Properties of the predator
The means by which predators capture their prey, or their hunting modes, are a pervasive driver of context dependence in NCEs (Preisser et al . 2007). Hunting predators, and their prey, are also characterized by a habitat domain, or the spatial extent over which individuals move while foraging (Schmitz 2005; Schmitz et al . 2017a). Together, these properties form the ‘hunting mode-habitat domain concept’, which aims to explain spatiotemporal contingency in the nature of predator-prey interactions. It can predict how foraging predators and prey should interact during the three phases as a consequence of contingencies in their spatial movement and overlap, the nature of which depends on how prey respond to the threat of predation across space.
Habitat domain size appears to be consistent among predators with similar hunting modes (Miller et al . 2014). At one extreme of a continuum, actively roaming/coursing predators typically have large habitat domains; at the other, sit-and-wait/ambush predators usually exhibit smaller domains. Notably, predators may switch hunting modes (Helfman 1990; Olson & Eklov 2005; Donihue 2016), which can change space use, habitat domain size, and contingency in the nature of interactions. Smaller prey may forage locally, whereas larger prey may roam widely depending on their forage requirements in relation to the distribution of plant (or other resource) quality and productivity (Haskell et al . 2002), creating contingency in prey movement and habitat domain size. Further contingencies could arise if prey have different habitat domain sizes as they adjust their movement behaviours to the type of predator they face (Fischhoff et al . 2007; Merrillet al . 2010; Miller et al . 2014).
The spatiotemporal nature of predator-prey movement and overlap may determine prey perception of predation risk (phase one). Sit-and-wait predators, by remaining sedentary in fixed locations, create a continuous presence within a narrow habitat domain (Schmitz 2007; Schmitz et al . 2017a). Consequently, prey facing sit-and-wait predators may have a heightened perception of risk because of the persistent point-source cue of predator presence. Actively hunting predators roam widely and thereby often produce diffused, moderate cues in any given location within their broad habitat domain, resulting in lower perception of risk by prey (Schmitz 2007). Consistent with this framework, Murie & Bourdeau (2019) observed that herbivorous snails (Tegula funebralis ) altered their distribution in an intertidal ecosystem in response to the purple sea star Pisaster ochraceus , which moves slowly within a narrow domain producing an acute and spatially localized acute risk signature. These snails did not alter their distribution when exposed to crab and octopus predators that hunt actively within larger domains and generate diffuse risk profiles. Hence, relative to sedentary predators occupying narrow habitat domains, active predators with large domains may be less likely to initiate direct and indirect NCEs that play out during phases two and three (e.g., Schmitz 2008).
Prey responding to predator cues (phase two) must weigh potentially considerable opportunity costs, in terms of energy and nutrient intake (up to 25% of daily energy expenditure: Schmitz [2005]) and survival, of remaining continuously vigilant given the likelihood of encountering and being captured by a predator. Thus, prey occupying landscapes with sit-and-wait predators may accept those costs and respond with chronically heightened apprehension. This response could involve heightened vigilance at the expense of reduced foraging, or seeking safety in refuges, or both, depending on the sizes of their habitat domain relative to their predator’s (Schmitz 2005). Alternatively, prey facing active hunting predators may encounter predators infrequently. Under these circumstances the prey should not be chronically apprehensive and incur a large energetic penalty. Rather, prey under these conditions should react acutely to imminent risk by simply evading predators upon encounter (Schmitz 2005). There is evidence that these divergent phase two scenarios can govern the nature of indirect NCEs in phase three. For example, Schmitz et al . (2017b) found that chronic avoidance of sit-and-wait spider predators by grasshoppers increased plant diversity while decreasing soil carbon retention, whereas a predator guild dominated by actively-hunting spiders failed to elicit grasshopper anti-predator behavior and, consequently, did not indirectly affect plant composition and soil carbon via a non-consumptive pathway.
Whether or not a predator-prey interaction during phase two is largely consumptive or non-consumptive will depend on the relative habitat domain sizes of predators and prey (Schmitz et al . 2004; Schmitz 2005). There are at least four contingencies that can arise, with non-consumptive effects being predominant in three of the four. Whenever prey and predator have overlapping, narrow habitat domains, prey will respond with chronic vigilance. Prey with narrow habitat domains that face widely roaming predators with broad habitat domains will likewise be chronically vigilant. Prey with broad habitat domains should seek refuge by shifting habitat use when facing predators with a narrow habitat domain. Finally, when prey and predators both have broad habitat domains, prey are less prone to exhibit habitat shifts or chronic vigilance, in which case consumptive effects (phase two) and their indirect consequences (phase three) should predominate (Schmitz et al . 2004; 2017a).
Predator state is also a factor that can shape NCEs. State variation can drive differences in a predator’s detectability (Scherer & Smee 2016) and motivation to seek (i.e., its activity and, consequently, spatiotemporal pattern of cue generation) and/or successfully attack (i.e., its lethality) prey (Brown & Kotler 2004; Brown et al . 2016). This variation can range from being highly dynamic, as when predator hunger elicits increased foraging activity (Hooten et al . 2019), to persistent, as when aggressive behavioral types are more likely to attack prey given an encounter (e.g., Michalko & Řežucha 2018). During phase one, dynamic changes to a predator’s feeding states can change how detectable it is to prey that rely on cues from depredated conspecifics as signals of danger. For instance, mud crabs (Panopeus herbstii ) detected and responded to predatory blue crabs (Callinectes sapidus ) that had recently been fed a mud crab diet at a greater distance than food-restricted blue crabs (Weissburg & Beauvais 2015). By implication, predator populations that rely on such prey species may be more likely to initiate NCEs that cascade through to phase three. In the Weissburg & Beauvais (2015) study, blue crabs that had fed recently on mud crabs indirectly reduced consumption of a basal resource (oysters) by mud crabs to a greater degree than their hungry counterparts.
During phase two, dynamic predator state changes can influence the strength of anti-predator responses by prey over short intervals (e.g., when hungry predators are perceived as more threatening; Box 1 ). Thus, prevalence of certain states within predator populations (e.g., compromised energetic state) could drive changes to overall prey risk taking that are large enough to affect propagation of indirect NCEs during phase three. More persistent differences in predator state can give rise to marked inter-individual variation in the anti-predator behavior induced by predators during phase two (Sih et al . 2012). For example, Winandy & Denoël (2015) found that goldfish (Carassius auratus ) with aggressive temperaments elicited greater reduction in newt (Lissotriton helveticus ) foraging than less aggressive conspecifics. By implication, the temperamental mix of predator populations could influence the nature of prey defenses during phase two and the likelihood of cascading NCEs in phase three.