Hydro-ecological relations in the Delta Waters

5.2 H- factors
Next to all fartors intrinsic to animals and ecosystem.? the occurrence
of htrds is also strongly influenced by man in a great variety of ways.
Disturbance, pellution and habitat loss are the major factors.
Disturbance by airplanea. beats, walking people, bait-diggiw. etc.
causes bitds to leave the feeding ar rerrfing areas fot some time, For
waders, disturbance distances have been measured. They differ greatly
between species. Curlews are most sensitive and fly away at leaat 250 m
from someone walking, Turnstones can be approacbad up to 1130 meter (Van
der &er. 1985). Disturbance causes a reduction in feeding time and
extra energy losses due to flying. This can cause the birds to leave
suitable feeding sites. Not only disturbance at feeding sides but the
availability of suitable rest sites can alss determine the occurrence
of some bird species. Thia certainly holds for breeding sites.
Pollution cab affect birds in tws distinct ways: rbrougb a
deteriozation of the food supply or through impairment of the
physiological processes in the animal itself. possibly leading CO
death. It is known that pollution sees chaagea the structure of
benthic populations a& cmmamities become daminated by a few small
sized $peCies (Oxay, 1982). Although their densities can be very high.
the biomss is on average quite low. Burthermore, few birds are able to
exploit efficient13 thme very small prey items. When the pollutfon
load increases and anoxic conditions oceur, the macrobenthic faona may
nearly completely dieappear, as is found in the eastern patt of the
Westerschelde CDevelter, et al. 1988; Meire and Kuyken, 1988).
Obviously birds will disappear here as well. It has been &own that
eutmphtcetion can improve benthic productivity (Bewkema and Cadee,
1986, 1987) and Van Impe (1985) associated the increase in bird numbets
In the eastern part of the Westersehelde between the fifeies and the
seventies to the improved feraging couditiells for the-e birds as a
eonsequenee of hlgher preductivity due to eutropwcation. As the
benthix fauna has collapsed in this areas by now the posititre effects
of the wtrophication were only very shartlived (Develter, et al. 1988;
Meire and Kuyhn, 1988).