Apparent competition couples donor and trophic control among prey of generalist predators

NICHOLAS GEORGIADIS , Douglas N Kamaru, Alfred K. Kibungei, Jacob R Goheen

Apparent competition refers to an indirect interaction between prey populations that affect one another's abundance through shared predators. Classically, prey are reciprocally coupled, such that each can sustain and be limited by predators via trophic (top-down) control. Here, we focus on a less-examined but potentially widespread form involving primary prey that are donor (bottom-up) controlled. These sustain predators but are not regulated by them; instead, their dynamics are governed mainly by donor processes. Consequently, the indirect interaction between prey populations becomes non-reciprocal. We refer to this as Apparent Competition via Donor Control (ACDC). Under ACDC, predation mortality is largely compensatory for primary prey but additive for secondary prey. Primary prey therefore function less as victims than as resources that maintain predators, which suppress secondary prey.

We explore the occurrence of ACDC using four qualitative criteria to diagnose 20 candidate ACDC systems from the literature. These systems span terrestrial, freshwater, and marine habitats and feature diverse taxa, suggesting that ACDC can arise across a wide range of ecological settings. Some cases had previously been described as subsidy-driven predation, hyperpredation, or disturbance-mediated apparent competition, highlighting donor control as potentially a shared causal mechanism. Thus, empirical examples of ACDC may have long been available, but primary prey were not always diagnosed as donor controlled. 

For systems with suitable population data, we then derive a diagnostic model framework to test whether dynamical consistency with alternative control regimes can be assessed from prey and predator population trajectories. Applied to a recovering African large-mammal community, the fitted models classified the primary prey (plains zebra) as donor controlled in all prey pairings, while four of eight secondary prey populations exhibited dynamics consistent with trophic control, and thus ACDC.

Mechanistically, ACDC decouples energy flow from demographic impact in food webs: prey that sustain predators need not be the prey most strongly regulated by predators. A critical uncertainty is the extent to which mixed donor and trophic control modes within communities is due to ACDC operating among the prey populations of generalist predators. We hope this emphasis shifts attention to assessing the importance of ACDC in structuring food webs.