An integrated framework for unifying our understanding of nonconsumptive predation risk effects

Andrew Thomas Davidson, Tal Avgar , Daniel MacNulty, Scott D Peacor

Predation risk can induce risk-induced trait responses (RITRs) – changes in prey defensive traits including behavior, morphology, life history, and physiology – thought to have profound effects on prey fitness and population dynamics (termed ‘nonconsumptive effects’). Yet, predicting the magnitude of RITRs and their fitness consequences remains difficult because outcomes depend heavily on ecological context and processes that influence the costs and benefits of defensive trait changes costs, all of which vary greatly. To address this, we present the Integrated Risk-Effects Framework (IREF), an intuitive graphical framework that incorporates fitness components related to the benefits and costs of the RITR to identify the optimal trait value and costs to prey fitness (NCEs) and effects on predation mortality (termed ‘consumptive effects’, CEs). The IREF is designed with the objectives of 1) guiding researchers in explicitly and quantitatively describing the biology underlying risk effects; 2) helping generate process-based, theory-driven hypotheses about the occurrence and magnitude of risk effects; and 3) using standardized and intuitive structures and vocabulary to communicate and synthesize hypotheses about risk effects across systems. Applying the framework to canonical cases (prey displacement and refuge use), we show that it recovers classic expectations (e.g., effects of predator hunting mode) while generating new, testable predictions. Specifically, we demonstrate that 1) the prey’s resource-processing rate shifts optimal defensive investment and the relative contribution of NCEs versus CEs to fitness outcomes; 2) prey satiation can transform predicted refuge use from boundary solutions to intermediate optima; 3) different factors that increase predation risk can move trait optima in opposite directions; and 4) in the same system, a predator density increase may lead to no, to large, to negligible increases in RITRs and NCEs, depending on initial predator density. By unifying trait costs, predation risk, and fitness consequences within a common graphical structure, the IREF provides a transparent tool for synthesizing risk effects and generating mechanistic hypotheses across ecological contexts.