Succession of bee communities on fallows

Wild bee communities were studied on one- to five-year-old set-aside fields with naturally developed vegetation (n = 20), and old orchard meadows (n = 4) to analyse effects of secondary succession on species diversity, resour ce use and associated life history traits. General theory predicts a steady increase of species richness with age of succession. In contrast, we found a first maximum in species richness of bees on two-year-old set-aside fiel ds and a second on old meadows. Successional changes of bee communities wer e related to changes of vegetation. The transition from pioneer successiona l stages, dominated by annuals, to early successional stages, dominated by perennials. resulted in the highest species richness of flowering plants in the second year within the first five years of succession. Species richnes s of flowering plants was the best predictor variable for species richness of bees, whereas the cover of flowering plants correlated with the abundanc e of bees. Annual plants were visited more often and perennials less often than expected from their flower cover. Halictidae tended to prefer flowers of annuals, whereas Megachilidae, Apidae and Anthophoridae significantly pr eferred perennials. In departure from successional theory, body size, propo rtion of specialised bees and proportion of parasitic bees did not signific antly increase with successional age, but number of generations and the pro portion of soil-nesting bees decreased with successional age. Comparison of different management types showed that set-aside fields with naturally dev eloped vegetation supported much more specialised and endangered bee specie s than set-aside fields sown with Phacelia tanacetifolia.