De Levende Natuur nummer 5 van 2014 (English summary)

25 years of nature development in Duursche Waarden

P.H.A.M. Dirks, G. Kooijman, A.M. Hottinga & W.G. Gerritse

Duursche Waarden was the first nature development plan in the Dutch part of the basin of the river Rhine as a result of ‘plan Stork’. In 1989 measures were taken to allow a more spontaneous development of the floodplain near Fortmond. Two branches of the river IJssel, one connected and one isolated at normal discharge, were made or enlarged and the management changed from mowing the grasslands to integral grazing by 25 Highland cattle and 15 Iceland pony’s on 133 hectares. After 25 years the vegetation has changed. Dry grasslands with short inundation periods remained and species diversity increased; the number of red list species had doubled, and the originally present red list species extended their range. Moist grasslands partially changed into a Crataegus monogyna scrub or Alopecurus geniculatus grasslands and the marshy vegetation disappeared almost entirely. The riverine forest mostly consisted of Salix species. It remained, except from parts that had to be cut for river-safety reasons in 2013. The Foot and Mouth Disease outbreak of 2001 necessitated the culling of the cattle. In fall of that year two new herds were introduced and territorial behavior caused a grazing gap between the two herds where the Crataegus scrub developed. Meadow birds like Limosa limosa disappeared altogether as large parts of the marsh bird population, including the rare Nyctocorax nycticorax and Ixobrychus minutus.  More common species like Acrocephalus scirpaceus only stayed at remote places where the cattle and pony’s don’t come. Birds of scrub and forest established or amplified and now reach densities uncommon to Rhine river basin area. Duursche Waarden shows us that low-dynamic river basin nature can disappear fast. Legislation like the Waterlaw and the Habitat directive focus on safety or stability, and this could result in the disappearance of those low-dynamic values. Space could be made available on the inside of the dikes to preserve and develop habitat for species adapted to the low dynamic river basin parts: it is time to introduce ‘plan Little bittern’.

Modest recovery of several species groups in The Netherlands?

A.J. van Strien, R.J.T. Verweij, M.P. de Zeeuw, L. van Duuren & L.L. Soldaat

To monitor the state of the biodiversity in The Netherlands, we developed an indicator that keeps track of the number of plant and animal species on so-called ‘Red Lists’ of threatened species. Although parallels can be drawn with the IUCN Red List methodology and indices, we followed population size and distribution criteria, as well as data mining protocols, specifically designed for the Netherlands. Data from both standardized and non-standardized observations by thousands of volunteers were used to determine the Red List status of 1771 species from 7 species groups. Although approximately 1/3 of all considered species is still threatened, we can now report a modest improvement: where the number of threatened species still increased between 1995 and 2005, this number decreased in the period thereafter (2005-2013). Additionally we observed an average shift from the ‘Critically Endangered’ category in the direction of the ‘Least Concern’ category, which already began before 2005. There were differences between species groups: vascular plants, dragonflies and mammals are performing best, whereas no improvement can be reported for butterflies and amphibians. Results for birds and reptiles were mixed. Further research is needed to verify the extent to which the observed improvement can be attributed to the different policy measures installed aimed at halting biodiversity loss. We intend to regularly update this indicator.

Do wild boars influence the survival probability of wood ants?

A.A. Mabelis

The effect of wild boars on the survival probability of red wood ants was investigated in the Estate of the Crown (9.700 ha, i.e. 23969 acres) in The Netherlands The density of wild boars was about 2 – 3 individuals/100 ha forest. In total about 294 nests of red wood ants were found: 225 nests of Formica polyctena, 68 of F. rufa and just one of F. pratensis. In the neighbourhood of 218 nests the soil was rooted up by wild boars, but only 45 nests (21%) were destroyed. Nearly all nests of  F. polyctena are part of a polydomous colony, which implies that if part of the nests of the colony will be destroyed the other nest populations may be able to restore the loss. However, destruction of a monodomous colony of  F. rufa or  F. pratensis  will have more serious consequences for their survival. Nevertheless, F. rufa seems to be able to survive in the area, due to the fact that the probability to colonize an area by means of flying queens is greater than in the case of  F. polyctena. This leads to the conclusion that F. rufa  is better adapted to habitat fragmentation than F. polyctena.  However, the distance between the fragments may not exceed the dispersal capacity of flying queens. For the survival of  F. polyctena, who disperse mainly by means of building daughter nests,  the distance between habitat patches should be within reach of walking ants, i.e. less than 100 m.

The findings suggest that it is not necessary to take measures for protecting nests of red wood ants against wild boars if the density of these mammals is rather low. In order to get sound judgement of the impact of wild boars on red wood ants their populations should be monitored regularly in different areas and under different circumstances.

Increase of nature quality in moorland pools in the province of Drenthe (Netherlands)

H. van Dam, G.H.P. Arts, R. Bijkerk, H. Boonstra, J.D.M. Belgers & A. Mertens

The moorland pools of the province of Drenthe are sites within the international Natura 2000 network. They harbor Red List species from oligo- and dystrophic water bodies and from early and late successional stages. Systematic inventories of management, water chemistry, vegetation, diatoms and desmids were performed in 18 pools in 1991, 2003 and 2011. In addition, data back to 1924 and more recent data about temperature, atmospheric deposition, hydrology and macro-invertebrates were retrieved from various historic and recent sources.

Since 1980 the mean summer temperature has increased by 1,6 centigrade, which causes a serious increase of sulphate reduction and denitrification. The atmospheric deposition of nitrogen and sulphur decreased considerably over this period. As a consequence the acidification of the pools decreased since 1980, expressed by increase of average pH from 4,0 to 5,5 and decrease of ammonium and sulphate, with positive consequences for the diversity of macrophytes, diatoms and desmids. However, in the last five years the pH-values in some pools increase to values well above 7, due to internal eutrophication, which has negative consequences for biodiversity. Characteristic macroinvertebrates declined between 1991 and 2011; however, characteristic beetles were re-discovered recently.

Measures against desiccation, e.g. filling up of drainage ditches, have increased the water level and improved biodiversity, particularly for vegetation gradients and succession series. Other actions with positive consequences for the biodiversity include removal of sources of eutrophication (agricultural drainage water, gull colonies, eutrophic sediments). The increased use of pools by roosting geese is a serious concern for water quality. Continued removal of organic material (e.g. cutting of peat and sods) is necessary for maintaining and increasing the nature quality of moorland pools.

Bees in Leeuwarden: diversity in urban environments

T. Gerritsen, D.A.W. Seegers, T. van der Sluis, R. Spijker, M. van Welsem, B. Franken, M. Rekers & A.M. Strijkstra

The decline in wild bees and honeybees has recently triggered efforts to conserve bee species in The Netherlands. To support policy making in Leeuwarden, a survey of the local bee fauna was conducted by 6 Wildlife Management Bsc students at University of Applied Sciences Van Hall Larenstein in Leeuwarden. Between the 24th of april and the 8th of august 2013 a total of 3188 bees have been observed, consisting of 57 species of which 11 species have not been observed before in Leeuwarden. Species richness analysis (CHAO2) indicated potential presence of 81 species, close to the 85 now listed for Leeuwarden. Factors that explained the number of bee species were identified using a General Linear Model analysis (GLM). Nesting opportunities (p=0.003), coverage of herb layer (p<0.001) and the number of flowering plant species (p<0.001) all had a positive effect on the number of bee species. Close proximity and continuity of nesting possibilities and presence of flowers appeared to be the key in enhancing bee species richness. Therefore, it is recommended that, to enhance bee diversity, local floral and nesting situation should be improved.