252
l) StabilIty Properties in thc Waddcll Sea
9.5.4
Migratory Birds
Variables of interest
Variables of interest considered are the abundance, species composition and spatial
distribution of migratory birds in the course of a year. But since the migratory
birds only spend more or less a short part of the year in the Wadden Sea, no conclusions are possible from the abundances recorded in the Wadden Sea about the
full population dynamics of the birds. Additional variables of interest studied by
ELA W AT include the consumption of individual birds and the estimated total
consumption of the entire avian community (Exo, Ketzenberg unpubl.).
Significance of the ingredient for the Wadden Sea
The significance of the Wadden Sea for migratory birds with arctic or subarctic
breeding grounds is well-known (Rose & Scott 1994). On their way from or to
their winter sites in the south, the migratory birds build or fill up their energy reserve necessary for their flight.
Less clear is the significance of the migratory birds for the Wadden Sea. In
view of the extremely high number of birds utilizing the Wadden Sea year after
year, one might well wonder about the extent to which total consumption by the
avian community controls the macrozoobenthos. For example, it has been hypothesized that the decline of Mytilus edulis in the East Frisian part of the Wadden Sea
may partly be a consequence of the increasing number of birds observed in recent
years (cf. Michaelis et a!. 1995).
Stability properties
The Wadden Sea is an important area for waders and waterbirds with about 10-12
million birds using the area over the year (Meltofte et a!. 1994). The phenology of
the abundance of the different species follows the same pattern each year. In the
studied tidal flats dominated by Lanice conchilega, the relative consumption of the
birds per month in autumn was estimated as 5-14 % of the biomass (Petersen &
Exo 1999). For certain types of prey and locations, however, consumption may be
higher. For example, it has been estimated for mussel beds in the North Frisian
part of the Wadden Sea that birds consume 32 % of the biomass (Nehls et a!.
1998). Feeding pressure exerted by birds seems to be relatively constant if considered on the time scale of years. In recent years, however, an increase in bird abundance has been observed. If this trend continues, consumption by birds may increase and consequently diminish the recruitment of macrobenthic species.
Observation after the ice winter in 1995/96 revealed that the birds are able to
cope with extreme climatic conditions when the quantity and quality of their prey
are considerably reduced or when the preferred prey species temporarily disappear
completely. This resistance is due to the plasticity of individual behaviour including temporary migration to ice-free areas. Whether resilience also exists is not
known because it could not be ascertained whether the ice winter actually had
l) StabilIty Properties in thc Waddcll Sea
9.5.4
Migratory Birds
Variables of interest
Variables of interest considered are the abundance, species composition and spatial
distribution of migratory birds in the course of a year. But since the migratory
birds only spend more or less a short part of the year in the Wadden Sea, no conclusions are possible from the abundances recorded in the Wadden Sea about the
full population dynamics of the birds. Additional variables of interest studied by
ELA W AT include the consumption of individual birds and the estimated total
consumption of the entire avian community (Exo, Ketzenberg unpubl.).
Significance of the ingredient for the Wadden Sea
The significance of the Wadden Sea for migratory birds with arctic or subarctic
breeding grounds is well-known (Rose & Scott 1994). On their way from or to
their winter sites in the south, the migratory birds build or fill up their energy reserve necessary for their flight.
Less clear is the significance of the migratory birds for the Wadden Sea. In
view of the extremely high number of birds utilizing the Wadden Sea year after
year, one might well wonder about the extent to which total consumption by the
avian community controls the macrozoobenthos. For example, it has been hypothesized that the decline of Mytilus edulis in the East Frisian part of the Wadden Sea
may partly be a consequence of the increasing number of birds observed in recent
years (cf. Michaelis et a!. 1995).
Stability properties
The Wadden Sea is an important area for waders and waterbirds with about 10-12
million birds using the area over the year (Meltofte et a!. 1994). The phenology of
the abundance of the different species follows the same pattern each year. In the
studied tidal flats dominated by Lanice conchilega, the relative consumption of the
birds per month in autumn was estimated as 5-14 % of the biomass (Petersen &
Exo 1999). For certain types of prey and locations, however, consumption may be
higher. For example, it has been estimated for mussel beds in the North Frisian
part of the Wadden Sea that birds consume 32 % of the biomass (Nehls et a!.
1998). Feeding pressure exerted by birds seems to be relatively constant if considered on the time scale of years. In recent years, however, an increase in bird abundance has been observed. If this trend continues, consumption by birds may increase and consequently diminish the recruitment of macrobenthic species.
Observation after the ice winter in 1995/96 revealed that the birds are able to
cope with extreme climatic conditions when the quantity and quality of their prey
are considerably reduced or when the preferred prey species temporarily disappear
completely. This resistance is due to the plasticity of individual behaviour including temporary migration to ice-free areas. Whether resilience also exists is not
known because it could not be ascertained whether the ice winter actually had