Despite the plethora of studies examining predation risk effects that result from prey changing traits in order to reduce predation risk, it is not well understood whether predation risk effects alter prey populations (i.e., non-consumptive effects, NCEs) or indirectly affect populations the prey interacts with (i.e., trait-mediated indirect effects, TMIEs) in natural settings (i.e., those not manipulated experimentally). As such, it is unclear if in natural systems NCEs and TMIEs are inconsequential relative to consumptive effects of predators. We performed a comprehensive review of the literature to screen published evidence of NCEs and TMIEs in natural settings, and to categorize the approaches used to examine this problem. We developed a framework to differentiate studies by (a) the type of predation-risk effect examined, and (b) the nature of the field component aspect of the study. We identified all studies of NCEs and TMIEs that had a field component, and differentiated whether papers explicitly examined a surveyed field patterns of demography, abundance or density.
Results/Conclusions
Our review from 1990 to 2018 produced >450 papers that examined the consequences of trait (phenotype) changes to predation risk (i.e., NCE/TMIE), with a 20-fold increase in publication rate over the time period. One third included a field component. Very few of the NCE studies examined an unmanipulated field pattern of prey abundance (< 1%), or of prey fitness measures such as fecundity (~6%). Although a bit higher, there were also few TMIE studies that examined a surveyed field pattern. Rather, the great majority of studies examined factors that influence risk effects in more controlled settings with some form of manipulation e.g. they examined the influence of prey density, evolutionary history, predation hunting mode, etc. While these latter types of studies are important, we argue that they are disproportionately represented and that demonstrating the contribution of NCEs and TMIEs to field patterns is being relatively neglected. Future research needs to shift more towards examining the importance of risk effects on natural populations, and in particularly those that evaluate the influence of risk effects on surveyed field patterns. We review and discuss the strengths and weaknesses of different approaches used to evaluate the contribution of risk effects generally, and more specifically to surveyed field patterns. Developing such approaches is critical to advance this exciting sub-field in ecology forward in order that it can be applied to management and conservation challenges.