Hydro-ecological relations in the Delta Waters
Hydro-ecological relations in the Delta Waters Hydro-ecological relations in the Delta Waters
that the predation pressure here is very high. This is also confinmd by the review paper of Baird, et al, 1988, and the tidal flats form a v- distinct ecosystem. Densities and comumptions of piscivores (tertiary consumers) are on average lower. Eemarkable is the high density and consumption in the saline lakes. These results fit more or lees in the concepts of ecological pyramids (Odm, 1872). From Bigure 5, however, we see that a food web is a better description of the reality, hence explaining some deviations from the expected pyramid structure. 5 PAIXOBS COMPWWING TAE RELUTON BETREEN DlSTRIBUTIDN AND FOOD SUPPLY From the previoudi section it is obvious that relatiens hetween bird density and food supply are hot always obvious. First of all the data of many spedes have been analysed rogecher. If wader densiries ii~ gener&l are related to grey densities, it ia very likely that this holds also far most of the abundant species. However, if no rerrelatian is found for all birds of a hitd group, this does not me= that the densities of iddividuaL species ma), not be closely link& ta their food supply. It is clear, however, that mt only food supply hut m), other variables may influence the distribution and density of birds. These factors may be purely ecological or resulting £ram human activities. We will try to briefly describe somg of them in order to show the qomplexity of the whole system and the difficulties one encounters in predieting effects of impacts oq the ecosystem. We hope this will make clear that in planning we shvuld he very conservative and use very large margins concerning enviromntal marters. 5.1 Ecologital factors Migratien, large-scale distribntion patterns of a species and climatic oonditions are very important in determining bird numbers. Indeed for several species the Delta area forms either che northern or the
scmthem bottueary o-f the winter distribncion (e.g. Lapwing, Avocet, Blaek-tailed Godwit and Barnacle Gomse, Bewick's Swan respeewvely). In cheee cases the severiq of the winter may strongly influence bird occurrence. In mild winters much larger numbers of some species, sensitive to frost, may be present; in severe winters large numbers of Birds, norntally winterfrrg.more to the north, are forced to &rate further southwards and peak numbers of wme species can be counted. Duriqg a cold spell the presence of open weter is extremly important and may attract birds from the large eurfoundin@. Pnrthermore bird riders in .n area are not ohly depertdent on the food supnly but also &ad on the qnality of other areas. Indeed if, for whatever resaon, the food supply at the begirming a£ the winter in an asea is eXUegK1y high or low, many mtroe or less birds may stay there, influencing the numbees in Other basu, where nethin$ in the fmed supply niisbt have changed. MIDWINTER NUMBERS OF GRNLAG GOOSE IN THE WESTERN SCHEUIT 1976-1 987 year Pigore 8 Evolution of the number of Oreylag Geee in tke Wesrersohelde
- Page 86 and 87: During the 19th century man starts
- Page 88 and 89: mudflats have retreated some 100-20
- Page 90 and 91: aq811~33a.e~ pue 3pTatlJS uxaJsafi
- Page 92 and 93: Erosion by waves of sandy shoals (c
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- Page 96 and 97: Implementafion of the Delta Project
- Page 98 and 99: ECOLOGICAL DEVELOWdENT OF SALT MAKS
- Page 100 and 101: fn tidal water systems sedimentatio
- Page 102 and 103: hierarchical position. Tn the egtua
- Page 104 and 105: Of course. tke environmental change
- Page 106 and 107: wind erasson, desaliuation, aeratio
- Page 108 and 109: Fonner tirtal flats with saltmarsh
- Page 110 and 111: Grevelingen. Lake Veere and Krammer
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- Page 116: BEEEXNX, W.D. and 80ZBf&. J. L9M. T
- Page 119 and 120: are determining the dfstrlbution af
- Page 121 and 122: this volume or in Duutsaa, et aL, 1
- Page 123 and 124: artificial waterflow was created af
- Page 125 and 126: of waterbirds occur regulsrly. The
- Page 127 and 128: . DISTRIBUXIOBI OF WATEJBIRoS OVER
- Page 129 and 130: small bird$ extludiag them vill net
- Page 131 and 132: In order to lsok for relatiouehips
- Page 133 and 134: Table 3 Biomass of macroroobenthos
- Page 135: freshwater basins. Clearly the pres
- Page 139 and 140: OCLURRENCE AND DIET QF B ~T .tUQ 80
- Page 141 and 142: 5.2 H- factors Next to all fartors
- Page 143 and 144: Bewick's Swans (Cygnus columbariusl
- Page 145 and 146: m's activiries do not always have t
- Page 147 and 148: measure affecting food supply or av
- Page 149 and 150: DOQiWT$QS, G. 4 TWISK, F., 1987. De
- Page 151 and 152: MEININGEB, P.L. and HAPEREX, A.M.M.
- Page 153 and 154: PISCIVORES Litfle Grebe Great Crest
- Page 156 and 157: IPYBRO-EGOLOOIOAL BELATIONB ZN THE
- Page 158 and 159: apart from upstream purification; -
- Page 160 and 161: Storm surge barrier Eastern Scheldt
- Page 162 and 163: - In addition to existing managemen
- Page 164 and 165: Nature is pliable. Belattvely young
- Page 166 and 167: Ro. 1. No. 2. SQ. 3. Investigations
- Page 168 and 169: 25. The relation between water quan
scm<strong>the</strong>m bottueary o-f <strong>the</strong> w<strong>in</strong>ter distribncion (e.g. Lapw<strong>in</strong>g, Avocet,<br />
Blaek-tailed Godwit and Barnacle Gomse, Bewick's Swan respeewvely). In<br />
cheee cases <strong>the</strong> severiq of <strong>the</strong> w<strong>in</strong>ter may strongly <strong>in</strong>fluence bird<br />
occurrence. In mild w<strong>in</strong>ters much larger numbers of some species,<br />
sensitive to frost, may be present; <strong>in</strong> severe w<strong>in</strong>ters large numbers of<br />
Birds, norntally w<strong>in</strong>terfrrg.more to <strong>the</strong> north, are forced to &rate<br />
fur<strong>the</strong>r southwards and peak numbers of wme species can be counted.<br />
Duriqg a cold spell <strong>the</strong> presence of open weter is extremly important<br />
and may attract birds from <strong>the</strong> large eurfound<strong>in</strong>@.<br />
Pnr<strong>the</strong>rmore bird riders <strong>in</strong> .n area are not ohly depertdent on <strong>the</strong> food<br />
supnly but also &ad on <strong>the</strong> qnality of o<strong>the</strong>r areas. Indeed if, for<br />
whatever resaon, <strong>the</strong> food supply at <strong>the</strong> begirm<strong>in</strong>g a£ <strong>the</strong> w<strong>in</strong>ter <strong>in</strong> an<br />
asea is eXUegK1y high or low, many mtroe or less birds may stay <strong>the</strong>re,<br />
<strong>in</strong>fluenc<strong>in</strong>g <strong>the</strong> numbees <strong>in</strong> O<strong>the</strong>r basu, where neth<strong>in</strong>$ <strong>in</strong> <strong>the</strong> fmed<br />
supply niisbt have changed.<br />
MIDWINTER NUMBERS OF GRNLAG GOOSE IN THE WESTERN SCHEUIT<br />
1976-1 987<br />
year<br />
Pigore 8 Evolution of <strong>the</strong> number of Oreylag Geee <strong>in</strong> tke Wesrersohelde
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