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

Abstract

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.