Structure of the parasitoid communities of grass-feeding chalcid wasps

The parasitoid assemblages associated with 18 species of chalcid wasps feeding on 10 grass species were sampled quantitatively between 1980 and 1992 at 24 sites in Wales and England to examine food web structure, the size and composition of the parasitoid complexes, and structures of the local communities. The complete food web included 87 species organized into five trophic levels. The lower two consumer trophic levels (the herbivores and primary parasitoids) were characterized by extreme host specificity, whereas the top two trophic levels (hyperparasitoids and tertiary parasitoids) comprised more generalized omnivores. The high levels of specialization resulted in a compartmentalized web. The regional species richness of the parasitoid complexes associated with Tetramesa species (Eurytomidae) depends primarily on measures of host abundance. Total species richness was significantly positively correlated with mean log host density (number per m 2 ) and the number of larvae per grass stem, explaining 70.5% of the variance in richness. Similarly, the number of specialist parasitoids was correlated with host density, whereas generalist species richness was most strongly correlated with host density and more weakly with host gall-formation and voltinism. The only determinant of local parasitoid species richness is regional richness (the latter explained 97.3% of the variance in the former). Further, there was no β-diversity in this food web, and every species found on a host species in Britain is found in virtually every local host population. The relatively few cases where fewer parasitoid species were found represent undersampling at particular sites. Limited evidence indicates that similar results apply to host populations on continental Europe. Thus, there is no evidence that the richness of local communities is constrained by species interactions. Variation in parasitism rates both between and within host species cannot be explained by the available variables, so the forces affecting host utilization rates remain unknown. We found that the formation of a gall had no influence on host mortality rates, offering no support for the hypothesis that galling provides protection from parasitoids. The parasitoids of most host species have consistent relative abundances over time and space. We could not identify any ecological correlates of the degree of consistency, but it may be due to the extremely widespread occurrence of all of the parasitoids coupled with their innate host-finding abilities.