Mark recapture estimates of dispersal ability and observations on the territorial behaviour of the rare hoverfly, Hammerschmidtia ferruginea (Diptera, Syrphidae)

In order to effectively manage habitat for fragmented populations, we need to know details of resource utilisation, and the capacity of species to colonise unoccupied habitat patches. Dispersal is vital in maintaining viable populations in increasingly fragmented environments by allowing re-colonisation of areas in which populations have gone extinct. In the UK, the endangered aspen hoverfly Hammerschmidtia ferruginea (Fall,n 1817) (Diptera, Syrphidae) depends on a limited and transient breeding habitat: decaying aspen wood Populus tremula L. (Salicaceae). Conservation management for H. ferruginea involves encouraging aspen expansion across Scotland, and ensuring retention, maintenance and continuity of dead wood where H. ferruginea has been recorded and in areas that may link populations. In order to do this effectively we need to know how far H. ferruginea can disperse. By taking advantage of the tendency of adults to group on decaying aspen logs, we estimated dispersal ability through mark recapture techniques. In the first year, 1,066 flies were marked as they emerged from aspen logs and 78 were re-sighted at artificially-placed decaying aspen logs up to 4 km from the release site. In the second year, of 1,157 individually marked flies, 112 were re-sighted and one was observed 5 km from the release site. Territorial behaviour was recorded at all (19) decaying aspen log locations. In total, 72 males were recorded defending territories, which overlapped with 68 % of recorded female oviposition sites. Among males only, wing length was positively associated with dispersal. While these results show H. ferruginea is capable of locating decaying logs up to 5 km away, most dispersing individuals (68 %) were recorded at 1 km, which should be taken into account in developing management protocols. If enough dead wood is available it should be distributed within a radius of 1-2 km, and where possible, as stepping-stones linking up aspen woodlands. We discuss the implications of our findings for the natural history of this species, and make recommendations for its conservation management.