Sally A. Entrekin

Distribution and dispersal of adult stream insects in a heterogeneous montane environment

The distribution and dispersal of adult mayflies, stoneflies and caddisflies were investigated in an approximately 5 ha area of montane forest and grassland-scrub in the South Island, New Zealand. Low-flying insects were collected in 28 Malaise traps set along stream channels, in dry valleys and on hillsides for 8 weeks in late summer. Adult assemblages differed in forest and grassland and along valleys depending on distance from streams that were their probable sources. Stoneflies and mayflies occurred predominantly in forest and were taken mainly above the stream or very close to it. In contrast, caddisfly species exhibited a wide range of distribution patterns with some restricted to forest and others being found in both forest and grassland up to 300 m from the nearest likely source. Many females of several species that were taken well away from their natal stream (e. g., Pycnocentria evecta and Aoteapsyche colonica), were not fully developed, reproductively, suggesting they were dispersing inland to mature, whereas others packed with eggs (e. g., most Hydrobiosis parumbripennis) were more likely to be searching for oviposition sites. Many more male and female caddisflies were trapped in valleys than on hillsides, even where surface-water was absent, suggesting that valleys provide important corridors for dispersal, at least of low-flying individuals.

The response of Chironomidae (Diptera) to a long-term exclusion of terrestrial organic matter

We examined the effects of a seven-year detrital exclusion on chironomid assemblages in an Appalachian headwater stream. We hypothesized that litter exclusion would lead to a reduction in all chironomids at both the subfamily and generic levels because organic matter serves as both food and habitat in these headwater streams. Tanytarsini total abundance and biomass significantly declined after litter exclusion. Before litter exclusion, Tanytarsini average abundance was 4271xa0±xa01135 S.E.xa0m−2 and 625xa0±xa098 after litter exclusion. Biomass was 3.57xa0±xa00.96xa0mg AFDM m−2 before litter exclusion and 1.03xa0±xa00.9 after exclusion. In contrast, Orthocladiinae abundance and biomass did not change because a psammanophilic chironomid, Lopescladius sp., and other Orthocladiinae genera did not decline significantly. Overall chironomid taxa richness and diversity did not change as a result of litter exclusion. However, Canonical Correspondence Analysis (CCA) of genus-level biomass did show a clear separation between the litter exclusion stream and a reference stream. Separation of taxa between the two streams was due to differences in fine (r2xa0=xa00.39) and coarse (r2xa0=xa00.36) organic matter standing stocks and the proportion of small inorganic substrates (r2xa0=xa00.39) present within a sample. As organic matter declined in the litter exclusion stream, overall chironomid biomass declined and the chironomid community assemblage changed. Tanytarsini were replaced by Orthocladiinae in the litter exclusion stream because they were better able to live and feed on biofilm associated with inorganic substrates.