The coastal dune landscape is endangered in many ways, from excessive urbanization to groundwater extraction. In order to evaluate this area on the basis of its Empidoidea fauna, the following characteristic dune habitats were selected: marram dunes, dry dune grassland, a dune slack, scrubby vegetations and 3 different dune woodlands. During two year cycles, 5 (1989) and 4 (1992) sites were sampled with white water traps at soil surface level. In addition, environmental variables such as temperature, light intensity, aerial and soil humidity and the vegetation and humus layer development were recorded. Data on the most abundant species were analyzed by means of multivariate analyses [DCA, CCA, (1); TWINSPAN, (2)]. In the CCA, both species and environmental data were used. A total of 10,894 dolichopodids and 2,585 empidids were collected over the two years of investigation, belonging to 54 and 41 species resp. Species diversity and abundances increased from dry to humid sites in dolichopodids and from open to canopied sites in empidids. The dry, sun and wind exposed areas demonstrated the poorest faunas including xerophilous dolichopodid species (Medetera micacea, M. pterophiloides). In the dune slack, a rich dolichopodid community was present, consisting of mainly small species (Chrysotus palustris, C. pulchellus, Medetera saxatilis, M. truncorum), whereas the minute soil-dwelling predator, Chersodromia cursitans, and Hilara lundbecki, a hunter on water surfaces, were the only abundant empidid species. Although the open areas are very rich in flowering plants during spring, no empidid nectar feeders such as Empids sp. and Rhamphomya sp. were obtained. Faunas of dry well-lit woodland sites were less diverse and species occurred in relatively low numbers. Despite its small size, the scrubby willow vegetation investigated showed characteristic dolichopodid inhabitants (Dolichopus migrans, Sciapus laetus) too. The empidoid fauna of humid woodland sites was very diverse comprising several large woodland-inhabiting dolichopodids (Dolichopus sp., Hercostomus sp.). The species composition of the 2 humid woodland sites sampled was very similar in dolichopodids but clearly different in empidids. In the well-lit sites, Hybos culiciformis, a hunter in flight, and the predatory Platypalpus excisus were dominant, whereas leaf-running predators such as Chelipoda vocatoria and several Platypalpus sp. and the presumably nectar-feeding Empis punctata reached their highest abundances in the densely covered sites. These wind and sun protected sites seem to act as a refugium for species such as P. strigifrons from where the adjacent areas can permanently be recolonized. On the one hand, each habitat type proved to house typical species and might act as a refugium for other species. Hence they contribute to the conservation of the species diversity in our dune landscape. At the other hand, ± 50% of all abundant dolichopodid species collected show a distribution in Belgium which is either entirely restricted to the coastal dune region or confined to the northern part of Belgium. Moreover, thus far large populations of these species were encountered almost exclusively in the dunes. It can thus be concluded that the dune area is essential for the survival of these species in Belgium.(I) M. POLLET and P. GROOTAERT (1987). Bull. Inst. Sci. Nat. Belg. Ent. 57:173-186.(2) M. POLLET and P. GROOTAERT (1991). J. Nat. History 25: 1297-1312. |
