Population biology of rare plants : the effects of ecological and genetic processes for the growth and viability of populations of three endangered floodplain violets
This work contains studies on various aspects of the population biology of the congeneric plants Viola elatior Fries, V. pumila Chaix and V. stagnina Kit..These are endangered and red listed in many countries. They show a continental distribution range. In Central Europe the species reach their western range margin. These violets have a complex life-cycle, a mixed mating system with chasmogamous (CH) and cleistogamous (CL) flowers and a persistent seed bank. Viola pumila and V. stagnina occur mainly in floodplain meadows, whereas V. elatior grows in alluvial woodland fringes bordering floodplain meadows.The main aim of this work was to apply an experimental demographic approach to the conservation of these three species. Specifically, I (1) analysed the influence of demographic, environmental, and genetic stochasticity, and deterministic processes (management, fragmentation) on population growth and viability, (2) described and compared the species life-cycle in central and marginal populations, (3) analysed the genetic population structure and the relative importance of genetic drift and gene flow in central and marginal populations, (4) analysed the effects of mating system and pollen source on seed production and offspring performance, and (5) tried to identify sensitive stages in the life-cycle.The investigation consisted of observations of the fate of marked individual plants in permanent plots, common garden and greenhouse experiments and genetic analyses of DNA collected from plants in natural populations.The main results of the thesis are that (1) deterministic processes, such as habitat destruction, decreased habitat quality, and succession play a major role for population viability and persistence of the study species, though environmental stochasticity will also strongly affect population stage structure and seedling recruitment. However, negative effects of fragmentation (isolation, small population size, edge effects), can be counteracted by suitable conservation management in the study species.(2) With respect the comparison of marginal and central populations, the predictions of the abundant centre hypothesis could only partly be verified for the study species. Population sizes were consistently smaller in marginal populations than in central ones, whereas the densities of seedling and total densities were higher in marginal populations. Stronger isolation and lower genetic diversity were found in marginal populations of V. pumila and V. stagnina.(3) Gene flow had a stronger relative influence on genetic structure than genetic drift (i.e. genetic stochasticity) in all species-region combinations with average distances <
0.5 km, whereas genetic drift was more important, if average distances were >
1 km. There was no equilibrium between gene flow and drift in the study species and genetic diversity was not significantly correlated with population size or isolation.(4) Studies on the pollination biology showed that the species are probably not pollen limited and there were no indications for inbreeding depression in. In general, CL seeds were significantly smaller than CH seeds, but seed mass and numbers of CH seeds did not differ significantly among pollen sources. Seedling growth was reduced in plants grown with the microbial community of the same species, as compared to control soils. Under these home -conditions, relative fitness of selfed seedlings of V. stagnina was significantly higher than that of crossed progeny.(5) Analysis of population growth rates demonstrated that differences between species and regions were largely due to strong variation of population performance of V. stagnina at the Rhine and the Dyje. In V. elatior and V. pumila, growth and survival were most important for population growth rate.In conclusion, the data presented in this work supplies important information for the conservation of the endangered study species by highlighting the important role of land-use management for the viability of populations. A lack of evidence for inbreeding depression in all three species and indications for possible outbreeding depression in V. stagnina provides important clues for the design of restoration programs. Data on the population genetic structure, the relative importance of gene flow and genetic drift and information on population divergence adds relevant background information for species conservation and restoration. Additionally, the study also sheds new light on various aspects of fundamental ecology, e.g. the relative role of stochastic and deterministic processes for population performance, and the relative importance of different genetic processes in central and marginal populations.
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