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Abstract

Environmental conditions in a lake vary on different time scales, from years to minutes. Organisms faced with environmental changes, which are ecosystem properties, must evolve survival strategies in order to persist in such a system. Predictability and velocity of environmental changes determine the most profitable (in terms of fitness) strategies. In response to predictable environmental variations, organisms can evolve fixed life-history traits; unpredictable changes should favor the evolution of phenotypic plasticity. However. recurrent moderately predictable conditions can also lead to shifts in genotypic frequencies in a population. Different strategies can co-exist if the environment fluctuates at varying time scales simultaneously. Daphnia is used as a model organism to explain various adaptations to changes in bottom-up and top-down forces. They live in a nutritionally dilute environment, thus reactions to sudden changes in food resources are essential. Phenotypic plasticity of the filter screens is used to optimize energy input. On the other hand, daphniids are subject to strong, size selective predation pressure by fish and invertebrate predators, which require different defenses. The relative importance of these predators and, thus, the optimum defense strategies vary during the season. Daphniids react to changes in predator pressure with life-history changes, e. g. variable size at first reproduction (SFR). ClonaI replacement. phenotypic responses to specific predator signals, and maternal effects determine the seasonal shift of SFR in the lake. Experiments in large indoor mesocosms are reported to demonstrate the interaction of phenotypic and genotypic responses in the expression of diel vertical migration (DVM), a behavioral strategy of avoidance of fish predation. Daphniids can gain the same fitness from either migrating deeper during the day or maturing at a smaller size. It is concluded that natural selection, although it must act on individuals, creates ecosystem properties through shaping the characteristics and interactions of populations