The effect of climate warming on future community composition is expected to be contingent on competitive outcomes, yet we currently lack mechanistic ecological understanding of how temperature affects competitive ability. Here, we combine resource competition theory with metabolic scaling theory and test hypotheses about how the temperature dependence of competitive ability changes with temperature. We find that the minimum resource requirement for growth, R* - an inverse indicator of

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... ve ability in phytoplankton - changes with temperature following a U-shaped pattern in all four species tested. The shape of temperature-dependence of competitive ability is systematically different from the temperature-dependence of population growth rates, both in our experiments and in collated data from previous studies. Our results suggest that exploitative competitive success is highest at temperatures that are sub-optimal for growth and declines rapidly at both cold and warm ends of the thermal performance curve.