In African savannas, are donor and trophic control of ungulate prey coupled by apparent competition?

Understanding how donor (bottom-up) and trophic (top-down) modes of population control shape food web structure and dynamics has long been a major goal of ecology, yet consensus about mechanisms is lacking. Two prevalent patterns hint at generality in mechanisms that shape predator-prey communities. First, within communities, herbivore biomass declines and plant biomass increases in the presence of predators, regardless of ecosystem productivity, evoking a trophic cascade. Second, across communities, predator biomass density increases sublinearly with prey biomass density, a ‘power law’ which often is assumed to arise from donor control. We show how both patterns can emerge simultaneously, using data from ungulate assemblages in African savannas. Within three savannas where mechanisms underlying trophic dynamics are understood (the Greater Serengeti ecosystem, Kruger National Park, and the Laikipia highlands), prey biomass is dominated by one or a few ungulate species that are donor controlled, yet they support most of the predators. The same predators also consume less abundant prey species that are trophic controlled. In effect, donor and trophic control of prey are coupled by generalist predators via ‘non-reciprocal apparent competition’ (NRAC). Within 56 African savannas where ungulate biomass densities but not dynamics are known, mean ungulate biomass rankings resemble those of Serengeti, Kruger, and Laikipia, and are therefore consistent with the hypothesis of NRAC. Under NRAC, total prey biomass declines within systems because trophic-controlled prey are suppressed by predators. Among systems, power law patterns emerge from the dynamics of dominant prey that are donor controlled. Given that coupled donor and trophic control can occur wherever there are generalist predators, NRAC is a candidate mechanism contributing to the prevalence in nature of both trophic cascades and predator-prey power laws.