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) and may rival or even exceed direct predation in terms
of their impacts on prey populations and ecosystems (Kotler & Holt
1989; Peacor & Werner 2001; Schmitz et al . 2004; Preisseret 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., Willems & Hill 2009; Burkholder et al . 2013; Basilleet al . 2015; Moll et al . 2016; Le Roux et al . 2018;
Smith et al . 2019; Valeix et al . 2019). This expansion has
shed new light on how NCEs manifest in communities of larger-bodied
species. Yet, it has also revealed the challenges associated with their
prediction in the field, 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 (Peacor et al . 2013; 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 (e.g., Preisser et al . 2007; Heithauset al . 2009; Creel 2011; Schmitz & Trussell 2016). Accordingly,
there have been several recent calls for these properties to be
characterized and leveraged to improve our understanding of the nature
and consequences of NCEs within ecological communities (e.g., Cresswell
2008; Creel 2011; Peacor et al . 2013; Moll et al . 2017).
Here, we address this need by: (i) conceptualizing the multi-stage
process by which predators may trigger direct and indirect NCEs; (ii)
reviewing key drivers of context dependence in NCEs; and (iii)
synthesizing these drivers into a general framework for predicting both
the nature and strength of direct NCEs. We then (iv) conclude with a
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).