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Abstract

Abstract Comparative study of the Plecoptera emerging from Breitenbach (near Schlitz/Hessen) during 5 years revealed significant differences in annual yield without noticeable changes in species composition. In a given year, most species together had high or low abundances, so that percentage-composition was only insignificantly altered. Species of Protonemura form an exception to this rule, as they apparently are sensitive to variations in external factors. Some species also show contrary trends (Amphinemura, Nemurella), possibly because of autecological particularities or because they fly at quite different times of the year than does the rest of the species (L. digitata in autumn, L. prima in late winter). There is a correlation to precipitation, low discharge during the quarter of a year subsequent to a species' flight period (while this species will be at the egg stage) will result in lower numbers emerging next year. In fact, abundance of L.prima and L.digitata depends on precipitation during the first quarter of a year, rather than during the 3rd, as is usually the case, thus providing evidence for the above-mentioned correlation. Annual production in highland streams appears to be largely dependent on discharge during the preceding late summer-autumn period. If this theory holds true, the amount of insects emerging could be predicted. Future investigations in our test streams will have to check on this.