A compromise between speed and efficiency of energy conversions gives the maximum power of useful energy conversion at intermediate efficiencies. Organisms are selected to maximize the power of energy conversions to a useful form. However, most species have very little capacity to vary the efficiency of their energy conversions in response to variation in the intensity of the environmental power supply. Plants can respond slowly to horizontal variation in the availability of energy by growth. The trade-off between power and efficiency which is dependent on the compromise between speed and efficiency of energy conversions does seem to apply to the relative efficiencies of successive species in a sere of secondary terrestrial plant succession. The application of the power trade-off to species in a sere predicts the common general properties of growth and reproduction in succession. The power trade-off may also help to explain reproductive patterns in animals that differ in the concentration of their food supply. The thermodynamic basis of causation provided by the power trade-off could be a valuable tool for connecting evol utionary ecology with community and ecosystem studies.