FEEDING PLASTICITY OF 2 DETRIVIORE-SHREDDERS

1. The feeding preferences of the trichopteran shredder Sericostoma pe rsonatum Spence and the amphipod shredder Gammarus pulex L. were studi ed using specimens collected in a springbrook where the major food sou rce was beech litter (Fagus sylvatica L.). Six food items were tested: conditioned beech leaves, conditioned alder leaves (Alnus glutinosa L .), conditioned Sitka spruce needles [Picea sitchensis (Bong.) Carr], fresh beech leaves, a fresh macrophyte (Potamogeton perfoliatus L.) an d a fresh filamentous green alga (Microspora sp.). 2. The overall pref erence pattern shown by the two shredders was the same: both preferred conditioned Alnus most, followed by fresh Microspora. The least prefe rred food items were conditioned Fagus leaves and Picea needles. 3. Th e feeding preference of the two shredders proved to be unrelated to fo od source fibre content, toughness, total phosphorus, C:N ratio and to tal nitrogen (P > 0.05). 4. Despite the overall similarity in their fe eding preferences, Gammarus was more selective than Sericostoma. The r eason for this difference is discussed.5. We interpret our findings as indicating that detritivore-shredders do not per se prefer leaf litte r, but in fact actively select other food items such as filamentous gr een algae or macrophytes, even when terrestrial leaf litter is abundan t. Most shredders in Danish forest streams thus seem to live on a grow th-limiting food resource that they do not prefer. This may have impor tant implications for secondary production in such streams.