Order and disorder in the river continuum: the contribution of continuity and connectivity to floodplain meadow biodiversity

Aim  Aspects of connectivity and continuity operating in the River Meuse were analysed for their contribution to the biodiversity of the floodplain. From this analysis of the diversity and composition of the meadow communities, we aimed to derive effective biodiversity conservation strategies. Location  The River Meuse is one of the larger rivers in the European Western Plains ecoregion. The alluvial plains of the river have a long history of cultivation, and for these plains the floodplain meadow vegetation is a highly appreciated and valuable nature conservation asset. Method  We sampled floodplain meadows from 400 km of the six geomorphic reaches of the middle to lower course of the River Meuse. For each, 50 vascular plant relevés were recorded, representing the spectrum of floodplain meadow communities of that reach. Beta diversity was calculated to quantify similarity in species pools between the reaches. A dissimilarity formula was used to determine the turnover between the reaches, and these dissimilarities were compared with a Mantel test to detect whether species composition of the floodplain meadows exhibited connectivity and continuity between the reaches. Species richness for the floodplain vegetation data of the reaches was compared with data for riparian invertebrate communities. The vegetation data sets were ordinated using detrended correspondence analysis (DCA) to reveal patterns in the floodplain meadow species composition, and the DCA axes were related to plant functional groups and population strategies. The axis scores of the species and plots were linked to river and plant species traits. Results  We did not observe an overall continuity trend in similarity, nor one in diversity in a downstream direction. Lateral connectivity was highlighted by the dissimilarity between the reaches and in the influxes of species from adjacent ecoregions. The DCA ordination showed statistically significant separations between reaches and between the plant functional groups. The second DCA axis related to the longitudinal gradient of the river, whereas the first axis showed stronger correlations with river and plant species traits. We termed this axis the ‘disorder axis’. Plant invaders and avoiders are located at the left side of the disorder axis, whereas the true river‐adapted categories of resisters and endurers are at the right extremity. Contributions to the disorder were identified in terms of connectivity with adjacent ecoregions and the physical disturbance regime of natural and anthropogenic perturbations, resulting in community changes between the reaches. Main conclusions  We concluded that a single overall strategy for biodiversity conservation of the river or its floodplains is not feasible. Strategies can, however, be derived for separate river reaches based on functional groups in the communities, the disorder characteristics of the reach, and the influence of surrounding ecoregions.