Sammendrag
The family Chironomidae is the most species rich group of insects in freshwater ecosystems of temperate regions and chironomid communities are well suited to characterise past and present environmental conditions. We used Malaise traps emptied regularly through the 1986 and the 2008 seasons to document possible changes in species composition along an alpine-boreal watershed in Central Norway. To test the comparability and the reliability of the sampling method, five parallel traps were placed along the main stream at 1105 to 1119 m a.s.l. with trap-distances of about 40 to 230 m. The similarity of the parallel collections is very high with correlations of 0.94 to 0.99 for a given week and location. The correlations between parallel samples are obtained by a bivariate Poisson lognormal species abundance distribution. Thus, within a small area, two Malaise traps collect approximately the same species in about the same frequency when positioned in the same way relative to water flow. In comparison, correlation of the samples from 1986 with samples from the same locality in 2008 and taken at the same time of the season is below 0.75 (between 0.56 and 0.75). The observed change in species composition includes both introduction of new species and alterations in abundance patterns. In addition, a shift of species towards higher altitude points to a community response to a warmer climate. This hypothesis is supported by a distinct increase in water temperature over the last 20 years.
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