Abstract
Global changes currently result in strong declines in species richness and population-genetic diversity. For maintenance of diversity in protected areas and developing new diversity in restored or newly created areas, immigration by long-distance dispersal (LDD) is of vital importance, as it allows for (re-)colonization by plant species of empty and
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disconnected patches and the maintenance of genetic diversity in small populations. Theoretical models predict that increasing habitat isolation may have large effects on inter- and intraspecific variation. Unfortunately experimental evidence is very limited, for seed plants and even more so for ferns. Occasionally, large-scale disturbances provide opportunities to directly test the results of long-distance colonization. An excellent example is provided by the Dutch IJsselmeerpolders, created in the 20th century. As their soil (in fact a former sea-bottom) hardly contained any viable diaspores, the polders acted as huge diaspore traps allowing for the development of new plant communities from scratch by means of immigration. Various woodlands were planted in the polders and gained a surprisingly large diversity of rare ferns, that must have arrived by LDD. My study focused on the impact of variation in LDD-related life-history traits on patterns of diversity in isolated sites. Key targets were: 1) quantification of LDD: the incidence of spore arrival and the diversity among the immigrants; 2) limitation of distribution and diversity of fern species in isolated habitats by a lack of arriving spores or by specific requirements for successful establishment; 3) the roles of dispersal and mating systems in shaping patterns in inter- and interspecific diversity in isolated habitats; 4) species differences in life-history affecting the structure and performance of recently established populations. In various polder forests of varying age the soil spore bank was analyzed. In order to identify the young plants emerging from the soil samples, I developed a novel method for molecular identification of fern species (DNA barcoding). Extensive spore reservoirs had already developed locally, and contained spores of various rare species not present anywhere near. So, habitat availability rather than dispersal may limit fern species diversities at large scales. Using a combination of laboratory breeding experiments and genetic analysis of natural populations, I studied the development of (patterns in) genetic diversity in the polders of four fern species: Asplenium scolopendrium, Asplenium trichomanes, Polystichum aculeatum and Polystichum setiferum. Species-specific microsatellite markers were developed for population genetic analyses. These analyses showed that, for each species, the polder populations together harbor a large part of the species’ European genetic diversity, although the variation in each population was very low. This strong spatial structure indicated, that populations resulted from independent colonization events. As potential mates were lacking, populations likely established from single spores via self-fertilization. Long-distance colonization likely selects for genotypes capable of selfing. Demographic studies for both Polystichum species indicated that strong differences in population dynamics may occur between closely related fern species. Besides, we showed that large variation may exist between such species in their demographic sensitivity to climatic variation.
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