DECLINE IN POPULATION-DENSITY WITH INCREA SING ECOSYSTEM PRODUCTIVITY- ANALYSIS OF DIAPHANOSOMA-BRACHYURUM (CRUSTACEA, CLADOCERA) DYNAMICSIN LAKES OF DIFFERENT TROPHIC STATUS

The non-selective predation (''top-town'' control) is usually consider ed as the main mechanism underlying shifts in the species composition of natural animal communities along productivity gradients. The goal o f the study is to test an alternative hypothesis that pronounced fluct uations of resources in productive habitats influence the community in a similar way suppressing the dominants of low-productive systems. Th e data on population dynamics of cladoceran species Diaphanosoma brach yurum from five lakes differed in primary production were used. An ind irect assessment of predation impact and variability in resource suppl y includes ten time series of midsummer changes in population birth an d death rates. Both factors are revealed to be responsible for lowerin g the population abundance in productive lakes. The effect of predatio n is partially compensated by an increase in the birth rate level. By contrast, resource fluctuations depress the abundance more strongly be cause periodical resource depictions simultaneously reduce the birth r ate and increase mortality. Destabilization of the planktonic communit ies in fertillized lakes can lead such a population to extinction. The role of resource fluctuations in limiting tile species diversity of p roductive ecosystems is discussed.