Bioindication using trap-nesting bees and wasps and their natural enemies:community structure and interactions

1, Results from four field studies show that communities of trap-nesting be es and wasps and their natural enemies are promising bioindicators for ecol ogical change or habitat quality. These small and easy-to-handle communitie s can be analysed with respect to (i) species richness and related paramete rs, and (ii) ecological functions or interactions. The communities comprise Hymenoptera (Apidae, Sphecidae, Eumenidae, Pompilidae) and natural enemies belonging to many insect taxa. Traps consisted of 150-200, 15-20-cm long, reed internodes, put into tills or plastic tubes of 13-15 cm diameter; wood en posts with 2-10 of such reed-filled tins were exposed ill the target hab itat. 2, Species richness and abundance of bees (but trot wasps) were closely rel ated to plant species richness of the habitat, a measure of the bees' food resource. However, availability of nest sites of above-ground nesting speci es was equally important: meadows with old trees supported greater populati ons than meadows without trees. A threefold increase in exposed traps resul ted in a twofold increase in species. 3. The sensitivity of this bioindicator system profits from the fact that e valuations rely not only on presence/absence data, descriptive population a ttributes or diversity indices, but also on interactions or ecological func tions. Monitoring ecological responses or multitrophic interactions, and th eir relationship to species diversity, is rarely done but much needed. Ecol ogical functions include (i) the percentage mortality of trap-nesting bees and wasps due to parasitoids and predators, which was correlated with the s pecies richness of these natural enemies; (ii) seed set of allogamous plant s due to successful pollination by trap-nesting bees; and (iii) biological control by the predacious wasps. 4, With increasing isolation of fragmented habitats (when traps were expose d in a cleared agricultural landscape), both species richness of natural en emies and percentage mortality (parasitism and predation) declined signific antly. In a comparison of habitat types (grasslands and field mal-gins), sp ecies richness of the trap-nest community correlated with plant diversity, but percentage mortality, due to parasitism and predation, with field age o nly. The threshold distance to the nearest habitat was 106-530 m for a 10 5 0% decrease in mean mortality, and the mortality increased greatly in habit ats that were older than 5 years. Accordingly, these studies emphasize the significance of a continuum of old habitat patches for the augmentation of nat ural enemies. 5, Exposure of standardized traps is an experimental approach with a small, interacting and reproducing community that can be easily characterized by simple parameters. Taxonomy and biology are well known, and quick evaluatio ns can be done using the close correlation between the number of occupied t raps and species richness. Species richness of trap-nesting bees and wasps was closely correlated with that sampled by sweep nets. Further criteria of indicator taxa that apply to this system are discussed in the text.