Semi-natural river system maintains functional connectivity and gene flow of the critically endangered gravel grasshopper (Chorthippus pullus)

River engineering and the subsequent loss and fragmentation of riparian habitats during recent centuries have had serious impacts on the distribution and population size of many riverine species. Thus it is important to identify the characteristics of semi-natural river systems that still support functional connectivity for vulnerable species. Chorthippus pullus is a critically endangered grasshopper that inhabits patches of gravel banks along Alpine river systems. The species’ distribution conceptually resembles a metapopulation. We analysed the patch occupancy and gene flow of a C. pullus population in a river system that had been historically drained in 1923 and improved 67 years later by allowing residual water flow. In 2010, we surveyed a total of 43 patches for occupancy, marked and recaptured 299 individuals and analysed DNA samples of 16 local populations using five microsatellite markers. We further correlated a measure of gene flow to landscape resistance matrices using least-cost-path modelling of the contemporary landscape. Our results show that occupancy patterns are positively related to habitat area and connectivity. Gene flow was restricted by forests and large river streams in the contemporary semi-natural situation. The observed high level of genetic diversity is potentially the result of the historical management over several decades. To preserve functional connectivity and gene flow among local populations of critically endangered gravel grasshoppers, habitat availability and suitability have to be maintained by ensuring appropriate temporal and spatial fluctuations in water level.