Genetic effects of habitat fragmentation on common species of Swiss fen meadows

The area of Caricion davallianae alliance in Switzerland has been considerably reduced and fragmented during the last 150 years. We assessed the genetic variability, inbreeding level, and among-population differentiation of two common habitat-specific plant species, Carex davalliana SM. and Succisa pratensis Moench, in 18 Caricion davallianae fen meadows subjected to fragmentation. We used a spatial field design of fen systems (six systems total), each consisting of one large habitat island and two small habitat islands. We used allozyme electrophoresis to derive standard genetic parameters (A, P, H-O, H-E, F-IS, F-ST). In Carex we identified a consistently lower A in isolated habitat islands; furthermore, H-E was lower in small habitat islands than in large habitat islands. In Succisa we identified a lower H-O in small habitat islands than in larger ones. Small habitat islands were marginally significantly differentiated (F-ST)from large islands for Succisa. For both species, no effects were evident for F-IS; therefore, we argue that genetic drift rather than inbreeding is the main cause of the observed differences. The genetic structure of Carex and Succisa in small habitat islands differed from that in large habitat islands, but differences were small. It appears that the observed differences in genetic variability among fen meadows correspond to observed differences in fitness and demographic traits. We show that habitat fragmentation affects not only the rare species in an ecosystem but also reduces the survival probabilities of common species. One of the main goals of conservation should be to mitigate fragmentation of natural habitats in order to increase population sizes and connectivity.