De Levende Natuur nummer 3 van 2011 (English summary)

Whitefin gudgeon: an invasive species in Rhine and Meuse?

F. Spikmans, J. Kranenbarg & N. van Kessel

In recent years several exotic fish species invaded the rivers in The Netherlands. Whitefin gudgeon (Romanogobio belingi) is one of these species and was first recorded in The Netherlands in 2004. Features to distinguish it from the resembling Gudgeon (Gobio gobio) are the presence of keeled scales, markings in the fins and the position of the anus compared to the pelvic fins. The habitat of Whitefin gudgeon is limited to large rivers, while Gudgeon also lives in smaller streams and lakes. Gudgeons used to be abundant in the larger dutch rivers until the 1980’s. Several surveys reveal the Whitefin gudgeon is nowadays abundant in the larger dutch rivers, whereas the Gudgeon is nearly absent in these rivers. Whitefin gudgeon is assumed not to be native to The Netherlands, based on its absence in museum collections. Colonisation of the river Rhine took place via the Main-Danube canal, or through introductions e.g. through ballast water from ships. The disappearance of Gudgeons in the larger dutch rivers might be a result of competition with the invasive Whitefin gudgeon.

Fertilizer on the salt marsh

J.P. Bakker & Y. de Vries

Salt marshes are naturally productive ecosystems as a result of sedimentation during high tides. The older the salt marsh, the thicker the sediment layer. The deposited sediment includes nitrogen. The amount of total and available nitrogen is positively related to the thickness of the sediment layer. Hence, an older salt marsh produces more plant biomass than a younger salt marsh. Despite the natural productivity, nitrogen is limiting for plant production, as is shown by short-term experiments with addition of inorganic nitrogen. Addition of fertilizer accellerates succession. On the long term, the vegetation changes by addition of inorganic fertilizer: salt-marsh species are replaced by ruderal inland species. After cessation of addition of inorganic fertilizer, salt-marsh plant species can re-establish. Winterstaging geese profit from the addition of inorganic fertilizer.

Recovering of water vegetation in the Reeuwijkse Plassen

C.A. van Gemeren, B.H.J. van Vliet & I.K. Dijkman

The Reeuwijkse Plassen are lakes in a bogland area in the west of Holland. In the past the water quality of the Reeuwijkse Plassen deteriorated, caused by the poor quality of the inlet water. This was nutrition rich water from the neighbouring rivers. As a consequence, the amount of water plants was strongly reduced. In the past five years we investigated the stimulation of the growth of the vegetation in ditches near the Reeuwijkse plassen. Certain ditches were selected, some shorter and some longer. By closing a ditch at one side, rain water got trapped at the closed part of the ditch, creating a gradient to the nutrition rich water from the adjoining lake. The so called ‘sweet water bubble’ that formed in the ditch proved to be a healthy environment for water vegetation to develop. The results can stimulate a better management of the  water systems in a bogland area. From an ecological point of view it seems better to close off ditches at one side.

The use of fauna passages by reptiles

R.P.J.H. Struijk

Fauna passages are frequently used to accommodate exchange between populations divided by roads or to prevent road mortality. An increasing number of green bridges, civil works and culverts have been installed (or adapted) for animals, including reptiles. Literature research and interviews made clear that all indigenous species of reptiles have made use of one or several types of fauna passages in The Netherlands and/or directly surrounding countries. Nevertheless many of the sightings are anecdotal and intensive research has almost never been conducted.

Information on effectiveness of fauna passages for reptiles is therefore almost unknown. However it is clear that fauna passages can create possibilities to increase, re-establish or create metapopulation structures. Besides this, they can also prevent road mortality. To accomplish this, it is essential that both fauna passages and guiding fences are of very high quality. Creativity and the urge to design more robust fauna passages instead of using the standard, often small (amphibian) culverts, will improve their quality. Above all, maintenance of both facilities and surrounding vegetations should take place on a regular base and should be taken very seriously. At the moment, almost every culvert/fencing system seems to be of inferior quality or not maintained properly.

Nature management, protection and habitat demands of three  species of snails Vertigo angustior, Vertigo moulinsiana and Anisus vorticulus.

A. Boesveld, A.W. Gmelig Meyling & R.H. de Bruyne

The European Habitats Directive (Council Directive 92/43/EEC) was adopted in 1992 as an EU response to the Berne Convention. It aims to protect habitats and species listed in the directive's Annexes. The directive has led to a network of protected sites across the European Union called Natura2000. About 1000 European plants and animals are protected. Among these only a few species of molluscs are protected in The Netherlands. For three species of snails, i.e.: Vertigo angustior, Vertigo moulinsiana and Anisus vorticulus, our country bears special responsibility. Comparison to other EU countries showed that in spite of substantially declining numbers in the last decades, The Netherlands still hold quite a number of populations.

As part of a special research program (called 'HabSlak-project') all known sites of the three snails are revisited and species-dispersion is examined. Because of this, our knowledge of the species and their Dutch populations and habitats has increased extensively since the start of the project in 2004. Regarding Vertigo moulinsiana the Dutch range of distribution showed to be much larger than previously assumed. Regarding Vertigo angustior our large dune area's hold quite a number of sometimes rather contiguous populations, with locally rather high densities. Regarding Anisus vorticulus the numbers and populations are still numerous within Natura2000 area's as well as outside these. In recent years there is intensified interest for these species and we notice rewarding results of measures to protect their habitats (especially within Natura2000 area's). However, in spite of this many populations are still at risk or even endangered. At many places it is hard to say whether or not the main objective, a favourable conservation status, is established. Especially in dune area's there are several types of management aimed at the protection of other species that conflict with measures beneficial to Vertigo angustior. There are however various, often rather simple alternatives. These are discussed.

At present more detailed research is still necessary and it is highly recommended for all three species to install monitoringprograms at several meticulously selected populations, especially in the most important habitats and populations.

Burrowing crayfish: where are they heading to?

B. Koese, E.P. Raaphorst, P.G.M. Heuts & A.E. Kolff

The invasion of American crayfish species is a relatively recent phenomenon in The Netherlands. Here we describe the presence of extensive burrows of the Red swamp crayfish (Procambarus clarkii) and Virile crayfish (Orconectes virilis) in the west of The Netherlands after reports of damage. Burrows of the Red Swamp Crayfish were observed in September 2010 and reconstructed in figure 2. Burrows of the Virile crayfish were observed in February 2011, but in contrast to the Red swamp crayfish, all specimens of the Virile crayfish had mostly (sub)vertical passages and specimens were found well below the water surface. We speculate that lowered water levels, the reproductive cycle or a combination of these factors might have caused the digging behavior of the Red swamp crayfish. In order to escape freezing, lowered temperatures might have caused the digging behavior of the Virile crayfish, although other factors could not be excluded. Observations of burrowing behavior by crayfish and damage caused by the behavior are likely to increase, since many species are still expanding and many potential areas are still void of species. However, once the interaction between burrowing behavior and factors stimulating the behavior are clarified, specific measurements have a high potential to suppress the behavior effectively.

The relevance of wild bees for pollination of crops

Jeroen Scheper, David Kleijn & Menno Reemer

Honey bees are generally considered to be the most important pollinators of insect-pollinated crops. However, the last decades have witnessed declining numbers of honey bee colonies and beekeepers in Europe, which may imply that the crop pollination services provided by wild bee species become increasingly more important. Relatively little is known though about the contribution of wild bee species to pollination of crops. Here, we examined the potential contribution of wild bees to the pollination of crops in The Netherlands. We compiled a list of insect-pollinated crops and used a database containing 19838 records of bee flower visits to assess which bee species were observed visiting what crop (analysed at crop genus level). We found that 78% of the bee species in the database were observed visiting at least one crop genus, and 57% of the crop genera on the crop list were observed being visited by at least one wild bee species. Most bee species (131) were observed visiting Rubus. The results show that a substantial proportion of the Dutch wild bee species can potentially contribute to the pollination of crops.