Potential Effects of Climate Warming on Density‐Dependent Growth and Mortality in a Cannibalistic Ectotherm ( Sympetrum vulgatum : Odonata)

To better understand how climate warming affects population dynamics, empirical studies are needed that examine how temperature, density‐dependent competition, and their interaction influence growth and mortality. In predatory aquatic invertebrates, such competition often manifests as cannibalism, making them valuable model systems for investigating complex ecological responses to environmental change. We conducted a laboratory experiment over 6 weeks to characterise the growth and mortality of larvae of the dragonfly Sympetrum vulgatum in response to intraspecific density (1, 2, 4 and 10 larvae per microcosm) and temperature (20°C and 26°C). We found strong negative density‐dependent effects on both growth and survival. Cannibalism was the primary cause of mortality and thus the main factor influencing survival, whereas non‐cannibalistic mortality was minimal. Temperature had a positive effect on growth but negatively impacted survival. A significant interaction between temperature and week indicated that the difference in growth rates between temperatures decreased over the experimental period. A significant three‐way interaction among temperature, density, and time suggested that these factors influence growth rates in complex, dynamic ways. Interestingly, no three‐way interaction was observed for cannibalism, indicating that this behaviour may be driven primarily by density rather than by temperature or time. Our results highlight the importance of density‐dependent interactions, particularly cannibalism, in shaping population responses to environmental change. To improve predictions of responses to warming, models should incorporate both abiotic factors including temperature and biotic factors such as intraspecific competition. This might be especially crucial for predatory invertebrates including aquatic insects.