Heterogeneity of mollusc communities between and within forest fragments in a much modified floodplain landscape

Abstract

The Rhine floodplain is much modified by human land use. What used to be the hardwood zone of the large floodplain nowadays holds only a few isolated, recent (mostly < 60 years) forest patches. The aim of the study was to check if the circumstances favoured homogeneity of the snail communities, usually observed within floodplains, or if heterogeneity prevailed. We inventoried molluscs (presenceabsence) in 21 sites within and 18 sites adjacent to a stretch of the Lower Rhine floodplain (total 39 sites). Communities from eight of the sites were quantitatively sampled for within-site analyses. Species turnover occurred with distance from the Rhine, partially along with lateral habitat turnover. Introduced species mainly occurred in young forests, in similar numbers within and beyond the floodplain. Old forests beyond the floodplain had a small number of introduced species and a small between-site heterogeneity, suggesting strong habitat filters (selection of species by environmental conditions) which might be associated with the prevalence of detritus-derived food. The occasionally flooded sites had the longest scatter along the first NMDS axis, indicating a local differentiation without a longitudinal gradient. The young forests that were occasionally inundated and those that were never flooded shared many species, resulting in the lack of significant differences based on the presence-absence data. The damp, regularly flooded forests differed from the occasionally inundated ones and the ones beyond the floodplain. Flooding introduces drift material and species to the sites. The drift is associated with an initially increased within-site heterogeneity. During the years after the flooding event, the within-site homogeneity of communities increases (suggesting local selection of species) in tandem with an increase in the between-site differences (local selection plus stochastic extinctions). key worDs: afforestation, drift dispersal, habitat fragmentation, introduced species, land use, snail fauna