T. Tscharntke et al., Bioindication using trap-nesting bees and wasps and their natural enemies:community structure and interactions, J APPL ECOL, 35(5), 1998, pp. 708-719
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.