My neighbours drive me cannibalistic: New cannibalism mechanism reduces population growth

Background/Question/Methods

1. Cannibalism is often positively density-dependent. Two mechanisms have been cited commonly for generating an increase in cannibalism with increasing population density: intensified resource limitation and increased encounter frequencies between cannibals and conspecific prey.
2. Using the insect Geocoris pallens, we demonstrate a third mechanism that can underlie density-dependent cannibalism. We subjected focal female G. pallens to different densities of neighboring females (0, 1, 2, 3, 5) and recorded egg cannibalism rates of focal females after 48 hours. Effects of density-dependent cannibalism are tested in the field by establishing populations of G. pallens founded by interacting or non-interacting females (high, and low cannibalism respectively).

Results/Conclusions

1. An increase in the perceived density of conspecifics elicited a 3-20 fold increase in cannibalism by adult females of their own eggs. Cannibalism by adult females did not discriminate between own eggs and non-own eggs. The perceived density of conspecifics may be used as an indicator of egg kinship, mortality risk to eggs or future risk of starvation.
2. Increase in egg cannibalism rates due to interactions between conspecific females reduce population growth in the field by ~30%.
3. Strong density-dependent cannibalism based on perceived conspecific density could constrain the population growth of predators even when prey resources are not limiting. The information provided by perceived conspecific density, its effect on cannibalistic behaviour, and the resulting consequences for population dynamics are understudied and warrant more attention.