Feeding plasticity of two detritivore-shredders

1. The feeding preferences of the trichopteran shredder Sericostoma personatum Spence and the amphipod shredder Gammarus pulex L. were studied using specimens collected in a springbrook where the major food source 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.) and a fresh filamentous green alga (Microspora sp.). 2. The overall preference pattern shown by the two shredders was the same: both preferred conditioned Alnus most, followed by fresh Microspora. The least preferred food items were conditioned Fagus leaves and Picea needles. 3. The feeding preference of the two shredders proved to be unrelated to food source fibre content, toughness, total phosphorus, C:N ratio and total nitrogen (P>0.05). 4. Despite the overall similarity in their feeding preferences, Gammarus was more selective than Sericostoma. The reason for this difference is discussed. 5. We interpret our findings as indicating that detritivore-shredders do not per se prefer leaf litter, but in fact actively select other food items such as filamentous green algae or macrophytes, even when terrestrial leaf litter is abundant. Most shredders in Danish forest streams thus seem to live on a growth-limiting food resource that they do not prefer. This may have important implications for secondary production in such streams.