The complexity of a “simple” predator–prey system: non-trophic positive interactions generate unsuspected dynamics and dependencies

Abstract. In natural ecosystems, many species engage simultaneously in both trophic and non-trophic interactions (NTIs), influencing each other's population growth and patterns of local coexistence. However, in coastal marine systems, where the larvae of most benthic adults disperse and frequently settle into populations distant from their origin, populations do not experience feedback from local reproduction. This implies an apparent decoupling between local dynamics and regional-scale dispersal processes. Here, we explore the consequences of positive NTIs for the coexistence and dynamics of a predator and its prey. Inspired by two species studied in the Chilean intertidal zone, we developed a predator–prey model in which the prey also facilitates the recruitment of and provides refuge to the predator, while larval subsidies externally control the population growth of both species. The predator–prey dynamic was simulated at different levels of species recruitment, with and without NTIs. Overall, NTIs led to density dependence of the predator on the prey, coupling their abundances across varying levels of larval subsidies. Furthermore, the impact of NTIs on predator abundance was non-additive, with the magnitude of these effects depending on recruitment rates. In addition to determining population growth, recruitment rates also modulate the extent to which the predator is facilitated by the prey. These results suggest that incorporating NTIs into dynamic models and ecological theory is necessary for a more complete understanding of the mechanisms of species coexistence and spatial variability. This knowledge is critical for understanding ecosystem responses to ongoing climate and global changes.