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When herbivore abundance is controlled by predators there may be an indirect positive effect on primary producers due to reduced grazing pressure, but the potential of predation refuges to modify such trophic cascades has rarely been studied. By experimentally manipulating substrate particle size and fish predation regime, we assessed the outcome of invertebrate grazer–biofilm interactions in streams. Locations at the center of larger substrate particles were predicted to pose a higher predation risk, and therefore be subjected to a lower grazing pressure. In our 52-day experiment in a New Zealand stream, small-sized substrates (terracotta tiles) remained virtually free of periphyton across their entire upper surface, whereas a thick periphyton mat was formed across large tiles with only edges remaining free. In channels containing fish (either native Galaxias vulgaris or exotic Salmo trutta), grazing on tiles was lower than in the absence of fish. A preference for grazing near to the edge of tiles was clearest in fish channels but was also evident even in the absence of fish, probably reflecting fish presence and/or fish kairomones in the stream from where the colonizing invertebrates had been derived. Total grazer density was similar across treatments with or without fish, suggesting that our results can be explained mostly by changes in the behavior of grazers. We suggest that refuge availability, interacting with grazer predator-avoidance behavior, may produce a context-dependent patchwork of trophic cascades in streams and other ecosystems.

The freshwater opossum shrimp Mysis diluviana can undergo extensive diel vertical migration (DVM) to feed in shallow, prey rich strata at night. Bright moonlight limits their night-time migration presumably due to predator avoidance. Using a linked, foraging-bioenergetics model, we evaluated the cost of avoiding predators by simulating the effects of prey density, water temperature, and light intensity on daily feeding and growth of M. diluviana in Lake Pend Oreille, Idaho, USA. We found that when mysid distribution was not limited by moonlight intensity, simulated food consumption (10.3 J day−1) increased 1.6-fold compared to estimated consumption (6.1 J day−1) based on their observed, vertical distribution. Moreover, simulated growth of mysids (0.61 mg day−1) increased 74% compared to that estimated from observed distribution patterns (0.35 mg day−1), when they were located in deeper, darker strata. Given recent insights into partial DVM by M. diluviana, we note that proximate factors associated with predator avoidance in pelagic (light availability) and benthic (hunger level, body size and reproductive status) habitats may convey complimentary benefits to M. diluviana fitness by reducing predation mortality and increasing metabolic efficiency.

Despite recognition of the significance of both food web interactions and habitat complexity in community dynamics, current ecological theory rarely couples these two processes. Experimental manipulations of the abundance of the two predators in an oyster-reef trophic cascade, and the structural complexity provided by reefs of living oysters, demonstrated that enhanced habitat complexity weakened the strengths of trophic interactions. The system of tri-trophic interactions included oyster toadfish (Opsanus tau) as the top predator that consumed the mud crab (Panopeus herbstii), which preys upon juvenile oysters (Crassostrea virginica). On reefs of low complexity, toadfish controlled mud crab abundances and indirectly determined the level of mortality of juvenile oysters. The indirect effects of toadfish on oysters emerged through their influence on how intensely mud crabs preyed on oysters. Augmentation of habitat complexity by substituting vertically oriented, living oysters for the flat shells of dead oysters disrupted both of the direct trophic linkages but did not alter the magnitude of the indirect effect of toadfish on juvenile oysters. This paradox can be understood by partitioning the mechanisms by which toadfish influence mud crabs and ultimately juvenile oysters. Trait-mediated indirect interactions (TMIIs; i.e., predator-avoidance behavior in mud crabs) accounted for 95.6-98.2% of toadfish indirect benefits to oyster survival and, consequently, were a much greater contributor than density-mediated indirect interactions (DMIIs; i.e., the reduction in crab abundance by toadfish). Avoidance behavior was unaffected by modification in habitat complexity. Complex reefs increased total oyster survival because added habitat complexity reduced mud crab predation on oysters. Additionally, the magnitude of this effect was much greater than the increase in oyster mortality as a result of complex reefs disrupting toadfish predation on mud crabs. This experimental demonstration of how habitat complexity modifies trophic interactions in a temperate reef community has fundamental implications for our understanding of species interaction webs and community structure. The influence of habitat complexity on the strength of a trophic cascade generally may depend upon whether physical complexity provides actual and perceived refuges for component predator-prey pairs.

Though numerous mammalian taxa exhibit cathemerality (i.e. activity distributed across the 24-h cycle), this includes very few primates, exceptions being species from Aotinae and Lemuridae. Four non-mutually exclusive hypotheses have been proposed to explain the ultimate determinants for cathemeral activity in lemurs: thermoregulatory benefits, anti-predator strategy, competition avoidance and metabolic dietary-related needs. However, these have only been explored in the frugivorous genus Eulemur, with some species increasing nocturnality as a possible response to avoid diurnal raptors and to increase their ability to digest fibre during resource-scarce periods. Since Eulemur lack specializations for digesting bulk food, this strategy would allow for processing fibres over the full 24-h. The folivorous lemurids, i.e. genus Hapalemur, provide a divergent model to explore these hypotheses due to gastrointestinal adaptations for digesting dietary fibre and small body size compared to Eulemur. We linked continuous activity data collected from archival tags with observational behaviour and feeding data from three groups of adult Hapalemur meridionalis from January to December 2013. We tested the effects of thermoregulation, predator avoidance and the weighted proportion of digestible dietary fibre on the daily diurnal/nocturnal activity ratio using a Linear Mixed-Model. Our best-fit model revealed that increased canopy exposure and dietary fibre predicted greater diurnality. Our findings partly contrast with previous predictions for frugivorous lemurids. We propose a divergent adaptive explanation for folivorous lemurids. We suggest that the need to avoid terrestrial predators, as well as longer digestive bouts during bulk food periods, may override cathemerality in favour of diurnality in these bamboo lemurs.

Many taxa possess a range of strategies to reduce the risk of predation, including actively seeking suitable refuge habitats; however, the global spread of invasive species may disrupt these behavioral responses. In lotic ecosystems, interstitial spaces in the substrate are important refugia for small organisms. Some predators are ecosystem engineers that exhibit zoogeomorphic agency—the ability to modify the geomorphology of their environment. It is therefore possible that direct ecological effects of predators on prey may be realized through modifications to the prey's habitat, including the availability of refugia, by predators that are zoogeomorphic agents or via external stressors such as fine sediment loading. This study examined three research questions in a mesocosm study across a gradient of sediment‐stress treatments: (1) What affects do predators (Pacifastacus leniusculus, invasive crayfish) and prey (Gammarus pulex, amphipods) have on the ingress of fine sediment into gravel substrates and therefore on available interstitial refugia? (2) Do prey taxa seek refuge from (invasive) predators in the form of vertical movement into subsurface sediments? and (3) How does fine sediment ingress influence predator–prey interactions and prey survival through predator avoidance behavior. Here, we provide direct evidence demonstrating that fine sediment ingress into gravel river beds can be facilitated by zoogeomorphic activity with P. leniusculus increasing the infiltration of fine sand particles (but not coarse sand) during foraging activities. Predator–prey interactions were found to be a primary factor mediating zoogeomorphic activity, with the isolation of crayfish from prey (G. pulex) leading to increased fine sand ingress. When present with signal crayfish, G. pulex displayed vertical avoidance behavior, entering subsurface substrates to evade predation by P. leniusculus. Coarse sand treatments resulted in higher predation rates of G. pulex, most likely due to clogging of interstitial pore spaces between gravels limiting the effectiveness of the prey's vertical avoidance behavior strategy. A new conceptual model that captures the interactions between predator, prey, zoogeomorphic processes and habitat availability is presented. This model highlights how predator–prey interactions can be strongly mediated by dynamic bi‐directional interactions between organisms and the physical environment they inhabit as ecological and geomorphological processes are intrinsically linked.

Predators impact prey in direct (lethal) and indirect (non-lethal) manners. Predator-avoidance models capitalize on the non-lethal effects of predators to study how predator-induced fear impacts prey behavior and physiology. Here, we aimed to develop a predator avoidance model to determine how predators alter feeding and anxiety-like behavior in the African clawed frog (Xenopus laevis). We determined (1) the repeatability of frog behavior over time, (2) the effect of a stimulus (nothing, a size-matched or a large conspecific [potential predator]) on frog behavior, and (3) the effect of a stimulus on frog behavior in the presence of food. Twelve juvenile experimental frogs were exposed to all three stimulus conditions over 1 week. We predicted that (1) frog baseline behavior would be repeatable, and that (2) the presence of the large frog, but not size-matched frog, would increase fear and anxiety-like behaviors (hiding and inactivity) and would decrease food consumption and the number of air gulps. In the presence of both food and stimulus, experimental frogs ate significantly less when exposed to a large (potential predator) vs. a size-matched and no frog and took more time to first contact the food. Time spent inactive and number of air gulps did not differ across conditions. Few frogs hid during the behavioral trials. Time spent exploring and inactive and tank locations were repeatable over time. Overall, our paradigm is a viable model for studying the effects of predators on prey behavior especially as it relates to feeding.

Predators often induce shifts in the traits of nearby prey, and these trait shifts are important in mediating a variety of evolutionary and ecological processes. However, little is known about the spatial and temporal scales over which predators induce trait shifts. We empirically determined the spatial scale of predator avoidance by measuring the habitat use and growth rates of snails (Physa acuta) held at varying distances from a caged pumpkinseed sunfish (Lepomis gibbosus). Refuge use was highest near the fish and gradually decayed to background level, with a characteristic response range of 1.0 m. Snail growth rates were negligible near the predator but increased with greater separation from fish. The dependence of behavior on the age of chemical cues was measured in a mesocosm experiment in which water was withdrawn from a tank holding pumpkinseeds and held for varying lengths of time before being added to experimental mesocosms with snails. Fresh cues elicited the strongest habitat shifts relative to well-water controls, and avoidance behavior decayed in an exponential manner with increasing cue age. The characteristic lifetime of avoidance behavior was 41 h. Taken together, these results allow us to begin to describe the behavioral landscape created by mobile predators.

Decision making in invertebrates often relies on simple neural circuits composed of only a few identified neurons. The relative simplicity of these circuits makes it possible to identify the key computation and neural properties underlying decisions. In this review, we summarize recent research on the neural basis of ultrasound avoidance in crickets, a response that allows escape from echolocating bats. The key neural property shaping behavioral output is high-frequency bursting of an identified interneuron, AN2, which carries information about ultrasound stimuli from receptor neurons to the brain. AN2's spike train consists of clusters of spikes - bursts - that may be interspersed with isolated, non-burst spikes. AN2 firing is necessary and sufficient to trigger avoidance steering but only high-rate firing, such as occurs in bursts, evokes this response. AN2 bursts are therefore at the core of the computation involved in deciding whether or not to steer away from ultrasound. Bursts in AN2 are triggered by synaptic input from nearly synchronous bursts in ultrasound receptors. Thus the population response at the very first stage of sensory processing - the auditory receptor - already differentiates the features of the stimulus that will trigger a behavioral response from those that will not. Adaptation, both intrinsic to AN2 and within ultrasound receptors, scales the burst-generating features according to the stimulus statistics, thus filtering out background noise and ensuring that bursts occur selectively in response to salient peaks in ultrasound intensity. Furthermore AN2's sensitivity to ultrasound varies adaptively with predation pressure, through both developmental and evolutionary mechanisms. We discuss how this key relationship between bursting and the triggering of avoidance behavior is also observed in other invertebrate systems such as the avoidance of looming visual stimuli in locusts or heat avoidance in beetles.

Prey species possess a variety of morphological, life history and behavioural adaptations to evade predators. While specific evolutionary conditions have led to the expression of permanent, non-plastic anti-predator traits, the vast majority of prey species rely on experience to express adaptive anti-predator defences. While ecologists have identified highly sophisticated means through which naive prey can deal with predation threats, the potential for death upon the first encounter with a predator is still a remarkably important unresolved issue. Here, we used both laboratory and field studies to provide the first evidence for risk-induced neophobia in two taxa (fish and amphibians), and argue that phenotypically plastic neophobia acts as an adaptive anti-predator strategy for vulnerable prey dealing with spatial and temporal variation in predation risk. Our study also illustrates how risk-free maintenance conditions used in laboratory studies may blind researchers to adaptive anti-predator strategies that are only expressed in high-risk conditions.

Larvae of the California newt (Taricha torosa) exhibit striking predator-avoidance behavior, escaping to refuges in response to a chemical cue from cannibalistic adults. In laboratory flow-tank experiments, stream water collected near free-ranging adults induced hiding responses in 100% of the larvae tested. Solutions prepared by bathing adults (in field and laboratory) also evoked strong hiding behaviors. Insensitive to adult feeding status (fed or starved), and clearly not an excretory product, the chemical cue was released from adult skin (i.e., in swabs of adult backs, sides, and bellies). Tetrodotoxin (TTX) was found in skin swabs of adults and in bathwater at 1 × 10-7 mol/L using reversed-phase high-pressure liquid chromatography (HPLC). Concentrations of 1 × 10-7 to 1 × 10-9 mol/L TTX standard, and equivalent dilutions of bathwater, triggered hiding behaviors in larvae, with no subsequent sublethal toxicity. The presence of TTX-sensitive cells within larval olfactory epithelium was confirmed by behavioral experiments and electrophysiological recordings. In contrast, larvae did not hide in response to two other, structurally mimetic compounds (saxitoxin and μ-conotoxin GIIIB). Ontogenetically, larval behavioral responses to TTX and bathwater were strongest during weeks 3—5, diminishing to nil during week 7. No longer susceptible to adult cannibalism, larval indifference to the cue coincided with their ability to climb out of water and onto land. Thus, newt larvae escape cannibalism by detecting a poison (TTX) well known as a chemical defense for conspecific adults. Eliciting a behavioral response in one case and inhibiting neural activity in the other, this compound results in opposing physiological effects, with avoiding predation as the common goal. Accordingly, TTX joins a select group of keystone molecules, each having critical, but different, ecological consequences at multiple trophic levels. The unique combination of bioactive properties makes a compelling case for asymmetrical selection as a force driving the evolution of adult—larval trophic interactions.