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