Abstract
Habitat fragmentation has been recognized as one of the major threats to plant population persistence. Fragmented small and isolated populations are expected to be seriously affected by inbreeding and genetic drift. Gene flow through seed and pollen dispersal may counterbalance the negative population genetic effects by increasing the level of genetic variation. However, in fragmented landscapes gene flow may be interrupted as interpopulation distances become too large to be bridged. At the same time species' dispersal ability may reduced due to severe inbreeding. If populations are to persist in fragmented landscapes, and to avoid inbreeding, successful dispersal will be crucial for plant species. Therefore we hypothesize that species that are adapted to long-distance seed dispersal might have an advantage over short-distance dispersing species, although the relationship between seed dispersal and the genetic effects of habitat fragmentation are complex and poorly understood. The studies described in this thesis aimed to clarify the complexity of genetic effects of habitat fragmentation on two plant species differing in dispersal ability and longevity (Succisa pratensis and Hypochaeris radicata). Crossing experiments in the greenhouse to determine the effects of inbreeding on species' dispersal ability, but also reciprocal transplant experiments to investigate adaptation to home sites in the field, and molecular markers to measure gene flow and genetic variation of fragmented populations were used to investigate the initial hypothesis. Synthesizing the results for S. pratensis and H. radicata, we can confirm our hypothesis and conclude that the long-lived, short-distance dispersing S. pratensis is unfavourably affected by habitat fragmentation, while the short-lived, long-distance dispersing H. radicata is not, as long as suitable sites for establishment remain available. Thus, it seems that in fragmented landscapes, long-distance seed dispersal in combination with the necessary habitat quality is more beneficial than longevity, although it must be noted that being long-lived may prevent rapid extinction of a species. In addition, recommendations are offered to improve the viability of the populations of both species and prospects for future research are presented.
