Ökologie und Biologie gefährdeter Stromtalpflanzen

Abstract

In Central Europe, more than 100 vascular plant species are exclusively or predomi-nantly confined to the basins of large rivers: the so-called river corridor plants. As the natural habitats along the rivers have been destroyed and degraded by the regulation of watercourses, the building of dikes, land reclamation and agricultural intensification, they may account for an above-average proportion of endangered species. Due to the fragmentation of their habitats, many populations are small and highly isolated and this may threaten the viability of populations. The degree and the way in which species are impacted can vary between species or groups of species, respectively, depending on their traits, such as their distribution pattern or their life-form. The aim of this study was therefore to find a general pattern reflecting the fragmentation effects on perennial river corridor plants and to advise conservation measurements for these species. The study was carried out in the highly fragmented floodplains of the Weser and Elbe systems in North-western Germany.The first objective of the study was to assess the effects of habitat fragmentation on the extinction of populations and the changes in population sizes of four endangered perennial river corridor plants: Euphorbia palustris, Pseudolysimachion longifolium, Sanguisorba officinalis und Senecio paludosus. High extinction rates of small populations (≤ 25 individuals) could be identified as the most probable cause for the decline of the species studied while the isolation of populations had no effect. Sudden environmental stochasticity induced by human activity, such as land reclamation, changes in land use, or utilization of pesticides, may have led to extinction in the short-term. If conditions stay unchanged, nearly all of the surveyed populations will have become in less than 120 years. This also includes the large populations (> 100 individuals) even though they have a much higher chance of survival or even growth to a size of 1000 individuals and more.In a next step, we analyzed whether the negative effects of habitat fragmentation would reduce reproduction in fragmented populations of Euphorbia palustris, Lathyrus palustis, Pseudolysimachion longifolium, Sanguisorba officinalis and Senecio paludosus and would thus lead to extinction of populations in the long-term, too. Reproductive success of all species studied was reduced in small but not always in isolated populations. This is possibly due to limited pollination and/or the loss of genetic variation. In a following investigation on E. palustris, the loss of genetic variation in small populations was, however, not clearly confirmed and this was probably caused by the longevity of the species.Overall, the effects of habitat quality, such as light availability, soil parameters and parameters of the surrounding vegetation, on the survival and the reproduction of the species studied were considerably less pronounced compared to the effects of population size. However, the recruitment of populations of E. palustris was determined by gaps in the vegetation and a relatively high content of organic matter in the soil, both indicating abundant winterly floodings. Consequently, most populations are static and aged since they are completely cut off from the dynamics of the river due to the construction of dikes.Further investigations on E. palustris over three years showed, that the relationships between reproductive success, population size, genetic variation, and habitat quality varied depending on the weather conditions of each growing season. Thus, studies concerning the impact of habitat fragmentation on the fitness of plant populations should preferably be based on surveys conducted over several consecutive years. In conclusion, river corridor plants are highly threatened by habitat fragmentation and will be further on the decline throughout the study area. The following management measures based on the findings of this study should be included in a conservation plan of this species group to stop or reverse this development: increase of population area and sizes, improvement of the connectivity between populations, re-attachment of habitats to natural river dynamics, utilisation of restored flood plains for reintroduction or introduction of species.