Ecological theory of predator-prey dynamics is substantially based on the Rosenzweig-MacArthur equations, which assume logistic growth of the prey in the absence of the predators. This implies that the predators do not influence the available amount of the resource which is limiting prey growth. This expectation is likely to be valid under light or space limitation. However, it does not hold under nutrient limitation, because both, predators and prey, contain nutrients stored in their biomass that are no longer freely available for uptake by the prey. Hence, the predators affect their prey negatively in two ways: by consumption and by emergent competition. We investigated the impact of this emergent nutrient competition on the stability of predator-prey dynamics and on the resulting likelihood of two predators coexisting on a single fluctuating prey through relative non-linearity in their functional responses. We further evaluated the influence of nutrient competition between predators and prey on the biomass distribution among the different species, and on the strength of trophic cascades in multi-trophic food web models.
Results/Conclusions
Emergent nutrient competition between predators and prey substantially influences the stability of predator-prey dynamics and the biomass distribution in multi-trophic food web models. In contrast to light or space limitation, predators and prey compete for the limiting resource if the prey is nutrient-limited, which strongly stabilizes predator-prey dynamics. As a result, the likelihood of predator coexistence on a single fluctuating prey species through relative non-linearity in their functional responses is substantially reduced. Therefore, fluctuation-dependent mechanisms of predator coexistence are more likely to occur under light or space limitation of the prey. Furthermore, nutrient competition between predators and prey promotes bottom-heavy biomass distributions and dampens trophic cascades in multi-trophic food web models, because the energy transfer efficiency towards higher trophic levels is reduced. Consequently, the energy transfer efficiency and the top-heaviness of biomass distributions in multi-trophic systems is likely to be higher under light or space limitation of the prey. Overall, our results uncover how the widely used Rosenzweig-MacArthur predator-prey equations do not reflect nutrient competition between predators and prey, when the prey is nutrient-limited. Hence, we recommend to use food web models with explicit nutrient dynamics when modelling natural food webs, given that autotrophs in natural systems are frequently nutrient-limited.