November 2009

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English summaries

    • When will we encounter the first silent spring? Predicting the fall of the Dutch Black-tailed godwits
    • J. Schroeder, M. Hinsch, J.C.E.W. Hooijmeijer & T. Piersma
    • To improve the bleak situation of breeding Black-tailed godwits in The Netherlands, we can either aim to improve survival or productivity. A more detailed knowledge of how much we have to improve either one can equip us with knowledge on where to place the levers that might enable us to stop the population decline. In this paper, we use empirically gathered demographic estimates to predict the future of Black-tailed godwits in The Netherlands with the help of an analytical population model. Under the best conditions (high adult survival and high reproductive output) the Dutch breeding population of Black-tailed godwits will drop under 15 000 breeding pairs as early as 2060, while when assuming more realistic starting values, we predict less than 10.000 breeding pairs by 2030. Survival is hard to modify, given the already very high annual survival of godwits. Therefore we aim at predicting how much reproduction has to improve to stabilize the population decline. Under the very best scenario, which is rather optimistic, reproductive success must reach values of at the very least 0.85 fledged young per pair and year for every godwit pair breeding in The Netherlands for every year. The realization of this will be difficult, as it has to be negotiated with respect to economical expectations of grassland owners. We conclude that the goal to stop the population decline of Black-tailed godwits breeding in The Netherlands is a very ambitious and heartening goal, but not a realistic one under the current conditions. In the light of the demands of agricultural politics and economics, it is questionable if we will be able to take the measures necessary to stop or at the very least slow the decline of the Black-tailed godwit in The Netherlands.

    • Does Prunus serotina always act as an invasive species?
    • M. Vanhellemont, L. Baeten, L. De Keersmaeker, K. Vandekerkhove, M. Hermy & K. Verheyen
    • Prunus serotina, a North-American tree species, is widely considered an aggressive invasive species in Western Europe. Opposite to prior studies, which focussed mostly on areas heavily invaded by P. serotina, we studied long-term (60 years) forest development in the Liedekerke forest reserve in Flanders, Belgium, located in an area with a low propagule pressure. Aerial photographs (1944–1986), forest inventory data (for 1986, 1996 and 2006), and tree ring data were used to reconstruct the invasion pattern of P. serotina, to infer the underlying mechanisms, and to predict the future role of P. serotina in the forest reserve. Long-distance dispersal events and windows of opportunity triggered the invasion of P. serotina. Colonization was directed by connectivity to seed sources and light availability. The presence of native shrub species, the quick canopy closure, and the recalcitrant herb layer hampered further establishment. Closing the windows of opportunity for invasive species might be an effective measure to mitigate their aggressive behaviour.

    • Changes in breeding birds and plant species of the Gelderse Vallei area
    • J.M. Gleichman, L.J. de Nijs & F. Berendse
    • During the last decades of the 20th century the biodiversity of the agricultural landscape in the Netherlands decreased strongly. To illustrate this process a comparison was made between data of the occurrence of breeding birds and plants in the northern part of the Gelderse Vallei (situated in the central part of the Netherlands) in the seventies of last century and in 2002. Inventories were made in 1 by 1 kilometer topographical grid cells. We found that especially shrub-nesting and common farmland birds (including meadow birds) have strongly decreased, while woodland-birds and several species of waterfowl have increased. We also found that the plant species of nutrient-poor habitats have strongly decreased and plant species of nutrient-rich habitats have probably increased. It is concluded that most negative changes in characteristic bird- and plant populations have been caused by major changes in the landscape due to agricultural development and urbanization. Our results coincide with general patterns in the rest of The Netherlands.

    • Friesche Veen: from Caltha palustris to Sphagnum species
    • J.P. Bakker & Y. de Vries
    • The nature reserve Het Friesche Veen was acquired in 1985. The reserve features a terrestrialization mire. Most of it consists of alder carr as a result of succession after abandoning from agricultural exploitation. A small part is still open mire, and the management authorities aim to maintain it by annual hay making. We recorded permanent plots during the past two decades of which the vegetation dynamics in the community dominated by Calamagrostis canescens are shown. The plots revealed similar trends for all communities dominated by Calamagrostis canescens, Phragmites australis and Carex lasiocarpa repectively. The number of plant species declined, whereas the cover of Sphagnum spp. increased to 100% (fig. 1), probably as a result of the increasing effect of rainwater lenses in the peat soil. Hence, the target of Sphagnum-rich reeds will be met in the future. It seems that the target of species-rich Caltha palustris fen meadows will not be reached.

    • White bacterial mats as an indicator for deterioration of lake Grevelingen
    • B.K. van Wesenbeeck, A. Nolte, S. Bouma, W. Lengkeek, A.M.T. Joosten & P.M.J. Herman
    • The ecological quality of lake Grevelingen is slowly decreasing. This is likely a result of oxygen depletion caused by lack of tidal movement which leads to stratification and accumulation of organic material in low dynamic areas. Observed effects include the presence of large white mats covering more than 10% of the bottom of the entire lake, the absence of benthic organisms and an increased turbidity. Until recently the origin of the white mats remained unknown. Sampling and microscopic research carried out in 2008 revealed that these mats are formed by at least four different species of Beggiatoa spp. This bacterial genus often occurs in marine sediments, especially with low levels of oxygen en high levels of organic compounds. Beggiatoa bacteria oxidize sulphide to sulphate. However, under low oxygen condition sulphide is transformed to sulphur (and H2O) and white sulphur mats become visible on the sediment surface. Thus, the white mats in lake Grevelingen are an indicator of low oxygen conditions. To halt the observed ecological decline in lake Grevelingen it is necessary to gain detailed knowledge about the presence of these white mats and the processes in and near the bottom leading to oxygen depletion (hypoxic conditions) resulting in the formation of these mats.