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 such life history traits affect consequences of priority effects in seasonal systems remains understudied. Here, we test how a key aspect of life history, numbers of generations in a growing season, interact 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. The early 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 more generations in a season led to smaller ranges of the calculated stabilization potentials and fitness differences between species and more positive stabilization potentials, indicating that overlapping generations buffered interactions against strong priority effects from different arrival times. These results suggest that effects of phenological shifts (e.g. due to climate change) on species coexistence mechanisms are contingent on the life-histories of interacting species.