Strengthening of negative density dependence mediates population decline at high temperatures

Temperature affects key life history traits of ectothermic individuals from metabolic rates to foraging behavior. However, any attempt to scale up temperature effects on individual-level processes to population-level consequences must assume a relationship between temperature and density dependence. Yet, we still know surprisingly little about how this relationship looks in nature, especially in heterotrophs. Here we experimentally isolated and quantified the thermal response of density-dependence, intrinsic rate of increase, and carrying capacity from population dynamics of the herbivore Daphnia pulex started with different densities and raised at four different temperatures, spanning most of the biologically relevant temperature range. We found that the strength of negative density dependence increased linearly with temperature, doubling every 7°C. Importantly, this increase continued beyond the thermal optima of the intrinsic growth rate. This differential response results in a unimodal relationship with peak carrying capacity at moderate to high temperatures. The results reveal that the population density decline at high temperatures is largely driven by the strengthening of negative density dependence relative to growth rates. Overall, the results provide a sorely needed test of theory and highlight the importance of accounting for thermal responses of indirect effects such as density-dependence that only emerge at that population level when forecasting the effects of global warming.