De Levende Natuur nummer 4 van 2006 (English summary)


DLN 2006-4

Habitat use and food choice of Harbour seals of the Wadden Sea, part 1

Brasseur, S.M.J.M., I. Tulp & P.J.H. Reijnders

In 2002 a study on habitat use and food choice in Harbour seals (Phoca vitulina) was started in The Netherlands. The background of this study is to identify to what degree Harbour seals and fisheries are competitors for the same fish. The distribution of foraging locations of the Harbour seal was investigated in the spring of 2003 by fitting seven seals with satellite transmitters. Information on location, depth and diving behaviour was collected between February and June. Most seals primarily used the North Sea, with few movements within the Wadden Sea. Trip lengths were highly variable with strong individual patterns. All seals alternated feeding trips lasting from one day to over one week, with resting periods close to the sandbank where they were captured.
The locations of the feeding seals were visited to sample fish. Main species present were: Dab, Plaice, Whiting, Flounder and Dragonet. Pelagic species such as Smelt, Sprat, Anchovy and Herring were not caught because we used fishing gear aimed at demersal fish. However these species are known to be common in coastal waters from other sampling programmes. Faecal analysis indicated that compared to historic data flatfish have become more common and gadidae have become less common.
In the follow up of this study we aim to characterise the diet more precisely by using fatty acid signatures in fish and seal fat. Once the diet is known it is possible to calculate the fish consumption by the Harbour seal population and relate this to the landings.

New fen meadows in the Krimpenerwaardl

Kerkhof, T.B.M.

Before 1900, fen meadows (Cirsio-Molinietum) were widespread in the Krimpenerwaard, a peatland region in the western part of The Netherlands. Fen meadows almost completely disappeared between 1900 and 1930 as a consequence of lowering of the water tables and the increased use of fertilisers. Since 1993 the organisation for nature conservation ‘Het Zuid-Hollands Landschap’ is trying to restore fen meadows by removing the mineralised and eutrophic top soils of former agricultural grasslands. The first results are promising. Red List species like Anagallis tenella, Carex hostiana and Gentiana pneumonanthe are colonising the new sites, partly from their seed bank. Some basiophilous species which are unlikely to be present in the seed bank, such as Dactylorhiza incarnata and Fissidens adianthoides, have also appeared, probably by air. However, due to the influence of stagnating and infiltrating rain water in the centre of the parcels, these parts are unmistakably acidifying eight years after top soil removal. Acidophilous plants, like Sphagnum species, are now spreading more rapidly than basiophilous species. To maintain a sufficient pH and base saturation for fen meadow species, the excavated meadows should be extensively inundated by base-rich, mesotrophic surface water during winter, as happened before 1880. It is also recommended to situate new restoration sites in the vicinity of existing nature reserves, to promote the dispersal of threatened species, which have not survived in the seed bank.

Is top soil removal a prerequisite for nature rehabilitation from agricultural soils?

Kemmers, R.H., A.T. Kuiters, P.A. Slim & J.P. Bakker

Two time series of nature rehabilitation from abandoned agricultural land on sandy soils were analyzed on vegetation and soil development over a period of 30 years. Both study sites are positioned in the upper course of brook valley systems of the Pleistocene sandy regions. In the initial situation the soils were strongly enriched with phosphates. In both study sites the top soils were not removed in order to deplete the excess of nutrients. The sites were managed by extensive grazing (Cranendonck) and by cutting and removal of grass (Loefvledder). Under both management strategies the productivity of the sward decreased and a clear shift of species from nutrient-rich to nutrient-poor soil conditions became apparent. Several species belonging to the aimed ‘target’ vegetation type have been recorded, but the real ‘sensitive’ species did not yet establish or could not be observed in the soil seed bank. The productivity of the vegetation in both study sites is restricted by potassium and/or nitrogen. Nitrogen in the top soil increased due to atmospheric deposition at both sites, whereas at Loefvledder an increase of phosphorus could be observed, probably attributed to the occurrence of a nutrient pump. From our study we conclude that rehabilitation of low productive grasslands from phosphorus-enriched agricultural soils is possible without top soil removal. A depleted soil seed bank probably is the main reason that target species have not yet established. We suggest to reconsider top soil removal and to give way to forest development as an alternative destination for abandoned agricultural fields on loam-poor sandy soils.

The stagbeetle in the dutch area ‘Het Rijk van Nijmegen’

Smit, J.T. & R.F.M. Krekels

The area ‘Het Rijk van Nijmegen’ is one of four key areas where the stagbeetle is found in The Netherlands. It is one of the two smaller areas, with a few isolated populations. In this paper the current knowledge on the stagbeetle in this area is summarized. Furthermore the management aspects as well as some specific suggestions for the management of the stagbeetle in this area are given. The main bottleneck is the isolation of the few, small populations.

Do have South African and Netherlands biology students different views on nature?;

Eijsackers, H.

Lecturing on nature conservancy at Vrije universiteit Amsterdam and Universiteit Stellenbosch offered the possibility to investigate if biology students in different countries and cultures have different views on nature and nature conservation. The results of a short questionnaire show a similar broad attitude towards nature and nature conservation in both university student groups. They all want action both with respect to global problems (global warming/climate change). But next to that there are also typical national problems like Dutch fisheries discussions and South African eradication of alien species and more attention for water and soil pollution. Moreover, there was a shared interest in more social aspects of nature conservation (education, consciousness, population growth). A major bias is that the answers express the views of well educated, white people; it would be most interesting to question young people of other cultural backgrounds.