Both water birds—such as coots, ducks, geese and swans—and fish have a negative influence on water and marsh plants. This does not mean that a single Carp (Cyprinus carpio) does any harm, but larger numbers may stir up the bottom to such an extent that vegetation cannot develop or existing valuable ones are destroyed. Owing to its strong ‘grazing’ and fertilising capacities, the grass carp (Ctenopharyngodon idella) should be completely banned. Indigenous fish species—excepting bream (Abramis brama) and carp—should not pose any problem.
Coots, ducks, geese and swans can cause real destruction. In small-scale situations they should not be encouraged. Coots (Fulica atra) will create problems even in small numbers. They manage to destroy older and deep-rooting white water lilies (Nymphaea alba) and yellow water lilies (Nuphar luteum) within a few years. The plants often do not succeed in rejuvenation by seeding, as erosion has often made the shallow banks they require for germination too deep. Chasing the coots away generally is of no use, since the water body will immediately be filled by other individuals from the too often overcrowded populations in urban areas. As with most noxious animals, the presence of coots has a positive side as well. They keep their territories free from wild city mallards who spend their days sleeping and defaecating on the banks, causing damage with their highly saline manure. Where flower meadows border on banks, the fertilising effect is serious but other vegetations may be influenced as well. Ericaceae and species such as Thelypterispalustris, Cochlearia spp. or Calthapalustris may suffer severely.
