Biodiversity of Gastropoda in European floodplains
Obrdlík, P.; Falkner, G.; Castella, E.
Riverine floodplain systems are remarkable for the close coexistence, often on relatively small surface area, of highly contrasted aquatic, semi-aquatic, and terrestrial biocenoses. The diversity of micro-habitats, the alternation of aquatic and terrestrial phases ("flood pulse", Junk et al. 1989), the facilitation of water transport of propagules, the increased development of land-water boundary conditions, as well as the occurrence of both progressive (e.g. terrestrialization, siltation) and regressive (e.g. rejuvenation by spates or deposition of bare sediment) successional processes (Amoros & Petts 1993) are often cited as determinant causes for this coexistence, and for the high number of stenoecious species (Gepp et al. 1986). As a consequence, high biodiversity and rapid species succession are generally considered among the major characteristics of floodplains in their most natural conditions (Gerken 1988, Terek & Obrdlík 1991). More than one century of modification of river systems, directly or indirectly induced by man (e.g. embankments, dams, flood protection, water abstraction, river-bed incision, hydropower generation, agricultural and forestry practices in the catchment) have caused a change in the original species composition of floodplain biotopes (e.g. Falkner 1991a). The former floodplains that are no longer flooded by surface water, but are still influenced by groundwater oscillations, often act as a refuge for Gastropoda (e.g.Valvata macrostoma, Gyraulus rossmaessleri) that are highly sensitive to being flooded by eutrophicated surface water (Falkner et al., in prep.) and for otherwise non-floodplain insects (Šustek 1994). The historical modifications of the river-floodplain systems in Western Europe are well documented (Bravard et al. 1986, Petts et al. 1989, Dister et al. 1990, Müller 1991, Müller et al. 1992). However, the consequences of these modifications on biotic communities have mostly been addressed for terrestrial vegetation and little is known of the consequences for invertebrates. A possible way to address this question is through the comparison of invertebrate assemblages in floodplains with different histories or degree of human impacts. Although Castella et al. (1991) attempted such comparison for aquatic invertebrate assemblages in two adjacent but contrasting floodplains, large scale comparisons of the composition and the state of floodplain invertebrate assemblages in Europe do not seem to exist, even for restricted taxonomic groups. Therefore, the objectives of the present contribution are: (i) to compare the biodiversity of an homogeneous taxonomic group, the Mollusca of the class Gastropoda (excluding slugs), in 12 European riverine floodplains representing a gradient of geographic locations as well as diverse hydrological regimes; (ii) to compare, for each location, the floodplain biodiversity with an available regional list of Gastropoda. Molluscs, which cover aquatic, semi-aquatic and terrestrial habitats have been used frequently in floodplain assessment studies (Schmid 1974, 1978, Kinzelbach 1976, Richardot-Coulet et al. 1987, Reichardt & Trüb 1990, Foeckler 1990, Foeckler et al. 1991) and the comparisons attempted in the present contribution are based upon both literature surveys and sampling programmes carried out by the authors.