Synthesis
Woven together, the ‘hunting mode-habitat domain’ and ‘evasion
landscape’ concepts produce a general framework for predicting the
nature and strength of NCEs on prey behavior during phase two. This
framework predicts that four different patterns of anti-predator
behavior can emerge depending on the degree of overlap between the
habitat domains of the interacting predator and prey species and spatial
variability in the efficacy of the prey’s evasion strategy (Fig.
4 ). NCEs manifest in all four scenarios and are expected to be
especially strong in three of them.
When predator and prey exhibit narrow, overlapping habitat domains, and
consequently encounters between them should be common, prey individuals
are predicted to chronically invest in anti-predator countermeasures
(Fig. 4a ). The nature of the investment, however, should depend
on the prey’s evasion landscape. If the prey’s evasion landscape is
spatially heterogeneous, enabling modification of the probability of
surviving an encounter situation, then it should be both chronically
vigilant and use space in a way that promotes the efficacy of its
evasion strategy (e.g., by seeking backgrounds against which it is more
camouflaged or, if the landscape lacks physical structure, grouping with
conspecifics) (scenario one). If its evasion landscape is homogeneous,
whereby the effectiveness of its evasion tactic is independent of
location, then the prey individual should be chronically vigilant but
only engage in evasion behavior such as fleeing when perceived risk is
elevated (i.e., from an encounter situation up to an attack; scenario
two). Risk effects and cascading indirect NCEs (phase three) under both
of these scenarios are expected to be strong given the opportunity and
energetic costs (Creel & Christianson 2008) and persistent changes to
prey foraging and distribution resulting from chronic defensive
investment.
When facing a predator with a narrow domain, prey individuals with broad
domains should seek predator-free space via spatial shifts (scenario
three; Fig. 4b ). These shifts should be chronic, given the high
potential for encounters associated with use of the predator’s domain,
and independent of the prey individual’s evasion landscape because
avoidance of predators in space obviates the need for escape behaviors.
This scenario should give rise to substantial risk effects and cascading
indirect NCEs because of marked increases in intra-specific competition
(e.g., from crowding in predator-free space) and changes to prey
distribution accompanying chronic predator avoidance.
When a narrow prey domain falls within a broader predator domain, the
predator should converge on the prey species, leading to high encounter
rates (Fig. 4c ). Under these circumstances, prey individuals
whose evasion landscape is heterogeneous should invest chronically in
vigilance and use space in a way that facilitates their evasion strategy
(scenario one), whereas those with homogeneous evasion landscapes should
exhibit chronic vigilance and engage in evasion behavior only when
perceived risk is heightened (scenario two). Risk effects and indirect
NCEs under these conditions are probable. However, the degree to which
predators converge on prey should depend on the relative importance of
the prey in question to the energy budget of the predator. Hence,
relatively simple predator-prey systems or situations in which the prey
species is highly profitable to the predator should produce the
strongest NCEs.
When predator and prey share broad, overlapping domains, encounters
should be infrequent (Fig. 4d ). Given that anti-predator
investment is not expected if the likelihood of predator-induced
mortality in the absence of countermeasures is low (Peacor et al .
2013), joint investment in vigilance and evasive behavior is predicted
under these circumstances (scenario four) only when the immediacy of
perceived risk is elevated (i.e., a predator has been encountered)
irrespective of the evasion landscape. Accordingly, CEs should
predominate under this scenario, with ephemeral NCEs emerging as the
result of a temporally dynamic landscape of fear for the prey.
In all scenarios, individual prey responses will be contingent on their
defensive repertoire and state. For example, prey individuals relying
exclusively on resistance, or that are constitutively defended, should
invest minimally in behavioral countermeasures no matter how immediate
the perceived risk cue is, save perhaps during an attack. Similarly,
prey individuals that are naïve to predators or in compromised
nutritional condition lack the experience, capacity, or incentive to
respond behaviorally to perceived risk. Thus, well-defended populations
or those with constrained opportunities for anti-predator investment
(e.g., by low food supply; Bolnick & Preisser 2005) should be subject
primarily to CEs.