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).