INTERACTIONS IN STREAM LEAF PACKS - EFFECTS OF A STONEFLY PREDATOR ONDETRITIVORES AND ORGANIC-MATTER PROCESSING

The influence of a common perlid stonefly predator, Diura nanseni, on the turnover rate of leaf material in leaf packs and their associated fauna was investigated in laboratory and field experiments in northern Sweden. In the laboratory experiment significant indirect effects of the predator were demonstrated upon leaf mass loss and the densities o f two nemourid stoneflies, Nemoura avicularis and Protonemura meyeri. There was evidence of non-lethal effects on the two species expressed as increased wound frequencies and reduced condition. In the field exp eriment predator influence on leaf turnover rates was supported but th ere were no discernible effects on prey densities. The latter observat ion is suggested to result from high mobility of prey swamping such pa tterns. Since P. meyeri was quantitatively dominant, and its numbers w ere both positively related to leaf mass loss and responsive to predat or presence in laboratory experiments, it was conceived that this spec ies together with Diura make up a particularly important predator-prey pair in the studied system, which is representative of many north Swe dish streams. Diura guts contained mostly nemourids and simuliids, sug gesting a higher degree of prey selectivity than expected. It is concl uded that the direct and indirect effects of Diura nanseni upon shredd ing species could importantly reduce leaf litter processing rate.