March 2012

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English summaries

    • pdf downloaden Towards possibilities for managing Pumpkinseed sunfish
    • H.H. van Kleef & J.J.C.W. van Delft
    • Pumpkinseed sunfish arrived in The Netherlands over hundred years ago. In recent decades the species has increased in number, becoming abundant in a number of nature reserves. Because it is an opportunistic carnivore, it can have a serious negative impact on populations of native invertebrates and amphibians. In order to develop suitable management options to reduce its numbers, a study was performed to identify natural mechanisms involved in controlling abundance of this species in the wild. Reproduction, growth, maturation and survival were studied in relation to Pumpkinseed densities and possible controlling environmental parameters (i.e. temperature, spawning habitat, water chemistry and abundance of other fish species). Pumpkinseed populations exhibit a density dependant feedback mechanism, where reproductive success decreases with increasing abundance. If in dense populations a large percentage of the fish would be removed, this feedback mechanism would stop operating. Reproductive success would increase due to increased egg production and juvenile growth and the population would be quickly restored to its previous state. Hence, even though removal of part of the fish is likely to be temporarily beneficial for other species, this measure has to be repeated regularly in order to be effective in the long term. Small Pumpkinseed populations showed a high juvenile mortality and densities were negatively correlated with the number of Northern pikes. So, introducing pike in combination with partial eradication of the Pumpkinseed population, is possibly a long term solution for controlling Pumpkinseed numbers. Other plausible methods for controlling Pumpkinseed are discussed. However, all these methods are still untested. It is therefore necessary to monitor the results when taking measures!

    • pdf downloaden Exotic and hybrid Phytophthora species, possible threats for natural ecosystems
    • Invasive pathogens can have a massive impact on populations of species. They can lead to big changes in crops and vegetations. Phytophthora species are notorious plant pathogens. Past introductions of species from this genus into new areas have lead to extensive damage in crops and vegetations. Recently,  an unknown Phytophthora species was discovered in seagrass. The possible role of this Phytophthora species in the decline of Zostera marina and Z. noltii (seagrass) in The Netherlands is discussed.

    • pdf downloaden Sold because of peace, wet heathland the Gorsselse Heide
    • H. Smeenge & A.J.M. Jansen
    • The wet heathland area Gorsselse Heide was used for military exercises and became redundant since the end of the cold war. The Government Service for Land and Watermanagement (DLG) and State Property Service (Domeinen) sold this area on the basis of a contract which includes the guarantee that the eco-hydrological conditions will be improved by specific measures. This wet heathland area is drained by many ditches and ponds, otherwise the area would be too wet for military exercises. Increased evapotranspiration due to gradual afforestation caused a further drop down of the groundwater levels and an increased nitrogen deposition. Therefore, the area became not only desiccated but also acidified. Consequently, characteristic species declined and could only survive along roads and paths because soil disturbance offers a more open vegetation and a slightly higher pH.

      A Landscape Ecological System Analysis revealed that local groundwater systems determine the site conditions of the plant communities in the area. Therefore, the removal of all drainage within the area will result in a remarkable rise of the groundwater levels, from which characteristic heathland plant communities and species will profit. The results of the Landscape Ecological System Analysis showed the necessity of measures outside the area. Only then a complete gradient and sound functioning local groundwater system can be re-developed. This gradient consists of wet heathlands (Ericion tetralicis), sward grass communities (Nardo-Galion) and fen meadows (Junco-Molinion). Parts of the area have been sod cut, due to which the semi permeable top layer has been removed locally. This was a threat to the functioning of the local hydrological system, which mainly is determined by these shallow loam layers. The required measure will be taken in co-operation with surrounding landowners. They will also implement the new management strategy, dependent on the state of the current vegetation (periodic sheep and cattle grazing, mowing and shrub removal). The key message is that a Landscape Ecological System Analysis gives a profound insight into the relation between the abiotic and biotic components of a landscape and a careful basis for taking successful and efficient restoration measures.

    • pdf downloaden ‘Cover up’: four scenarios for soft coastal defence in The Netherlands
    • M.J. Baptist & W.A. Wiersinga
    • Under the influence of climate change and sea level rise, coastal defence has a new urgency in the low-lying delta of The Netherlands. These topics were therefore part of The Nature Outlook 2011, in which a number of possible future directions for nature and landscape in the year 2040 were established, responding to developments in society and climate change. The preferred coastal defence strategy for The Netherlands is to “cover up” the sandy shore with sand nourishments. This soft strategy fits in well with the sandy coast of the Netherlands and the dynamic nature of its coastal zone, beach, dunes and salt marshes. The strategy is a form of adaptive management: monitoring makes it possible to respond to rising sea levels and current developments. The strategy has been successful because the number of exceedances of the basic coast line is significantly reduced. As part of the Nature Outlook 2011, four scenarios for soft coastal defence until the year 2040 were assessed. The results present how the nature of the shallow coastal sea in 2040 may look as a result of sand nourishment strategies. A conceptual model is presented in which return periods of nourishments along the 350 km sandy Dutch coast are compared to biological as well as morphological recovery times, indicating what nourishment volumes are feasible and sustainable. Two types of nourishments are distinguished, i.e. foreshore nourishments, which are smaller in size but have to be applied more frequently, and mega nourishments, which are larger in size, and have to be applied less frequent. The model results show that for future nourishment volumes exceeding 50 Mm3/yr, mega nourishments have to be applied, otherwise the biological system cannot recover from the frequent impacts. Yet, even the application of mega nourishments is limited to a volume of 100 Mm3/yr. by the recovery of the morphological system.

    • pdf downloaden Dutch wildness; The Vera hypothesis compared with prehistory
    • L.P. Louwe Kooijmans
    • In the eighties of the last century an alternative view on the primeval conditions in Lowland Europe was developed and formalized in the well-known study of Frans Vera (1997). It challenges the established palaeoecological idea of a predominantly wooded landscape and promotes that of an open woodland pasture maintained by large herds of grazing ungulates. While the former vision remained in use in the earth sciences and archaeology, the latter was adopted in nature management and applied as a basis of projects aimed at restoring natural ecosystems. This paper critically reviews two predictions of the hypothesis: the archaeozoological data set and the palaeobotanical evidence. In each case the hypothesis fails and needs to be adjusted