The many faces of fear: A synthesis of methodological variation in characterizing predation risk from carnivores

1. Predators affect prey by killing them directly (lethal effects) and by inducing costly antipredator behaviors in living prey (risk effects). Recent research in carnivore-ungulate systems has shown how risk effects can strongly influence prey populations and cascade through trophic systems. A crucial prerequisite for assessing risk effects is characterizing the spatiotemporal variation in predation risk.
2. Carnivore-ungulate risk effects research has experienced rapid growth. However, preliminary assessments of the resultant literature suggests that researchers characterize predation risk using a diverse variety of techniques. This methodological variation complicates inference about risk effects and confounds comparability between studies due to an evident lack of clear benchmarks.
3. We couple an extensive literature survey with a hierarchical framework, developed from established theory, to quantify the methodological variation in characterizing risk from carnivores.
4. We detected substantial variation in methods characterizing risk from carnivores, with 243 metrics of risk from 141 studies falling into at least 13 distinct subcategories within 3 broader categories. Most studies characterized predation risk in relatively simplistic terms, often using a single metric to represent risk. We also documented a strong focus in the literature on a specific trophic interaction (wolf Canis lupus – elk Cervus elaphus).
5. Our synthesis suggests that the gaps in our understanding of carnivore-ungulate risk effects are due, at least in part, to the methodological variation in characterizing predation risk and an overarching research focus on wolf-elk systems. We provide recommendations to guide future work, including calls to evaluate risk effects related to a greater diversity of carnivore species and for studies to strategically characterize risk so that key, unifying hypotheses regarding carnivore-ungulate risk effects can be adequately tested.