Arthropod populations occurring on the banks of the tidal part of the river Scheldt: distribution patterns and threats
Hendrickx, F.; Maelfait, J.-P.; Steeno, R. (1998). Arthropod populations occurring on the banks of the tidal part of the river Scheldt: distribution patterns and threats. Biol. Jb. Dodonaea 65: 152-153
Beeckman, T.; Caemelbeke, K. (Ed.) (1998). Populations: Natural and manipulated, symposium organized by the Royal Society of Natural Sciences Dodonaea, University of Gent, 29 October 1997. Biologisch Jaarboek (Dodonaea), 65. Koninklijk Natuurwetenschappelijk Genootschap Dodonaea: Gent. 257 pp., more
In contrast to other biota of the river Scheldt the terrestrial arthropod communities occuring on its banks were until recently very poorly known. After some preliminary work (May-June 1992), an extensive sampling campaign was organised during July and August 1995 and May 1996. In 1992 all kinds of open habitats (non-willow scrubs) were included. In 1995-1996 48 reed belts were investigated. This was done in a standardised manner with half an hour of hand- and 'pooter'-collecting as a sampling unit. Hereafter we summarise the results obtained for the spiders and the terrestrial amphipods. On the whole we collected more than 80 species. There is a pronounced difference between the assemblages living in the marshes along the freshwater part and those occurring in the brackish marshes north of the city centre of Antwerp. Some very rare spiders, still occurring in these marshes such as Tmeticus affinis, Baryphyma duffeyi, Clubiona juvenis and Pardosa purbeckensis, show distribution pattern clearly associated with the salt gradient of the river. The communities along the freshwater part seem to have lost more species in comparison with those of the brackish marshes. Good indicators for the transition brackish/freshwater are also the talitrid amphipod Orchestia gammarellus (brackish marshes) and its sister species Orchestia cavimana (freshwater marshes). In contrast to the very high abundance of the former species, 0. cavimana has a rather patchy distribution and never reaches high densities. To inspect for the effects of habitat fragmentation and of habitat deterioration, the genetic structure of some populations of both species was assessed by means of cellulose acetate electrophoresis. In seven investigated populations six loci of O. gammarellus were polymorphic and showed a relatively high degree of heterozygosity. On the other hand, the four sampled populations of the fresh- water species O. cavimana showed very little genetic variation. Only three out of ten scored loci were polymorphic. Of those four populations, the two occurring along the freshwater part of the Scheldt-estuary clearly showed less variation than the one sampled at Schelderode, situated at the non-estuarine part of the river, and the one sampled along the river Yzer. As well as from the species distribution patterns as from the genetic diversity assessments it can be concluded that the populations from the littoral habitats along the freshwater part seem to have been much more affected by habitat fragmentation and deterioration than the populations from the brackish marshes.
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