Dokument

Summary:
The ongoing anthropogenic impact on our landscape, e.g. by intensification of agricultural use and abandonment of extensively used farmland, strongly affects the persistence of many plant and animal species and populations. Many formerly connected habitats were destroyed or became fragmented. As a consequence many populations became extinct and remnant populations became smaller in size and more isolated. These small and isolated populations are at a higher risk of extinction because they are more sensitive to demographic, environmental and genetic stochasticity. Genetic stochasticity, i.e. genetic drift, and inbreeding in small and isolated populations might lead to reduced individual fitness in the short term. In the long term the ability to react to changing environmental conditions might decrease. Moreover, the environmental conditions in remnant habitats might be of lower quality, e.g. due to increasing edge effects. As species become restricted to remnant habitats, effective management for long-term conservation requires a quantitative understanding of the effects of habitat fragmentation on population viability. Most recent studies of the effects of habitat fragmentation were carried out at relatively small geographical scales. In this thesis I present four studies of the effects of habitat fragmentation on plant performance that investigate offspring performance, quantitative genetic variation and local adaptation in populations at a large geographical scale. Populations were studied in several European regions (in Sweden, The Netherlands, Germany, Britain, Czech Republic, Luxembourg and Switzerland). The target species Carlina vulgaris L. and Hypochoeris radicata L. have contrasting life histories with respect to dispersal ability and the frequency of flowering, but both species are widespread in species-rich semi-natural grasslands throughout Europe. Species-rich grasslands have dramatically declined in number and size in the last decades and are thus an excellent system to study effects of habitat fragmentation. In Chapter 2 I study the effects of habitat conditions and population size and isolation on the performance of C. vulgaris from 74 populations in seven European regions, both in natural populations and in a common garden. In the common garden several measures of performance were reduced in plants from small populations and plant size decreased with increasing isolation of the populations, indicating genetic drift and inbreeding depression in small and isolated populations. In the field, only seed set was reduced in small populations. Environmental variables explained most of the variation in plant size among populations in the field. The results suggest that reproduction and offspring performance may be reduced in small populations of C. vulgaris across Europe due to genetic deterioration, and because pollen quality and quantity might be reduced. In Chapter 3 I analyse the variation in several quantitative genetic traits in offspring grown in a common garden from 74 populations of Carlina vulgaris from seven geographical regions and 32 populations of Hypochoeris radicata from three geographical regions. In both species, most traits varied considerably among seed families within populations, among populations within regions and among regions. The overall proportion of phenotypic genetic variation among populations was higher in C. vulgaris (75.8 %) than in H. radicata (50.7 %), suggesting less gene flow between populations in the poorly dispersed C. vulgaris. The results suggest that at the small scale genetic drift or environmental differences that are not related to geographical distances drive population differentiation, whereas at the large scale selective forces closely related to geographical distances are most important for population differentiation. In Chapter 4 population differentiation and local adaptation in Carlina vulgaris is studied at the European scale. Reciprocal transplant experiments were carried out among (regional scale) and within (local scale) five European regions. Only at the regional scale, the individual fitness of C. vulgaris and several other measures of the plant performance were highest if plants grew in their home region. Furthermore, performance of plants decreased with increasing transplant distance. At the local scale, genotype x environment interactions were far less pronounced and were not related to geographical distance between the population of origin and the transplant site. The results of our study suggest that C. vulgaris consists of regionally adapted genotypes and that distance is a good predictor of the extent of adaptive differentiation at large (> 200 km), but not at small scales. Thus, patterns of local adaptation have to be taken into account for the efficient preservation of genetic resources. In Chapter 5 I present a study on the effects of cross-proximity on seed production and offspring performance of Hypochoeris radicata from Germany, Czechia and The Netherlands. Crosses were carried out within plants (selfing), and between plants from the same seed family, from the same population, from different populations of the same region and from different regions. Seed set and germination after inbreeding and crosses between populations were lower than after within population crosses, indicating inbreeding and outbreeding depression. However, crosses between regions resulted in higher seed set than within population crosses. For late traits, there was regional variation in the effects of inbreeding and interpopulation crosses. Our results suggest that outbreeding depression does not necessarily increase with interpopulation distance and that the sensitivity of populations to introgression may vary among regions.

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