Cannibalism has its limits in soil food webs

Cannibalism imperfectly recycles resources back to the same species and so decreases trophic transfer efficiency in food webs. As such, viable populations have some limit on how much of their diet can come from cannibalism. We applied a Lotka-Volterra model to derive a theoretical maximum for the proportion of the diet coming from cannibalism. This proportion is set by the food conversion efficiency for both cannibalism and alternative prey. We apply the result to sixteen published soil food web models and find that cannibalism cannot exceed 20% of the diet of most organisms, which includes eating conspecifics that were already dead. However, predators can show a strong (>80%) preference for cannibalism because encountering conspecifics is rare. Cannibalism increased carbon and nitrogen mineralization in fifteen soil food webs and had non-monotonic effects in the remaining one. Our estimates map a physiological parameter (conversion efficiency) to an ecological one (cannibalism) to help to improve model fit and to help soil ecologists identify taxa where cannibalism may be most important.