Similar conditions with opposite effects: Predation-risk effects on prey abundance are highly contingent

Experiments have shown that predation risk effects on prey fitness can be highly contingent on environmental conditions, suggesting a potential difficulty in generalizing risk effects on prey abundance in natural settings. We examine this problem with a novel approach. Rather than study the influence of a particular controlled factor, we examined the influence on risk effects of study conditions that would likely be deemed inconsequential, i.e., to factors incidental to a study. If such incidental factors influence can influence the magnitude and even direction of risk effects, this suggests risk effects on prey population density are profoundly influenced by context in natural communities. We compared results of multiple experiments conducted under similar conditions, objectives, measurables and implementation, and which captured much of the complexity of natural systems (e.g., they were performed with diverse prey assemblages (≥11 taxa) over multiple prey generations). There was consistently no risk effect of fish on some zooplankton prey abundance, but great variability effect on other prey including combinations of negative, absent or positive effects on the same prey’s abundance. We review mechanisms that could underlie these results. Our findings highlight the need to understand the mechanisms linking trait responses to fitness and, ultimately, to abundance of prey, to understand risk effects across studies and systems.