The nonlinear effect of increasing cannibalistic predator density on heterospecific prey predation

In cannibalistic interactions, the same chemical cue may signal either threat or hunting opportunity, depending on the receiver's perspective. In this study, we conducted a series of behavioral experiments to assess how varying concentrations of kairomones and alarm cues from injured conspecifics influence predation pressure that cannibalistic dragonfly larvae Brachytron pratense exert on heterospecific prey (Daphnia magna). Exposure to all chemical cue treatments increased larval mobility, but the response was non-linear: at low concentrations of conspecific cues, larvae displayed increased activity within refuges, leading to reduced hunting efficiency and prey consumption-suggesting a self-defensive behavioral response. In contrast, higher cue concentrations prompted more exploratory movement outside refuges without a corresponding increase in heterospecific prey capture. These findings suggest that B. pratense modulates foraging strategy in response to conspecific chemical cues, prioritizing cannibalistic interactions over heterospecific predation. This shift leads to density-dependent, nonlinear predation pressure and highlights the overlooked indirect effects of predator cannibalism on predator-prey interaction dynamics.