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Phenological shifts can alter the relative arrival time of competing species in natural communities, but predicting the consequences for species interactions and community dynamics is a major challenge. Here we show that differences in relative arrival time can lead to predictable priority effects that alter the outcome of competitive interactions. By experimentally manipulating the relative arrival time of two competing tadpole species across a resource gradient, we found that delaying relative arrival of a species reduced the interaction asymmetry between species and could even reverse competitive dominance. However, the strength of these priority effects was contingent on the abundance of the shared resource. Priority effects were generally weak when resources were limited, but increased at higher resource levels. Importantly, this context dependencycould be explained by a shift in per capita interaction strength driven by a shift in relative body sizes of competitors. These results shed new light into the mechanisms that drive variation in priority effects and help predict consequences of phenological shifts across different environments.