The ecology and genetics of central and peripheral populations of Carduus defloratus
The aim of this thesis was to test several of the predictions of the abundant centre model (ACM) by comparing central and peripheral populations of the model species Carduus defloratus along a gradient from the centre towards the periphery of the distribution of the species. The ACM predicts that be...
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|Zusammenfassung:||The aim of this thesis was to test several of the predictions of the abundant centre model (ACM) by comparing central and peripheral populations of the model species Carduus defloratus along a gradient from the centre towards the periphery of the distribution of the species. The ACM predicts that because of increasingly unfavorable and stressful conditions populations become less frequent, smaller, and less dense towards the range edges. Further predictions for peripheral populations derived from the model include lower reproduction of organisms, higher temporal variability of demographic transitions and of population growth rates, higher genetic differentiation among populations and lower within-population genetic diversity. Most of these predictions of the ACM were supported in C. defloratus. The size of populations, their density and reproduction, but also the proportion of seeds damaged by insects decreased from the distribution centre of the species towards the periphery. The number of flowering plants in a population influenced all components of reproduction. Plants in large populations initiated more seeds, aborted less seeds, and produced more and larger seeds per plant. This indicates pollen limitation and increased inbreeding in small, peripheral populations. The strongly reduced reproduction in combination with the lack of suitable, open rocky habitats and poor dispersal of the seeds limits the abundance of C. defloratus towards its northern range limit. Demographic studies in 14 populations of C. defloratus along the central-peripheral gradient did not indicate significant differences between peripheral and central populations in the asymptotic growth rate (λ) of the populations, of the temporal variability in λ, or in the extinction risk of populations of a certain size. However, the variability of several demographic transitions like seedling survival and stasis of vegetative plants decreased toward the periphery, but changes in these transitions compensated each other. These results are in contrast to the hypothesis of increasing demographic variability towards the periphery of the distribution of a species and an increased extinction risk of peripheral populations. Both population types differed significantly in particular demographic transitions, the contribution of particular transitions to λ, their stage structure, and the life span of plants. The fact that demographic features of C. defloratus showed clinal variation related to gradients in centrality and thus climate, suggest that it might be possible to predict general demographic features for individual populations based on their environment. An analysis of the genetic variability and genetic structure of 78 populations of C. defloratus based on AFLPs indicated that genetic variability within populations decreased towards the periphery whereas the genetic differentiation between populations increased. A strong increase of genetic differentiation between pairs of populations with their geographic distance (isolation by distance) indicated gene flow between neighbouring populations. The pattern found is likely to have been formed during the last glaciation, because the populations of C. defloratus outside of the Alps are very isolated and gene flow between them has been very unlikely for a long time. This result together with the small effect of current population size on genetic variability indicates that the population genetic structure of the long-lived species is mainly influenced by historical processes. Like the genetic diversity of molecular genetic markers, that of several quantitative traits decreased from the distribution centre towards the range margin. However, in contrast to the molecular genetic differentiation, the differentiation in quantitative traits did not increase towards the periphery. Quantitative and molecular genetic diversity were not correlated significantly and correlations between quantitative and molecular genetic differentiation were either weak or not significant. The quantitative genetic differentiation of several traits (QST) between 32 populations of C. deflorauts was stronger than the differentiation between molecular markers (PhiST) and some traits showed clinal variation with regard to environmental gradients, indicating that divergent selection acts on quantitative traits. The clinal variation in quantitative traits indicates that the observed differences are adaptive. The northern peripheral populations are likely to contain alleles that may become important for the adaptation of the species to a warmer climate. The northern range limit of C. defloratus rather results from lack of suitable open, rocky habitats than from poor adaptation to climatic conditions in the north. Altogether, the combination of different studies on a single model species and the results of this thesis contribute to a better understanding of the role of current and historic, and of demographic and genetic processes for the differentiation between central and peripheral populations. Moreover, these studies contribute to the discussion about the conservation value of small, peripheral populations in a time of climatic change.|