• Population size;
  • metapopulation;
  • isozyme polymorphism -genetic variation;
  • establishment;
  • Pteridophyta

Three small, naturally isolated populations of Asplenium septentrionale were investigated. The plants grow on erratic boulders in the Swiss lowlands north of the Alps. The distance to the main distribution area is about 40 km. The three populations can be separated from each other by at least one different isozyme phenotype. Two populations do not show any variability among individuals. The third, however, contains four different phenotypes. Different hypotheses are discussed to explain the establishment of the habitat and the fate of the populations. Predictions from metapopulation theory are compared with our results. According to theory we assume that the extinction rate of the populations is very low. Our results from naturally small and isolated populations are considered to be important in the context of conservation biology.