Consequences of phenological shifts are determined by the number of generations per season.

The order and timing of species' arrival in a community can often change species interactions. Termed priority effects, this phenomenon arises in communities with drastically different life histories (e.g., bacteria, insects, and amphibians), yet how their life history traits affect the consequences of priority effects in seasonal systems remains understudied. Here, we test how the length of one growing season, which can accommodate different numbers of overlapping generations, interacts with priority effects using two competing flour beetles, Tribolium castaneum and Tribolium confusum. We manipulated the sequence and timing of arrivals and collected all adults after one, two, or three generations, simulating phenological shifts in communities with different growing season lengths with less or more overlapping generations. The early-arriving species always reached a higher population, and in general, its per capita competition towards the late arriver was stronger, indicating priority effects. However, fitting competition models to the data revealed that for both species, longer experimental durations (which included more overlapping generations) led to less dramatic changes in competitive outcomes with differences in arrival time, indicating that overlapping generations of each species provided a buffer against strong priority effects from different arrival times. These results suggest that changes in the order and timing of species arrival (e.g., due to climate change) can impact species coexistence, but such effects could be contingent on the life history, such as the number of generations of interacting species, or abiotic factors, such as the duration of a growing season.