Persistent effects of experimental heatwave duration on population dynamics

Heatwaves are becoming longer, more frequent and more intense under climate change. However, we still have a limited understanding of the longer term (multi-generational) impacts of heatwaves, and of the demographic mechanisms that underly these effects. To address these gaps, we conducted an experiment using a model system of flour beetles (Tribolium castaneum) that tested the effects of heatwave duration on population dynamics. We subjected replicate beetle populations to a single 38°C heatwave event of one of four durations (0, 5, 10 or 15 days) and then tracked population sizes, birth rates and mortality rates for 7 months (approx. 7 generations). We found that, initially, heatwaves reduced population growth rates by temporarily lowering birth rates (via reduced fecundity), but that later in the experiment, the impact of heatwaves on population dynamics occurred via increased mortality rates. Together, these demographic effects resulted in a sustained suppression of population sizes throughout the experiment, as populations exposed to longer heatwaves (10-day and 15-day) experienced reduced carrying capacities for the duration of the experiment. Our findings indicate that heatwaves can have persistent negative impacts on population dynamics that could be underestimated by focusing solely on short-term responses.