Knowledge gaps and areas for synthesis in experimental studies of context-dependent predation risk

A growing number of experimental studies address whether the anti-predator trait responses of prey are associated with costs to prey fitness (non-consumptive effects) or to the fitness of other species (trait-mediated indirect effects). Because ecologists remain keen to determine when these risk effects matter, there is a need to know how much of this literature has also evaluated the context-dependence of risk effects. Such knowledge would identify both understudied research directions and richly investigated areas whose synthesis could advance general understanding of predation risk effects. As a guide for ecologists, we reviewed publications from 1990 to 2021 that experimentally manipulated both predation risk (presence/absence of risk) and another aspect of biotic or abiotic condition (context). We classified each experiment according to a common scheme: the manipulated context was assigned to a Focus (predator, prey, resource, or environment) and Category (community, population, or individual for biotic contexts; temperature, water, habitat, or anthropogenic disturbance for abiotic contexts). A finer categorization of Specific Context (e.g., predator identity, resource abundance) was assigned within each Focus and Category. We also identified the System (marine, freshwater, terrestrial) and Venue (laboratory or field) of each experiment. We classified 433 manipulations (339 publications) falling within 49 different Specific Contexts. Many experiments (63%) manipulated aspects of the predator and/or prey Foci, independently and factorially. Additionally, several Specific Contexts have many published papers and could be suitable for synthesis, including predator and prey identity, predator diversity, prey and resource abundance, habitat structure, and temperature. Meanwhile, manipulations of resources of the prey are relatively lacking. Because most experiments (51%) featured freshwater systems, more marine and terrestrial experiments are needed. Regardless, an imbalance between laboratory (69%) and field (31%) experiments highlights the need for more field investigations. We discuss potential contributors to biases in the literature and consider how understudied contexts should be prioritized. Consequently, our review undergirds future work on the context-dependency of predation risk effects. As many biotic and abiotic contexts are impacted by anthropogenic activities, optimizing the integration of experimental approaches with other research efforts is crucial to furthering the development and application of predation risk theory.