The effects of fragmentation of groundwater systems on the distribution of characteristic fen species in Dutch polders

摘要

Abstraction of drinking water, intensive agriculture and the intake of Rhine water in the Dutch polder area have caused a decline of natural wetland ecosystems and a loss of their biodiversity. In the fen ecosystems of the Vecht river plain, human interference in hydrology as well as eutrophication and acidification have caused a decrease in suitable habitat for nutrient poor groundwater dependent fens. Additionally, fragmentation of groundwater systems resulted in fragmentation of the remaining species' habitats, causing a decrease in habitat area and therefore in the size and the connectivity of the remaining subpopulations. This paper considers the abiotic and biotic key-factors that determine the occurrence of characteristic fen species. It is hypothesised that habitat degradation is primarily responsible for the decline in the distribution of characteristic fen species, but that the decreased connectivity and population sizes caused by fragmentation have an additional negative effect. A multiple regression analysis (GLM) with species presence/absence as a dependent variable was performed, with analyses undertaken for two fen species (Carex diandra and Menyanthes trifoliatd) and a wet grassland species (Lychnis flos-cuculi). The results indicate Calcium and Iron to be good predictors of species occurrence, indicating the importance of restoring the original hydrological situation with seepage water discharge. Furthermore, variables relating to habitat fragmentation were also good predictors of species occurrence, confirming the expectation that additional to habitat loss and degradation, habitat fragmentation has a negative effect on species performance. We conclude that the model presented in this study can support decision making on the conservation of rare fen species in relation to the spatial configuration of their habitat.