May 2010

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English summaries

    • A relation between butterfly decline and flower abundance
    • M.F. Wallis de Vries, C.A.M. van Swaay & C.L. Plate
    • Recent studies have highlighted the declining trends of even common butterfly species. Causes of these declines are still unclear but the loss of habitat quality across the wider countryside is thought to be a major factor. Nectar supply represents one of the main resources determining habitat quality. Yet, data on changes in nectar abundance are rare. In this study, we link the trends in butterfly species richness and abundance to changes in nectar supply. We used transect counts from the Dutch Butterfly Monitoring Scheme and data on the abundance of flowering plants along the same transects. We compared the results over two time periods: 1994-1995 and 2007-2008. The results show that butterfly decline can indeed be linked to a decline in overall nectar abundance, and the abundance of specific nectar plants such as thistles and knapweed. The link between butterflies and nectar sources shows species-specific preferences for different flowering plants. We argue that excessive nitrogen is a main cause for the decline of flowers and butterflies.

    • The effects of Graylag geese on Black Tern colonies in Dutch marshlands
    • J. van der Winden
    • The Dutch Black Tern population declined significantly during the second half of the 20th century due to human disturbance in the chick period, declining availability of nest substrates and chick food. Recent management efforts addressing these issues have resulted in a stable population trend or even local recovery. However, breeding Greylag and other geese species have colonised the Dutch marshlands and are now present in many areas in substantial numbers. Black Tern colonies were recorded to be negatively affected by Greylag geese visiting tern colonies at night. The geese destroy clutches and nests with small chicks as they swim through the colonies. In nine colonies studied in the period 1994-2009, the breeding success was significantly lower at sites with geese, than at sites without. At this moment, the overall breeding success in Dutch marshlands is estimated as just high enough to maintain the current population size. However, if geese populations will increase in number and distribution, the population effects might be more substantial and will nullificate the recent successes of tern protection.

    • Dutch goose policy versus European Nature 2000 policy
    • R. Kleefstra
    • Since the winter of 2004/05 The Netherlands adapted a new goose policy by designing 80.000 ha of foraging areas for wintering geese on agricultural land. Before this new policy geese were able to feed wherever they wanted to. According to the evaluation of this new policy in 2008 there appears to be enough foraging areas near Nature 2000-areas (where geese spend the night in shallow water) to comfort the intended numbers of geese on a regional scale in these European protection areas. This conclusion is based on a calculation of the carrying-capacity of the foraging hectares in a 5 km radius around the Nature 2000 areas. Yet on a local scale there seems to be a shortage of foraging areas near at least some Nature 2000 areas. It also resulted in the appearance of hunters close to the roosts.
      The monitoring of goose numbers on roosts in the province of Friesland by volunteers of the FFF-Roost Survey Group in the period 1998/99-2008/09 shows significant declining numbers of Pink-footed geese and European white-fronted geese in the Nature 2000-area of ‘Witte en Zwarte Brekken’. Around this area the availability of foraging grounds dropped with 85%.
      The number of European white-fronted geese increased until the season of 2004/05 but showed a significant decrease since then. The mean season maximum for the Pink-footed geese in the period 2004/05-2008/09 (2059) was significantly lower than the mean maximum in 1998/99-2003/04 (8802). Witte en Zwarte Brekken used to be the second best roost in The Netherlands for the species, but after the decline the numbers dropped below the aimed numbers and the roosts became far less important.
      In this article the effect and the evaluation of the new Dutch goose policy is discussed as well as the discrepancy between the Dutch goose policy and the European Nature 2000 policy. Also on roosts in other Nature 2000 areas numbers of geese are dropping (f.e. lake Sneekermeer). For other areas this is simply unknown, because roosting geese were not counted.

    • The role of dispersal in freshwater restoration
    • K. Didderen & P.F.M. Verdonschot
    • Successful restoration of freshwater bodies not only depends on the availability of suitable habitat for aquatic organisms, but also on the ability of organisms to reach the new habitat via dispersal. Little is known about water assisted dispersal of aquatic invertebrates.
      Research on possibilities of aquatic dispersal through culverts showed that current and the design of culverts influenced dispersal rates. The importance of pre assessment of available species source pools and the degree of connectivity for macroinvertebrate populations in newly created wet habitats is emphasised. Heterogeneity of connected and isolated habitat could be beneficial and might lead to higher levels of gamma biodiversity.

    • Changes in the invertebrate fauna in aging field margins
    • J. Noordijk, C.J.M. Musters & G.R. de Snoo
    • The implementation of sown field margins is a commonly applied agri-environment scheme. Such margins are often installed for one, two or six years, and initiated by sowing a flower seed-mixture. Subsequent management entails the occasional mowing of the vegetation without removal of plant biomass. Consequently the composition of the vegetation changes dramatically; low vegetated margins with sown plants and wild annual species changed rapidly into margins heavily covered with nitrophilous and rhizomatous species. We examined invertebrate group diversity and functional feeding group abundance in such margins in the Province of Zeeland. A steady increase in the number of invertebrate groups was observed with increasing age of the margins. The abundance of invertebrate functional groups showed different patterns. Predators decreased, probably due to the limited amount of sunshine able to reach ground level in older field margins. Herbivores and detritivores increased in time, due to the increase of plant biomass en the build-up of a litter layer. We conclude that longer-lasting (or permanent) field margins are desirable from a biodiversity point of view. To improve the margins for the benefit of farmers (and to keep them accepted by them), we suggest to apply a hay-making management to suppress rhizomatous plants and to promote ground-dwelling predators.

    • Case; Contribution of feces of waterfowl to the dispersal of Desmids
    • J. Meesters
    • Viable desmid cells were found recently in frozen feces that waterfowl dropped on the ice-plateau of the Dutch lake Naardermeer. The handling of the droppings collected is described and a comparison is made with other ways of algal dispersal. The conclusion is that desmids can survive quite extreme circumstances and that it is plausible that waterfowl contribute effectively to the dispersal of micro-algae that pass alive their digestive tract.