Hoverfly (diptera: syrphidae) richness and abundance vary with forest stand heterogeneity: preliminary evidence from a case study in a montane beech fir forest

Hoverflies (Diptera: Syrphidae) provide crucial ecological services and are increasingly used as bioindicators in environ- mental assessment studies. Information is available for a wide range of life history traits at the species level for most Syrphidae but little is recorded about the environmental requirements of forest hoverflies at the stand scale. The aim of this study was to explore whether the structural heterogeneity of a stand influences species richness or abundance of hoverflies in a montane beech-fir forest. We used the catches of Malaise traps set in 2004 and 2007 in three stands in the French Pyrenees, selected to represent a wide range of structural heterogeneity in terms of their vertical structure, tree diversity, deadwood and tree-microhabitats. We assessed hoverfly assemblages by recording species richness, abundance and functional diversity. Malaise traps caught 2,374 hoverflies belonging to 104 species. The catches of hoverflies fluctuated both in species richness and abundance between 2004 and 2007. Strictly forest species were only caught in the most heterogeneous stand. Species strictly associated with fir were not recorded in the least heterogeneous stand although fir was present. Although most of the functional groups were recorded in the three stands, species richness, abundance and functional diversity decreased dramatically from the most heterogeneous to the least heterogeneous stand. However, the species assemblages in the less heterogeneous stands were not perfect subsets of that in the most heterogeneous stand, as some additional species not found in the most heterogeneous stand were also present