An estimation of the natural value of dune habitats using Empidoidea (Diptera)

The dune landscape along the Belgian coast was evaluated on the basis of its Empidoidea fauna by investigating the faunal composition in different habitat types. The sites selected for sampling were marram dunes, dune grassland, dune slack, scrubby vegetations and three different dune woodlands. White water traps at soil surface level were used to sample five sites from the end of April until November 1989, and four sites from April to August 1992. Data on the most abundant Dolichopodidae, Empididae and Hybotidae were analysed by means of the following multivariate analyses: detrended correspondence analysis (DCA), canonical correspondence analysis (CCA) and two-way indicator species analysis (TWINSPAN). Most sites showed a distinctive species composition, and several species were confined to particular habitat types. Empidoid communities from canopied and open habitats showed the largest contrasts during 1989. Six environmental variables were measured in July and August 1992, and correlations indicated that aerial humidity and, to a lesser extent, soil moisture were probably the main factors explaining dolichopodid distributions. In Empididae, on the contrary, the presence of canopy by scrub or trees appeared to be important for the occurrence of many species. Thus species diversity and abundance increased from dry to humid sites in dolichopodids and from open to canopied sites in empidids. On the other hand, canopied sites appeared to function as home-bases from which certain empidid species colonized more sun-exposed areas. With the Dolichopodidae it was possible to examine the recorded distribution of all species throughout Belgium. Nearly 75% of the dolichpodids appeared to be typical lowland species and in eight species, largest Belgian populations were established in the dune region. Moreover, seven species could be considered as true coastal species as they were almost entirely restricted to this region. These results clearly stress that not merely the coastal dunes themselves but the full diversity of the associated habitats needs to be maintained in order to protect the characteristic dune empidoid communities.