Introduction
The idea that predators might influence prey non-consumptively by eliciting trait changes has a long history. Indeed, Darwin (1839) hypothesized that prey escape responses cost time and energy to maintain and, consequently, should attenuate in the absence of predators. Today, these predator-induced trait changes, or non-consumptive effects (NCEs), have a strong conceptual basis (Charnov et al . 1976; Lima & Dill 1990; Lima 1998, Creel & Christianson 2008) and are thought to rival or even exceed direct predation in terms of their impacts on prey populations and ecosystems (Kotler & Holt 1989; Peacor & Werner 2001; Werner & Peacor 2003; Schmitz et al . 2004; Preisser et al . 2007). Once the purview of laboratory and short-term field experiments involving small-bodied taxa (Kotler 1984; Preisser et al . 2005; Weissburg et al . 2014), NCEs and their broader consequences are increasingly being explored in large vertebrate systems (e.g., Dill et al . 2003; Willems & Hill 2009; Burkholderet al . 2013; Middleton et al . 2013; Basille et al . 2015; Moll et al . 2016; Le Roux et al . 2018; Courbinet al . 2019; Smith et al . 2019; Valeix et al . 2019). This expansion has shed new light on the extent to which NCEs scale up to communities of larger-bodied species. Yet, it has also drawn more attention to contingencies in NCEs. Such contingencies currently defy a coherent explanation, underscoring the need for standardized methodology for evaluating these phenomena across species and environmental contexts (Ford & Goheen 2015; Prugh et al . 2019) and conceptual clarity (Schmitz et al . 2017a; Gaynor et al . 2019) to guide research.
A growing literature suggests that contingency in NCEs hinges on key properties of the organisms involved as well as the environments in which they interact (Preisser et al . 2007; Heithaus et al . 2009; Wirsing et al . 2010; Creel 2011; Schmitz & Trussell 2016). Accordingly, there have been several recent calls for these properties to be characterized and incorporated into a general framework for predicting the nature and consequences of NCEs within ecological communities (e.g., Cresswell 2008; Heithaus et al . 2009; Creel 2011; Moll et al . 2017). Here, to facilitate the development of such a framework, we (i) conceptualize the multi-stage process by which predators may trigger direct and indirect NCEs; (ii) review key drivers of context dependence in NCEs; and (iii) quantify the extent to which prey trait plasticity in response to predation risk hinges on prey energetic state to demonstrate how considerations of contingency are critical to understanding the role of NCEs in complex communities. We then (iv) conclude with a synthesis and prospectus for future work. Our review spans aquatic and terrestrial ecosystems, addresses invertebrates and vertebrates, and focuses on a prevalent form of prey trait plasticity that is often implicated in the transmission of NCEs,anti-predator behaviors . We emphasize, however, that many of the sources of context dependencies that we address likely also apply to other forms of predator-induced trait modification (e.g., prey development, morphology, and physiology).