River bed restoration boosts habitat mosaics and the demography of two rare non -aquatic vertebrates

Abstract

Many river revitalization projects are currently under development, but we still lack proper assessments of their effects on biodiversity, especially regarding non-aquatic vertebrates. We analysed the progressive changes in habitat mosaics, and the demographic response of two rare species of wading birds along a Swiss section of the Rhone river over a period of three decades. During the period preceding revitalization, the diversity of the habitat mosaic had been steadily declining. This impoverishment could however be reverted after 1994, when the revitalization was initiated: in 2006, the re-established habitat matrix was more diverse than at any time during the previous 27 years. The formerly channelized stream underwent radical transformation as it mutated into a heterogeneous braided river boosting pioneer riverine habitats. This has led the two wading bird species to establish significantly more territories than before the revitalization operations. Population size of Actitis hypoleucos and Charadrius dubius increased by 83% and 20%, respectively, between 1995 and 2007. During the same period, A. hypoleucos declined dramatically in Switzerland as a whole, whilst C. dubius showed no distinct population trend. The response of the two waders may have resulted from an increase in the number of lateral arms of the river, where the availability of productive temporary ponds provides excellent foraging opportunities. A significant proportion of these habitats was created by a targeted extraction of sediments with the objective to promote biodiversity while maintaining economic activity and improving security against floods. This suggests that gravel exploitation is not incompatible with efficient river restoration.