January 2019

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English summaries

    • What determines the succes of Common House Martin houses?
    • L. van den Bremer, J. van der Wal, W. de Jong, W. van Berkel, S.J. Vreugdenhil, L.H. Louwe Kooijmans, C. van Turnhout, J. Nienhuis, & R. Fopppen
    • The Common House Martin has suffered major losses since the 1970s (–75% or more) and has almost vanished from highly urbanised areas. Loss of nesting opportunities and depleting food supplies are major factors contributing to the collapse of breeding populations. To help Common House Martins people have started providing man-made nesting structures (houses). A Common House Martin house consists of ‘roof overhang’ where artificial nests are placed, mounted on a pole. This article describes the results of an analysis of the occupation of Common House Martin houses in the period 2009-2015 in the Netherlands. We explore the extent to which the occupation can be explained by environmental variables and other factors, such as the close proximity of colonies. Between 2010 and 2015 the occupation rate has increased from 3% (n=36) to 15% (n=141). The results show that the underlying motivation for placing a Common House Martin house appears to be an important explanation whether a house is occupied. When the original nesting location has disappeared or has been made inaccessible (house is placed as compensation), the chance of occupation of a house that is placed in the vicinity of the original nesting location is more than two times greater than when this is not the case. Of the environmental factors studied, soil type, grassland and ditch length were of significant importance. The chance of occupation is higher on sand than on clay. A larger area of grassland within 500 meters of the house has a positive influence on the occupation rate. An explanation can be that Common House Martins show a clear preference for foraging above open ground, such as grasslands, because on average more suitable preys (flying insects) can be found here. Why the amount of ditch length in the vicinity of a house seems to have a negative influence on the occupation we can not explain properly. From the experiences with Common House Martin houses in the past ten years, it appears that the placement can be especially succesfull as a replacement nesting facility (compensation). Our analysis offers only limited points of reference for increasing the occupation probability of Common House Martin houses based on environmental facors. Literature and experiences from abroad show that distance tot the nearest colony is particularly important. Despite the increase in the occupationrate over the years, the probability of success is still low at 15%, certainly in comparison with other countries. Conservation efforts should focus on protecting existing nests and taking measures aimed at increasing the food supply (insects) such as reducing large-scale use of agricultural pesticides or boosting biodiversity in rural areas.

    • The effect of ecology on Lyme borreliosis risk
    • L.A.G. van Duijvendijk & T.R. Hofmeester
    • The sheep tick (Ixodes ricinus) is spread throughout the Netherlands and can transmit Borrelia burgdorferi s.l., which can cause Lyme borreliosis in humans. The density of infected nymphs affects Lyme borreliosis risk. The ecological aspects that affect this density are, however, only partly understood. This article focusses on a number of studies on these ecological aspects. The studies were conducted between 2012 and 2016 for our PhD theses. Host communities of different forests were determined with the use of camera traps and life traps. Density of questing ticks and Borrelia burgdorferi s.l. infection rate were also determined. We found that rodent density affects the density of infected nymphs one year later when the density of questing larvae is relatively high. Larval tick burden is aggregated amongst rodents and tick burden is affected by the species, sex, weight and infection status of the rodent. Wood mice (Apodemus sylvaticus) have higher tick burdens than bank voles (Myodes glareolus) and I. ricinus take larger bloodmeals from wood mice than bank voles. Larval tick burden on rodents increases when deer density increases, and decreases when the density of predators increases. About 50% of the infected rodents acquire the infection via the bite of an infected larva and the other 50% via the bite of an infected nymph. The ecology of B. burgdorferi s.l. appears to be very complex and the effects of ecological interferences to reduce Lyme borreliosis risk are therefore difficult to predict. Increasing the number of predators reduces the activity of rodents, which reduces the tick burdens on rodents and thereby rodent infection rate and the density of infected nymphs. The best method to reduce Lyme borreliosis risk is, however, still to reduce tick bite risk by putting your pants in your socks and check yourself after a visit to tick habitat.

    • Rewetting without soil removal in the ‘Onderlaatse laak’ brook valley
    • J.P.M. Lenssen, R. Ketelaar, W.J.A. Drok, S.P.J. van Delft, M. van Mullekom & A.J.P. Smolders
    • In 2009 about 90 hectares of heavily fertilized agricultural pastures along the brook ‘Onderlaatse Laak’ were transformed to natural hayfields. Simultaneously, the bottom of the brook and its tributaries was raised to achieve a higher groundwater table and stronger seepage flow to the rhizosphere. Contrary to common practice the top soil was not removed, in order to preserve the geomorphological structure of the area and the iron deposits in the upper soil layers. The measures have resulted in a 5 to 20 cm increase in groundwater levels. Particularly in winter and spring the area is now considerably wetter. The increased upward groundwater flow has also restored iron- enrichment of the rhizosphere. Iron deposits in the rhizosphere limit the amount of soil–phosphorus available for plant growth and may thus strongly determine the opportunities for species-rich hayfields. Since the iron deposits were mainly constrained to the top soil layers all of it would have been removed if common practice was blindly copied in this project. We calculated that, given the present-day iron concentrations in the groundwater, it would have taken several thousands of years to reach the present-day iron levels in the top soil. Vegetation developments are promising. There is no all over dominance of Juncus effusus. Instead Juncus acutifloris and Lotus pedunculatus, the latter indicating nitrogen-limitation, dominate most of the wetter areas. The fields also gradually get colonized by species like Carex disticha, Caltha palustris and Dactylorhiza praetermissa, suggesting succession towards species-rich hayfields. 

    • Unveiling the secrets of deep man-made lakes
    • L.M.S. Seelen, J.H.P. Bruinsma, T.M.F. Huijsmans & L.N. de Senerpont Domis
    • Intense sand and gravel mining has created numerous man-made lakes around the world in the past century. These small lake systems (1-50 ha) are usually hydrologically isolated, often deep (6 – 40 meters) and stratified during summer and in cold winters. Our study area is located in the catchment area of the rivers Meuse and Rhine, in the southern part of the densely populated Netherlands, the province of Noord-Brabant. Due to their small size, these deep man-made lakes are usually not included in the regular monitoring campaigns, such as monitoring required for the European Water Framework Directive. Therefore, not much is known about their ecological functioning. During two summers, we measured the macrophyte diversity and a range of physio-chemical and biological parameters including pH and phosphate concentration in the water column in 51 deep man-made lakes. Comparing these deep lakes to the surrounding shallow water bodies, these deep systems add a good water quality and ecological quality to the total landscape. Man-made lakes are often described as dead and empty underwater deserts, but out results show this is not always the case. To preserve the good quality waters, care should be taken when selecting deep man-made lakes for storing e.g. dredging material. We urge water managers to not only judge the quality of the deep man-made lakes by the biological quality of the surface water, but include deep water quality as a key parameter to determine their suitability for storing materials.

    • Opportunities for the Water Soldier (Stratiotes aloides) in eutrophic peat meadow areas
    • A.J.P. Smolders, E.C.H.E.T. Lucassen, S.F. Harpenslager, F. van Schaik, J.G.M. Roelofs & L.P.M. Lamers
    • Water soldier is an aquatic macrophyte characterised by alternating floating and submerged life stages. Because of its ability to form dense, floating mats and produce high amounts of organic matter, water soldier is an important species in the process of terrestrialisation and the succession of fens. Throughout the Netherlands, the species declined since the 1950s due to a combination of eutrophication and alkalinisation. Eutrophication and enhanced sulphate deposition can lead to ammonium or sulphide toxicity and iron deficiency, both of which can impact root formation. Alkalinisation, on the other hand, reduced the amount of carbon dioxide available for photosynthesis during submerged stage and thereby lowers buoyancy and growth. A recent survey shed light on the ability of this species to actively alter its environment to support its own growth as a so-called ecosystem engineer, which can be useful for management purposes. When reintroducing the species, sufficient numbers should be transplanted to ensure self-facilitation. Furthermore, while waterways should be regularly cleared to avoid build-up of toxic ammonium or sulphide, care should be taken that a dense stand remains.