Impact of increasing landscape openness on waders and avian predators in the Netherlands
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Between 1990 and 2011, of the bird species groups from Europe the largest decline is observed for wader species. Since the Netherlands harbors a relatively large proportion of the global population of these wader species, it has a responsibility for their persistence. Designated, as one of the major negative influences on meadow bird breeding success is a decrease in openness, which is related to an increase in predation. In order to reduce predation, reed, brushwood and potential nesting trees for the Common Buzzard, Northern Goshawk, Sparrowhawk, White Stork, Grey Heron and Kestrel are removed from meadow bird breeding area. In this review I addressed the following question: What are the necessity and the effects of removing tall vegetation (reed and woody plants) from Dutch meadow bird areas on waders and avian predators? Here I reviewed scientific and grey literature and identified knowledge gaps. Increasing the openness of meadow bird areas will possibly have positive effects on meadow bird populations. Since wader populations are still decreasing negative impacts on their reproductive success should be improved. However, many other factors contribute to the negative trends of wader populations, such as water level, mowing date and herbal diversity. Since tall landscape elements such as in the green-blue network (in Dutch ‘groenblauwe dooradering’) have several ecological functions and many species depend on them I would recommend to improve other important habitat demands before a large-scale increase of openness in meadow bird areas. Especially, since predation levels are highly variable over years. According to the provinces, management of tall vegetation is done on a relatively small scale (in meadow bird core areas and surrounding grassland areas). The Flora- and Fauna Act regulations restrict the removal of existing bird nests and removal of tall vegetation is restricted by cultural historical value, Natura 2000 designation, Forest Act and APV log regulation. Logging permits may or may not be granted after consideration of various interests (including the openness benefits for meadow birds). Not vulnerable avian predator species that are distributed throughout the Netherlands will, when tall vegetation is lost from the area, possibly be able to relocate to surrounding areas. For relatively rare species such as the Harriers and Purple Heron it is important to monitor population differences closely. The large-scale removal of reed might have negative effect on Marsh Harrier populations and many reed inhabiting singing birds. Removing reed can also have reverse (positive) impact on predation, since reed provides habitat for alternative prey (Coots, ducks). It is therefore important not to mow all reeds in the (meadow bird) area. It is also important for the conservation of these species to create vole rich places as an alternative food source. When these places are located strategically outside the meadow bird area this will most likely lower predation pressure in the area. In short, more insight is needed in the effect of increasing openness in meadow bird areas on the green network of tall vegetation in the Netherlands. This is needed in order to prevent fragmentation of tall vegetation areas, which might lead to a collapse in population numbers of many species (not only avian predators). Therefore, the effects of increased openness in meadow bird areas should be carefully monitored and compensation of the loss of tall vegetation by strengthening the green network is recommended.