquatic emergence and implications for terrestrial spiders in these sites. Seasonal abundance and biomass of adult emergent aquatic insects, riparian arthropod abundances and spider densities were compared between conifer- and hardwood-dominated sites in headwater streams of the Oregon Coast Range. Collections of insects were taken from emergence and sticky traps along with hand collected spiders from May 25 to July 10 and from Aug 1 to Sept 15. Comparisons of emergent insect orders responding to riparian composition and seasons were made using two way anova. Plecoptera and Diptera emerged more abundantly from hardwood-dominated sites. Ephemeroptera and Plecoptera biomass was higher in hardwood dominated streams. Seasonal abundance and biomass estimates indicated that emergent Ephemeroptera, Plecoptera and Diptera were more abundant in spring compared to summer. Riparian distributions of Ephemeroptera, Plecoptera and Trichoptera were more concentrated at stream edge, while riparian Diptera were more evenly distributed. Riparian orb-web spiders also were more numerous at stream edge. Diptera, Ephemeroptera, Plecoptera, Trichoptera and terrestrial prey collected on riparian sticky traps were more abundant in hardwood-dominated sites. Spider collections were dominated by the families of Araneidae, Linyphiidae and Uloboridae. Orb-web weaving spiders were more abundant in conifer-dominated sites; and higher abundances were associated with vine maple, rather than salmonberry. Whereas spider densities were higher during summer, aquatic and terrestrial prey were more abundant in spring.