The secret lives of cannibals: Modelling density-dependent processes that regulate population dynamics in Chaoborus crystallinus

Population regulation is mainly driven by density-dependent processes. In many species, mortality caused by cannibalism is an important density-dependent factor, which has often been overlooked when investigating population dynamics. In this study, we want to show the significance of cannibalism in regulating the population densities of the phantom midge, Chaoborus crystallinus. To this end, an integrative approach was used that combined experimental data with an individual-based population model of C. crystallinus. In laboratory experiments, density-dependent cannibalism rates of first and fourth larval stages preying on first instar larvae were quantified. The model was parameterised with laboratory and mesocosm data and subsequently validated using independent, outdoor semi-field experiments. Thus, population dynamics and structure of C. crystallinus could be accurately simulated at different temperatures and food regimes. The comparison of simulated to measured population dynamics in outdoor mesocosms revealed that cannibalism was mainly responsible for the high overall mortality (>80%) in C. crystallinus populations when using cannibalism rates for the simulations measured in the laboratory. Our results suggest that cannibalism also acts as a density-dependent compensatory mechanism by regulating population dynamics at higher larval densities and reducing population vulnerability at lower larval densities.