Determinants of parasitoid abundance and diversity in woodland habitats.
Insect parasitoids comprise a large fraction of terrestrial biodiversity. Because of this diversity, species-level conservation of most parasitoid species is impractical and habitat conservation must substitute. However, habitat indicators of parasitoid abundance and diversity are poorly known. To identify such habitat indicators, parasitoid wasps in four ichneumonid subfamilies were sampled in the field herb layer of 15 woodlands in the Vale of York, UK, using Malaise traps. The catch was related to vegetation characteristics. A total of 1543 individuals in 60 species was recorded, representing 36% of UK species in the taxa sampled. Parasitoids tended to be more abundant and species rich in woodlands with a high broadleaf content and tree species richness. This pattern was observed in the ichneumonid subfamilies Pimplinae, Poemeniinae and Diacritinae. However, the ichneumonid subfamily Diplazontinae was found to vary in abundance and richness within rather than between woodlands and showed no association with measured habitat variables. Reserve selection analyses indicated that coniferous woodlands, and woodlands with a low abundance and richness of parasitoids, none the less can contribute to maximizing parasitoid diversity at the landscape scale. Synthesis and applications. At the individual woodland scale, broadleaved woodlands with high tree species richness appear best for conserving parasitoid abundance and diversity. At a landscape scale however, a variety of woodland habitat types can maximize diversity of all parasitoid taxa. We hypothesize that the degree of association between parasitoid abundance and diversity, and characteristics of the vegetation within habitats will decrease with an increase in the number of trophic links that separate them.