Processing of two detritus types by lake-dwelling shredders: species-specific impacts and effects of species richness

1. In studies of processes in ecosystems, species are often merged into functional groups. The concept of functional groups requires that species have a similar function with respect to some process. However, in many cases little is known whether the actual species are sufficiently similar. 2. In the present study, species-specificity among lake-dwelling detritivores was investigated. Microcosms were used to test 10 species of shredders regarding ability to process alder (Alnus glutinosa) and oak (Quercus robur) leaves. Taxonomically, nine species were caddisfly larvae (Trichoptera Limnephilidae) and one a freshwater louse (Isopoda Asellidae). 3. The shredders differed significantly in ability to process leaf litter. The most common shredder in lakes of the study area (SE Sweden), the isopod Asellus aquaticus, was shown to be inferior both in terms of capacity per individual and per g animal biomass. Also among the trichopterans, significant differences were found. 4. Consequently, these species cannot be treated as a unit with equal properties in the decomposition process. 5. In addition, 12 multispecies combinations of the shredders were tested for synergistic effects breaking down alder and oak leaves. No positive or negative effects due to species richness were found; it was possible to predict the yield of multispecies treatments using the decomposition output from the single species experiment.