Ardea
Official journal of the Netherlands Ornithologists' Union

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Lebret T. (1964) Oecologische successie en waterwildconcentraties. ARDEA 52 (1-2): 48-92
PrFont34Bin0BinSub0Frac0Def1Margin0Margin0Jc1Indent1440Lim0Lim1Ecological successions and wildfowl concentrations. I. For two reasons the problems of wildfowl conservation in the Netherlands have arrived in a new phase. 1. For centuries the estuaries of the rivers Rhine and Meuse have been silting up very gradually. On their banks and on mudflats in the middle of the streams extensive tidal marshes developed into excellent wildfowl habitat. When a marsh was well above the high tide mark, it used to be surrounded by a dike and changed into agricultural land ('polder'). The wild geese and ducks, however, easily found new habitats elsewhere in the same estuary. The process was a very slow one and there was plenty of room for wildfowl for many centuries. The flood catastrophe which ravaged the 'Delta-area' (southwest of the Netherlands) in 1953 induced the government to decide that the estuaries should be closed by 'super-dikes'. This work is planned to be finished in 1978. As far as wildfowl is concerned the closing of the estuaries will finish the process described above. 2. An equally important area for wildfowl in the Netherlands is the IJsselmeer area. The wildfowl habitats here are relatively young, as they are only dating from 1932, when the IJsselmeer was created. A considerable part of these wildfowl habitats are now in their optimal ecological stage of development. Here and there the pioneer vegetation is still conquering new 'grounds'. Some other parts, however, show a tendency to impoverish, which stage is characterized by an increase of reeds (Phragmites). Moreover some creeks which formerly isolated the sand bars with their extensive vegetation of Scirpus and Carex, are now rapidly silting up. These processes are a matter of concern. Not only do they predict that on the long run a considerable part of the IJsselmeer coast may lose its importance for wildfowl, but similar processes must be expected to start in the Delta-area. A special management program for wildfowl habitat in the Netherlands is therefore urgently required. II. Factors favouring the concentrations of wildfowl are the following: a sufficient food supply, the food being in accessible situations for the species concerned; the latter factor particularly being often underestimated; shallow waters; waters of sufficient size,. cover and absence of disturbance. Several of these factors are related in various ways: Food plants e.g. depend on shallow waters; here they are accessible for wildfowl. In shallow waters disturbance by angling and yachting is usually absent or negligible. Extensive waters usually are safe day-time resting places for nocturnal feeders. The presence of cover may compensate the effect of hard wind on extensive waters. III. The habitat selection of a number of wildfowl species is discussed. The species are classified in four groups according to their specialisation in habitat selection and their capacity to adapt themselves to agricultural land use. The following groups are tentatively recognized: A. Food specialists 1. Brent Goose (Branta bernicla) attached to tidal marine marshes. 2. Red-crested Pochard (Netta rufina) attached to Chara-vegetation. 3. Barnacle Goose (Branta leucopsis) attached to grasslands of the Agropyr-Rumicion crispi type, though other types of grassland may be accepted. B. Food specialists who are vulnerable because their habitat is both liable to rapid ecological succession and to drainage by human interference. 4. Greylag Goose (Anser anser) feeling on tidal Scirpus pioneer vegetation, though grassland may be accepted. 5-7. other species of grey geese, which are no food specialists, but which are vulnerable to over-disturbance caused by shooting. 8-12 Surface-feeding ducks others than Wigeon and Mallard. This group is to some extent attached to seeds of pioneer vegetations of the same type as the Greylag Goose. 13-15. Swans are somewhat less dependent from very shallow waters. C. Diving ducks, which are much more independent from shallow waters than any of the species of group B. D. Non-specialists. Coot (Pulica atra), Wigeon (Anas penelope) and Mallard (Anas platyrhynchos) have many feeding techniques they can rely upon, and seem quite capable of holding their own. IV. The status and type of development in a number of wildfowl areas in the Netherlands is described. The present status is summarized in Table 1, in which capitals indicate top numbers. Four types of development have been distinguished: A. Areas which are practically stable. Among these the meadow areas which during the winter are inundated rank first. Winter inundations, however, are present only from December to March, while wildfowl in the Netherlands is numerous from July to the beginning of May. B. Areas with pioneer vegetation developing at a slow rate. The fresh and brackish tidal waters are most important for Greylag Geese and surface feeding ducks. The marine tidal marshes in the north (Wadden) especially harbour Brent Geese, Barnacle Geese, Wigeons and Eiders. C. The IJsselmeer area is very important. The succession is developing at a rather rapid rate. Appropriate steps should be planned and executed in time. V. Wildfowl concentrations are largely depending on pioneer vegetations. In the Netherlands there is not much room left where new pioneer vegetations can start afresh, when the existing ones will be worn out. Wildfowl conservationists in the Netherlands are therefore faced with the problem of keeping pioneer vegetations in existence. Special attention should be paid to the areas where large dike-works are undertaken by the government (IJsselmeer, Delta and Lauwerszee).


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