Species composition, feeding specificity and larval trophic level of flower-visiting insects in fragmented versus continuous heathlands in Denmark

Heathlands of Northwest Europe, which once covered extensive areas, are now reduced to small and large fragments in an agricultural landscape. In this study, we investigate the effect of fragmentation on the flower-visiting insect fauna in relation to species composition, level of specialization/generalization at the adult and larval stages, and larval trophic level. Two study sites were used: a large continuous tract of heathland, and a fragmented mosaic area. The assemblages of entomophilous plants at the two sites were similar. However, of a total of 256 species of flower-visiting insects observed, only 108 were shared between the two sites. The fragmented heath supported a taxonomically and ecologically more diverse assemblage of flower-visitors, compared to the extensive heath. At both sites, the majority of adult flower-visiting insect species were generalists, consuming floral resources from a variety of flowers. Larval types encompassed both specialists and generalists, represented by a spectrum of trophic groups, feeding classes and substrates. Species presence was dependent on the availability of larval resources and habitats rather than the requirements of imagines. No difference in level of specialization of adults or larvae was found between sites. Fragmentation affected trophic-level structure among larval specialists, significantly more high trophic-level species being found at the continuous heath tract, compared to the fragmented area. In contrast, no effect was observed among larval generalists, supporting the trophic-level hypothesis of island biogeography. Thus, although species richness is not negatively affected, composition and trophic structure is markedly changed by fragmentation of heathlands.