1 Fragmentation reduces size and increases isolation of plant habitats, and increases the ratio between edge and centre area. Consequences of habitat fragmentation have rarely been studied for common plants, and edge effects are rarely studied for plants in general. We studied density, population structure, fitness components and biotic interactions in the locally abundant distylous fen plant Primula farinosa in the centres and at the edges of 27 Swiss fen habitats of different size and degree of isolation.
2 Population sizes ranged from 80 to 106 450 flowering plants and were larger in larger fen habitats than in smaller ones.
3 The densities of plants were lower in more isolated habitats (by 26-46% depending on developmental stage). In the less isolated habitats, the density of seedlings and juveniles was higher in larger habitats. Plant densities were lower at the edge than in the centre of habitats (34-55%), and edges had fewer plants of younger developmental states. Densities of reproductive plants differed only in the centres of habitats, where they were higher in larger habitats, while at the edges these densities were independent of habitat area.
4 Flower morph proportions were independent of size and isolation of habitats.
5 Plants in larger habitats had larger rosette diameters and tended to have more flowers.
6 At edges, seed set was on average 11% lower, and occurrence and degree of herbivory more than 50% higher, than in centres. Capsule frugivory was less likely in larger habitats. Infection by the smut fungus Urocystis primulicola was more likely in larger habitats.
7 We conclude that size, isolation and edge to centre ratio of fen habitats all affect the abundant P. farinosa and recommend that edge effects and more common species are given more attention.