March 2007

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English summaries

    • Viability of remnant Antennaria dioica populations in The Netherlands
    • The dioecious clonal plant Antennaria dioica has shown a dramatic decline in The Netherlands. Currently only six populations, or population complexes, remain. We investigated population size and structure and habitat quality at each of these locations. Where possible, we compared habitat characteristics between extant sites and sites from which populations had recently disappeared. The observed differences were generally not consistent among locations. There was no consistent indication that soil acidification has been a major cause of extinction. At two locations, extinct sites showed significantly taller vegetation. Except for two larger population complexes, populations were generally small and most comprised only female plants or did not flower at all. Pollination supplementation experiments in two larger subpopulations showed that seed production was significantly pollen-limited, as seed set increased threefold after experimental pollen addition to flower heads. Our results suggest that reduced habitat quality only partly explains the recent extinctions of Antennaria. Most populations are now unisexual, and so small and isolated that they are unable to recover, even when habitat conditions are good. These can only be rescued by means of reinforcement. The two larger population complexes can be considered viable, but also need extra management attention.

    • Will the recent Crassostrea gigas reefs replace the former Ostrea edulis beds in the Wadden Sea?
    • Ostrea edulis beds, with their high biodiversity on which Möbius established his ‘biocoenosis’ concept in 1877, have gradually disappeared from the Wadden Sea in the late 19th and early 20th century due to overfishing. The last oysters disappeared during the severe winter of 1963. The Pacific oyster Crassostrea gigas was introduced in the Wadden Sea in 1976 and started to increase and forming reefs in the 1990’s. C. gigas is hardly eaten by molluscivorous birds and as filterfeeders they compete with the main food for these birds: cockles and mussels. However, C. gigas also forms new biostructures, which provide shelter for a host of other species, including molluscs, crustaceans and worms. As between and around these structures cannot be fished, they provide also shelter for fishes. They seem to have a positive effect on the biodiversity in the Wadden Sea comparable to that of the lost O. edulis oyster beds.

    • Swallow abundance on organic and conventional farms
    • Lubbe, S.K., S. Kragten, E. Reinstra & G.R. de Snoo
    • This study sought to identify differences in Swallow (Hirundo rustica) abundance between organically and conventionally managed dairy and arable farms in The Netherlands, by examining three possible explanatory factors: farm buildings, food availability and farmer attitude towards swallows. Organic and conventional dairy and arable farm holdings were compared in pairwise set-up. No significant differences were found in the number of swallows on organic and conventional farms. Nor was there any significant difference found in farmer attitude, farm buildings or food availability between the two types of holding. Our results show that the adopted regime of farm management (conventional vs. organic) has no influence on the number of breeding pairs of Swallows.

    • Effects of floodplain restoration on breeding birds
    • Turnhout, C.A.M. van, M.J.T. van der Weide, G. Kurstjens & R.S.E.W. Leuven
    • Over the period 1989-2003 approximately 1900 hectares of agricultural floodplain area along the large rivers Rhine and Meuse were transformed into new nature areas, where natural processes such as flooding, morphodynamics, semi-natural grazing and spontaneous vegetation development were reactivated. The effects of the first ten years of this large-scale floodplain rehabilitation on 47 species of common breeding birds are analysed, using their population developments in 129 sampling sites, of which 40 concern ecological rehabilitation projects. For 20 species positive effects of floodplain rehabilitation on linear trends were found, of which seven species are Red-listed. For five species we found negative effects, but none of these are Red-listed. Seven species showed increasing numbers in rehabilitated sites, whereas their numbers in regular floodplains declined. Of these, four species are Red-listed and their national populations decreased with more than 50% since 1960. For the remaining species with positive effects, increases were stronger or decreases were less strong in rehabilitated floodplains compared to regular floodplains. Especially breeding birds typical for pioneer habitats, scrubs and woodland seem to benefit from rehabilitation measures, whereas effects for water, marshland and meadow birds are more heterogeneous.
      Half of the species with positive linear trends, however, show decreasing numbers at the end of the ten year study period. Also a number of species without linear trends show initial increases followed by decreases, especially species of pioneer habitats and water, marshland and meadow birds. Therefore, we recommend that the developments in rehabilitated floodplains are closely monitored in future. Possibilities to optimise rehabilitation measures are discussed.

    • The House sparrow is one of the disappearing bird species inhabiting towns and villages. The decline of the species in The Netherlands is ongoing since 1990 and recent inventories in Amsterdam indicate a decline of at least 90%. Possible causes of the
    • Apeldoorn, R.C. van, C. Klok, L. Hemerik & D.A. Jonkers
    • The House sparrow is one of the disappearing bird species inhabiting towns and villages. The decline of the species in The Netherlands is ongoing since 1990 and recent inventories in Amsterdam indicate a decline of at least 90 %.

    • Declining Adder and Viviparous lizard populations due to fragmentation and water draw-down
    • Strien, A.J. van, A. Zuiderwijk, B. Daemen, I. Janssen & M. Straver
    • Heathland is the most important reptile habitat in the Netherlands as six out of seven native species can be found there. The once vast heathlands have vanished during the 20th century. Nowadays mainly fragmented and degenerated patches of heathland remain. This study focuses on the Viviparous lizard (Zootoca vivipara) and the Adder (Vipera berus), both typical species of Dutch (moist) heathland that are affected by the negative impact of fragmentation. The effect of fragmentation, water draw-down and other factors on their populations have been quantified through a causal-analytical study with data from the Dutch Reptile Monitoring Network. Species numbers were found to decline when the effective habitat area is smaller than 250 ha (Adder) or 100 ha (Viviparous lizard). Also the water draw-down of the habitat is adversely affecting both species, whereas the trend of the Viviparous lizard also was negatively influenced by encroachment of trees. The study shows clearly how results from a monitoring network can be used in causal-analytical studies.

    • Underwater racket: fish and noise pollution
    • Opzeeland, I.C. van, H. Slabbekoorn, T.C. Andringa & C.J. ten Cate
    • Anthropogenic activities such as commercial shipping, recreation and construction activities (i.e. piling and drilling) have made underwater noise pollution an increasing factor in the aquatic environment. Two-thirds of all freshwater fish species are so called hearing specialists and very sensitive to sound. It is to be expected that these hearing specialist species are affected negatively by the increased underwater sound levels. Depending on the noise source, this may affect overall health, acoustic communication, reproduction, distribution patterns, migration and possibly the survival of populations. This paper reveals an important gap in what is known about the acoustic characteristics of various underwater noise sources and the effects of underwater sound on freshwater fish.