Increased Refuge Use Research Articles

Local scale population risk determines the adaptive responses of larval salamanders to predator kairomones

Abstract Organisms can exhibit plasticity in phenotypic responses to environmental stimuli but the response can vary based on the evolutionary history of a population. Numerous studies have found that prey will express adaptive responses when subjected to various predators in experimental settings. For species with a large distribution, such as Ambystoma maculatum (spotted salamander), it is not surprising that phenotypic variance can be high across their range. This variance has been hypothesized to be because of different predator regimes of populations. While A. maculatum preferentially oviposit eggs in low-risk ponds (temporary fishless) to decrease egg and larval mortality, Missouri populations still use high-risk ponds (permanent with fish) for reproduction. Using a series of experiments, we investigated how A. maculatum population risk influenced adaptive responses to both native and novel predator kairomones. For natural predators, we used larval A. opacum (marbled salamanders) and Lepomis macrochirus (bluegill) and for novel predators we used Siren intermedia (lesser siren). We found that larval salamanders generally responded with no differences in morphological traits to all three predators. However, head width was larger for low-risk populations. One hypothesized benefit of larger heads (increased foraging efficiency) was not fully supported. All larval salamanders also increased refuge use with predators but this decreased over time. Our results suggest that predation risk of a population may influence the degree of phenotypic expression in response to larval predators. Overall, local adaptation may dictate the ability of prey to respond to environmental conditions within a life stage.

Evaluating adaptive, carry-over, and plastic antipredator responses across a temporal gradient in Pacific chorus frogs.

The development of antipredator traits is dependent on the frequency and intensity of predator exposure over evolutionary and ecological time. We hypothesized that prey species would respond with increasing accuracy when exposed to predators across generational, ontogenetic, and immediate time scales. We assessed larval Pacific chorus frog (PSRE; Pseudacris regilla) individuals that varied in population sympatry, embryonic conditioning, and immediate exposure to stocked populations of rainbow trout (Oncorhynchus mykiss). Using PSRE populations from sites with and without resident rainbow trout, we conditioned embryos to trout odor, PSRE alarm cues, trout odor in combination with alarm cues, or control water. After being hatched and reared in control water, individuals were exposed to the four predator cue treatments using a fully factorial design. Tadpoles from populations with resident rainbow trout did not behave or develop differently than tadpoles originating from fishless sites. However, we found evidence that PSRE reduced predation risk with a combination of carry-over effect (i.e., transfer of information across life history stages) and within-life stage phenotypically plastic mechanisms. We found both developmental and behavioral carry-over effects: tadpoles conditioned with trout odor as embryos grew more slowly and took refuge more often than control animals. Within-life-stage behavioral plasticity was observed in tadpoles from all treatment groups, responding to predator cues with increased refuge use. Potentially additive effects of predator exposure on prey response should be considered when predicting the ability of prey to recognize novel threats.

Anxiety-like behaviour increases safety from fish predation in an amphipod crustacea.

Anxiety is an emotional state generally expressed as sustained apprehension of the environment and elevated vigilance. It has been widely reported in vertebrates and, more recently, in a few invertebrate species. However, its fitness value remains elusive. We investigated anxiety-like behaviour and its consequences in an amphipod crustacean, using electric shock as aversive stimuli, and pharmacological assays. An anxiety-like state induced by electric shocks in Gammarus fossarum was expressed through increased sheltering behaviour in the absence of predation risk, thereby showing the pervasive nature of such behavioural response. Increasing the number of electric shocks both increased refuge use and delayed behavioural recovery. The behavioural effect of electric shock was mitigated by pre-treatment with LY354740, a metabotropic glutamate receptor group II/III agonist. Importantly, we found that this modulation of decision-making under an anxiety-like state resulted in an increased survival to predation in microcosm experiments. This study confirms the interest in taking an evolutionary view to the study of anxiety and calls for further investigation on the costs counterbalancing the survival benefit of an elevated anxiety level evidenced here.

Cues from a common predator cause survival-linked behavioral adjustments in Mojave Desert tortoises (Gopherus agassizii)

Animals are expected to engage in behavioral decision-making that minimizes their risk of predation; these decisions can cause non-lethal predator effects to behavior and spatial use. Our goal was to determine whether non-lethal effects of a common predator, coyotes (Canis latrans), could affect the behavior of a declining reptile, the Mojave Desert tortoise (Gopherus agassizii), hereafter tortoise. We exposed captive tortoises to olfactory and visual cues of coyotes and measured their resulting foraging behavior, burrow use, and other antipredator behaviors. We also measured risk of coyote predation in wild tortoises based on burrow use. Willingness to feed and time spent feeding were unaffected by the presence of coyote urine, but higher air temperatures at the start of feeding caused a decrease in time spent at their food dish. Captive tortoises chronically exposed to coyote urine did, however, spend more time in their burrows than when exposed to rabbit urine. When exposed to a coyote decoy, captive tortoises showed more general antipredator behaviors than those exposed to a control stimulus (deer decoy). Wild tortoises were less likely to be depredated by coyotes, if they were encountered in burrows more frequently. Predator olfactory and visual cues exerted non-lethal effects on captive tortoises through behavioral adjustments to their refuge-seeking behavior, which was a behavior linked to decreased coyote predation risk for wild tortoises. Human-subsidized predator populations may negatively affect declining species, including reptiles, as a result of behavioral modifications in response to predator cues. Predators can affect behavior of animals through olfactory and visual cues, which can have measurable effects on prey populations. Changes in prey behavior in response to predator presence can occur at the expense of opportunities to forage or reproduce, leading to decreased survival and reproduction. We tested behavioral responses of captive Mojave Desert tortoises to olfactory and visual cues of coyotes, which are a common predator in the Mojave Desert and subsidized by human food sources. Foraging behavior of captive tortoises was strongly affected by air temperatures at the start of feeding, but not coyote cues. Captive tortoises did, however, increase the amount of time they spent in burrows when coyote urine was present and showed greater fearfulness at the presentation of coyote urine combined with coyote models. Increased burrow use was associated with lower risk of coyote predation in wild tortoises, which suggested an advantage to increased refuge use when coyotes were detectable. Responses by tortoises suggest that coyote cues on the landscape affect tortoises through behavioral pathways that result in increased refuge use.

Parasite-induced alteration of odour responses in an amphipod–acanthocephalan system

Odour-related behaviours in aquatic invertebrates are important and effective anti-predator behaviours. Parasites often alter invertebrate host behaviours to increase transmission to hosts. This study investigated the responses of the amphipod Hyalella azteca when presented with two predator chemical cues: (i) alarm pheromones produced by conspecifics and (ii) kairomones produced by a predatory Green Sunfish (Lepomis cyanellus). We compared the responses of amphipods uninfected and infected with the acanthocepalan parasite Leptorhynchiodes thecatus. Uninfected amphipods reduced activity and increased refuge use after detecting both the alarm pheromones and predator kairomones. Infected amphipods spent significantly more time being active and less time on the refuge than uninfected amphipods, and behaved as if they had not detected the chemical stimulus. Therefore, L. thecatus infections disrupt the amphipods’ anti-predator behaviours and likely make their hosts more susceptible to predation.

Effect of predation threat on repeatability of individual crab behavior revealed by mark-recapture

The persistence of behavioral types in situ and the drivers of persistence are central to predicting the ecological effects of intraspecific behavioral variation. We surveyed individual refuge use of mud crabs (Panopeus herbstii), a behavior related to the strength of a trait-mediated trophic cascade in oyster reefs, in the absence and presence of toadfish (Opsanus tau) predation threat. We then released these crabs into the field and using mark-recapture, measured the repeatability of this behavior in the absence and presence of threat, and how behavioral change was affected by time in the field (a month on average, up to 81 days), crab size, and sex. Because crabs exhibited some evidence of a circatidal rhythm in refuge use, we also tested how tidal height during observation influenced behavioral change. Predation threat increased refuge use, and small crabs used the refuge more than large crabs, particularly under threat. In recaptured crabs, refuge use was more repeatable under threat. Neither time in the field, crab size, crab sex, nor tidal height had any effect on behavioral change. Our results support the non-mutually exclusive hypotheses that (1) prey organisms in the presence, rather than absence, of predation threat should exhibit less behavioral variability because the fear of dying (a severe fitness consequence) should take precedence over less immediately important influences on behavior (e.g., hunger) and that (2) individual behaviors tied to fixed traits (e.g., the body size dependence of refuge use under threat in this study), rather than variable traits, should be more repeatable over time.

Parasite-induced alteration of plastic response to predation threat: increased refuge use but lower food intake in Gammarus pulex infected with the acanothocephalan Pomphorhynchus laevis

Larvae of many trophically-transmitted parasites alter the behaviour of their intermediate host in ways that increase their probability of transmission to the next host in their life cycle. Before reaching a stage that is infective to the next host, parasite larvae may develop through several larval stages in the intermediate host that are not infective to the definitive host. Early predation at these stages results in parasite death, and it has recently been shown that non-infective larvae of some helminths decrease such risk by enhancing the anti-predator defences of the host, including decreased activity and increased sheltering. However, these behavioural changes may divert infected hosts from an optimal balance between survival and foraging (either seeking food or a mate). In this study, this hypothesis was tested using the intermediate host of the acanthocephalan parasite Pomphorhynchus laevis, the freshwater amphipod Gammarus pulex. We compared activity, refuge use, food foraging and food intake of hosts experimentally infected with the non-infective stage (acanthella), with that of uninfected gammarids. Behavioural assays were conducted in four situations varying in predation risk and in food accessibility. Acanthella-infected amphipods showed an increase in refuge use and a general reduction in activity and food intake. There was no effect of parasite intensity on these traits. Uninfected individuals showed plastic responses to water-borne cues from fish by adjusting refuge use, activity and food intake. They also foraged more when the food was placed outside the refuge. At the intra-individual level, refuge use and food intake were positively correlated in infected gammarids only. Overall, our findings suggest that uninfected gammarids exhibit risk-sensitive behaviour including increased food intake under predation risk, whereas gammarids infected with the non-infective larvae of P. laevis exhibit a lower motivation to feed, irrespective of predation risk and food accessibility.

A suite of prey traits determine predator and nutrient enrichment effects in a tri‐trophic food chain

Predation, predation risk, and resource quality affect suites of prey traits that collectively impact individual fitness, population dynamics, and community structure. However, studies of multi‐trophic level effects generally focus on a single prey trait, failing to capture trade‐offs among suites of covarying traits that govern population responses and emergent community patterns. We used structural equation models (SEM) to summarize the non‐lethal and lethal effects of crayfish, Procambarus fallax, and phosphorus (P) addition, which affected prey food quality (periphyton), on the interactive effects of behavioral, morphological, developmental, and reproductive traits of snails, Planorbella duryi. Univariate and multivariate analyses suggested trade‐offs between production (growth, reproduction) and defense (foraging behavior, shell shape) traits of snails in response to non‐lethal crayfish and P addition, but few lethal effects. SEM revealed that non‐lethal crayfish effects indirectly limited per capita offspring standing stock by increasing refuge use, slowing individual growth, and inducing snails to produce thicker, compressed shells. The negative effects of non‐lethal crayfish on snails were strongest with P addition; snails increased allocation to shell defense rather than growth or reproduction. However, compared to ambient conditions, P addition with non‐lethal crayfish still yielded greater per capita offspring standing stock by speeding individual snail growth enabling them to produce more offspring that also grew faster. Increased refuge use in response to non‐lethal crayfish led to a non‐lethal trophic cascade that altered the spatial distribution of periphyton. Independent of crayfish effects, snails stimulated periphyton growth through nutrient regeneration. These findings illustrate the importance of studying suites of traits that reveal costs associated with inducing different traits and how expressing those traits impacts population and community level processes.

Impacts of a large-bodied, apex predator (Alligator mississippiensis Daudin 1801) on salt marsh food webs

Large-bodied apex predators (e.g., sharks, wolves, crocodilians) are believed to regulate food web structure and drive ecosystem processes, but there remains relatively little experimental evidence. Here we use field surveys and a mesocosm experiment to evaluate the cascading effects of an apex predator (American alligator) on a salt marsh food web. Consistent with previous studies (n=10), field surveys revealed blue crabs (Callinectes sapidus Rathbun 1896), an important marsh mesopredator, were a frequent component of estuarine-occurring alligators' diet (mean±SD, 47±20%, n=1384). In mesocosms, we examined potential consequences of this interaction in a simplified salt marsh community. We experimentally isolated alligator effects on the abundance (consumptive effect) and behavior (non-consumptive effect) of blue crabs and on blue crab consumption of plant-grazing snails and ribbed mussels. Alligators reduced blue crab abundance by ~40% over 3days and induced behavioral changes, resulting in decreased foraging activity and increased refuge use by blue crabs. The combined effects of reduced crab abundance and altered behavior translated into increased survival of both a keystone grazer (snails) and a mutualist (mussels) within the salt marsh food web. Our findings experimentally demonstrate that a large-bodied, apex predator has the potential to 1) generate a trophic cascade, 2) elicit behavioral changes (i.e., non-consumptive effects) in mesopredator prey, and 3) indirectly affect the potential for both grazing and mutualism to occur in this food chain. Our results generate testable hypotheses regarding the broad-scale effects of alligator presence and top-down forcing in salt marsh ecosystems.

Predator biomass determines the magnitude of non-consumptive effects (NCEs) in both laboratory and field environments

Predator body size often indicates predation risk, but its significance in non-consumptive effects (NCEs) and predator risk assessment has been largely understudied. Although studies often recognize that predator body size can cause differing cascading effects, few directly examine prey foraging behavior in response to individual predator sizes or investigate how predator size is discerned. These mechanisms are important since perception of the risk imposed by predators dictates behavioral responses to predators and subsequent NCEs. Here, we evaluate the role of predator body size and biomass on risk assessment and the magnitude of NCEs by investigating mud crab foraging behavior and oyster survival in response to differing biomasses of blue crab predators using both laboratory and field methods. Cues from high predator biomass treatments including large blue crab predators and multiple small blue crab predators decreased mud crab foraging and increased oyster survival, whereas mud crab foraging in response to a single small blue crab did not differ from controls. Mud crabs also increased refuge use in the presence of large and multiple small, but not single small, blue crab predators. Thus, both predator biomass and aggregation patterns may affect the expression of NCEs. Understanding the impact of predator biomass may therefore be necessary to successfully predict the role of NCEs in shaping community dynamics. Further, the results of our laboratory experiments were consistent with observed NCEs in the field, suggesting that data from mesocosm environments can provide insight into field situations where flow and turbulence levels are moderate.

Fleeing towards death – leech‐induced behavioural defences increase freshwater snail susceptibility to predatory fish

Prey species are often exposed to multiple predators, which presents several difficulties to prey species. This is especially true when the response to one predator influences the prey’s susceptibility to other predators. Predator‐induced defences have evolved in a wide range of prey species, and experiments involving predators with different hunting strategies allow researchers to evaluate how prey respond to multiple threats. Freshwater snails are known to respond to a variety of predators with both morphological and behavioural defences. Here we studied how freshwater snails Radix balthica responded behaviourally to fish and leech predators, both separately and together. Our aim was to explore whether conflicting predator‐induced responses existed and, if so, what effect they had on snail survival when both predatory fish and leeches were present. We found that although R. balthica increased refuge use when exposed to predatory fish, they decreased refuge use when exposed to predatory leeches. When both predators were present, snails showed a stronger response towards leech than fish and responded by leaving the refuge. This response made the snails more susceptible to fish predation, which increased snail mortality when exposed to both fish and leech compared to fish only. We show that predators that have a relatively low predation rate can substantially increase mortality rates by indirect effects. By forcing snails out of refuges such as rock and macrophyte habitats, leeches can indirectly increase predation from molluscivorous fish and may thus affect snail densities.

Wolf in sheep's clothing: Effects of predation by small-bodied fish on survival and behaviour of salamander larvae

Fish can affect amphibian larvae directly through consumption or indirectly by eliciting antipredator behaviours that incur fitness costs. Because of predation, long-toed salamanders (Ambystoma macrodactylum) typically show an allotopic distribution with fish. Linnet Lake (Waterton Lakes National Park, Alberta, Canada) represents the unusual situation in which salamanders coexist with a small-bodied cyprinid, lake chub (Couesius plumbeus). However, the salamander population resident to Linnet Lake has declined in recent years. Our objective was to assess the role of lake chub in the decline of this salamander population by combining field observations and laboratory experiments. We used mark-recapture techniques to estimate the size of the 2 populations. We found little evidence of successful salamander reproduction, and the adult population had decreased by 60% in 14 y. In contrast, a large population of lake chub was present. Experiments showed that lake chub between 70 and 100 mm could consume salamander larvae < 40 mm in length. Larvae responded to the presence of lake chub by reducing activity and increasing refuge use, especially during the day. Our study is one of few to document the ability of a native small-bodied fish to consume amphibian larvae. The coexistence of fish and salamanders may result from a dynamic interplay between periodic extirpation of fish by winter hypoxia and of salamanders by predation, punctuated by episodes of re-colonization or strong recruitment.

Prioritized phenotypic responses to combined predators in a marine snail

Although many species face numerous predators in nature, the combined impact of multiple predators on the inducible defenses of prey has rarely been studied. Prey may respond with an intermediate phenotype that balances the risk from several sources or may simply respond to the most dangerous predator. I examined the separate and combined effects of the presence of shell-breaking (crabs, Cancer productus) and shell-entry (seastars, Pisaster ochraceus) predators fed conspecific snails on the defensive shell morphology and antipredator behavior of a marine snail (Nucella lamellosa). When exposed to each feeding predator separately, snails responded with a combination of morphological defenses that reflect the attack mode of the predator and a generalized behavioral response. Snails responded to feeding crabs by increasing refuge use and producing a thick, rotund shell. Snails responded to feeding seastars with increased refuge use but produced elongate shells with high spires that allowed for greater retraction of the soft tissue. Seastar-induced phenotypes reduced susceptibility to seastars relative to crab-induced phenotypes, but crab-induced phenotypes did not significantly reduce susceptibility to crabs, indicating an asymmetrical functional trade-off. When feeding predators were combined, snails produced a morphological phenotype similar to that expressed in the presence of the predator that imposed the highest mortality at the population level, suggesting that predator-induced morphology was prioritized according to predation risk. These results suggest that prioritizing conflicting defenses according to predator danger may be a common strategy for prey responding to combined predators, particularly in conjunction with generalized behavioral responses that reduce overall risk in multiple-predator environments.

Differential influence of two acanthocephalan parasites on the antipredator behaviour of their common intermediate host

Fish acanthocephalans can modify the antipredator behaviour of their intermediate hosts in response to cues from fish predators. However, it is still unclear whether such behavioural changes are adaptive, or are just the consequence of infection. We addressed this question through studying two acanthocephalans, Pomphorhynchus laevis and Polymorphus minutus, and their intermediate host, the amphipod Gammarus pulex. Pomphorhynchus laevis completes its cycle in a freshwater fish, whereas P. minutus exploits waterbirds as final hosts. We first assessed vulnerability of infected and uninfected gammarids to predation by bullheads, Cottus gobio. Pomphorhynchus laevis-infected gammarids were more susceptible to predation than uninfected ones when a refuge was available, whereas no selective predation on P. minutus-infected individuals was recorded, independently of refuge availability. We then quantified refuge use when a bullhead was present in an enclosure or when the enclosure was empty. Individuals of each group significantly increased refuge use in the presence of a bullhead. However, a larger proportion of P. laevis-infected gammarids remained out of the refuge in the presence of a predator, compared with uninfected controls, whereas no such effect was observed in P. minutus-infected ones. Finally, we assessed reaction to bullhead olfactory cues, using a Y-maze apparatus. Pomphorhynchus laevis-infected gammarids spent significantly more time in the predator-scented arm, whereas the reverse was observed in uninfected ones. Polymorphus minutus-infected individuals, however, did not differ from uninfected controls. We discuss our results in relation with the adaptiveness of host manipulation by parasites.

Pregnant female lizards Iberolacerta cyreni adjust refuge use to decrease thermal costs for their body condition and cell‐mediated immune response

Lizards often respond to increased predation risk by increasing refuge use, but this strategy may entail a loss of thermoregulatory opportunities, which may lead to a loss of body condition. This may be especially important for pregnant oviparous female lizards, because they need to maintain optimal body temperatures as long as possible to maximize developmental embryos rate until laying. However, little is known about how increased time spent at low temperatures in refuges affects body condition and health state of pregnant female lizards. Furthermore, it is not clear how initial body condition affects refuge use. Female Iberian rock lizards forced to increase time spent at low temperatures showed lower body condition and tended to show lower cell-mediated immune responses than control females. Therefore, the loss of thermoregulatory opportunities seems to be an important cost for pregnant females. Nevertheless, thereafter, when we simulated two repeated predatory attacks, females modified refuge use in relation to their body condition, with females with worse condition decreasing time hidden after attacks. In conclusion, female lizards seemed able to compensate increased predation risk with flexible antipredatory strategies, thus minimizing costs for body condition and health state.

Refuge use: A conflict between avoiding predation and losing mass in lizards

Prey often respond to predation risk by increasing refuge use, but this strategy may entail a loss of body condition. Factors responsible for this loss of body condition remain unclear. Also how prey deal with refuge use to cope with predation risk without incurring costs of body condition, and how initial body condition affects refuge use remain barely known. We analyzed in the field whether adult Iberolacerta cyreni lizards modify their escape strategies and refuge use in areas with different levels of habitat deterioration and ecotourism pressure, which represent different levels of predation risk, and the consequences of changes in antipredator behavior to body condition. Lizards inhabiting deteriorated areas, where risk is higher, remained closer to refuges, but decreased time spent hidden in refuges after attacks, probably to maintain similar body condition than lizards inhabiting natural areas. We performed two laboratory experiments to isolate potential costs of refuge use that might affect the body condition of male lizards: a) a decrease of the efficiency of digestion due to low temperatures inside refuges and/or b) a reduction in food intake. Results suggest that refuge use is costly in terms of body condition due to reduced food intake, but the loss of efficiency of digestion seems unimportant. Lizards modified refuge use in relation to their body condition, with lizards with worse condition decreasing time hidden after predatory attacks. We concluded that lizards compensated for increased predation risk with flexible antipredatory strategies, coping with risk without incurring costs for body condition.

Thermal dependence of chemical assessment of predation risk affects the ability of wall lizards, Podarcis muralis, to avoid unsafe refuges

Prey often respond to predators by increasing refuge use, but some refuges may expose prey to other types of predators. Because predators are not always visible inside refuges, their chemical stimuli may provide early warning of their presence. However, in ectotherms, chemosensory detection of predators may be thermally dependent. Lizards often hide in cold refuges, where their body temperature (Tb) may decrease, and this might affect their ability to detect chemicals of saurophagous snakes that ambush inside refuges. We examined the effects of Tb of wall lizards, Podarcis muralis, on their ability to detect chemicals of smooth snakes, Coronella austriaca. Differential tongue flick (TF) rates and behavioral patterns of lizards in response to scent of smooth snakes showed that lizards with optimal Tb discriminated and avoided the snake's scent, whereas lizards with suboptimal Tb did not. We also examined the importance of this effect on movement patterns and refuge use by lizards in terraria with safe (odorless) or unsafe refuges (snake-scented). While results of this experiment were variable, there was evidence of snake avoidance in that lizards at optimal Tb spent less time in snake-scented refuges relative to odorless refuges. Therefore, this study provides evidence that chemosensory discrimination of snake chemicals is thermally dependent, and, thus, suboptimal Tb impedes a lizard's ability to avoid snake-scented refuges.

Behavioral responses to a novel predator and competitor of invasive mosquitofish and their non-invasive relatives (Gambusia sp.)

Attributes of the recipient community may affect the invasion success of arriving non-indigenous organisms. In particular, biotic interactions may enhance the resistance of communities to invasion. Invading organisms typically encounter a novel suite of competitors and predators, and thus their invasiveness may be affected by how they cope with these interactions. Behavioral plasticity may help invaders to respond appropriately to novelty. We examined the behavioral responses of highly invasive mosquitofish to representative novel competitors and predators they might encounter as they spread through North America. We compared the behavior of invasive Gambusia holbrooki and G. affinis to that of two close relatives of lower invasive potential (G. geiseri and G. hispaniolae) in order to elucidate whether responses to novelty related to invasiveness. In short-term assays, female Gambusia were paired with a novel competitor, Pimephales promelas, and a novel predator, Micropterus dolomieu. Behavioral responses were measured in terms of foraging success and efficiency, activity, refuge use, predator inspections, and interspecific aggression. Contrary to a priori predictions, invasive and non-invasive responses to novel interactions did not differ consistently. In response to novel competition, both invasive species increased foraging efficiency, but so did G. geiseri. In response to novel predation, only G. holbrooki decreased consumption and activity and increased refuge use. No antipredator response was observed in G. affinis. We found consistent differences, however, between invasives and non-invasives in foraging behavior. Both in the presence and absence of the competitor and the predator, invasives foraged more efficiently and consumed more prey than non-invasives.

Thermal dependence of chemical assessment of predation risk affects the ability of wall lizards, Podarcis muralis, to avoid unsafe refuges

Prey often respond to predators by increasing refuge use, but some refuges may expose prey to other types of predators. Because predators are not always visible inside refuges, their chemical stimuli may provide early warning of their presence. However, in ectotherms, chemosensory detection of predators may be thermally dependent. Lizards often hide in cold refuges, where their body temperature (Tb) may decrease, and this might affect their ability to detect chemicals of saurophagous snakes that ambush inside refuges. We examined the effects of Tb of wall lizards, Podarcis muralis, on their ability to detect chemicals of smooth snakes, Coronella austriaca. Differential tongue flick (TF) rates and behavioral patterns of lizards in response to scent of smooth snakes showed that lizards with optimal Tb discriminated and avoided the snake's scent, whereas lizards with suboptimal Tb did not. We also examined the importance of this effect on movement patterns and refuge use by lizards in terraria with safe (odorless) or unsafe refuges (snake-scented). While results of this experiment were variable, there was evidence of snake avoidance in that lizards at optimal Tb spent less time in snake-scented refuges relative to odorless refuges. Therefore, this study provides evidence that chemosensory discrimination of snake chemicals is thermally dependent, and, thus, suboptimal Tb impedes a lizard's ability to avoid snake-scented refuges.

Trade-offs in the choice of refuges by common wall lizards: do thermal costs affect preferences for predator-free refuges?

Prey often respond to predator presence by increasing refuge use. However, some types of refuges may expose prey to other types of predators. In addition, in selecting refuges ectothermic animals may have a conflict between safety and thermal suitability. In this paper we examined in the laboratory whether common wall lizards, Podarcis muralis (Laurenti, 1768), (i) prefer to use warm refuges to cold ones, (ii) prefer safe refuges to those with chemical cues of a saurophagous snake, and (iii) whether lizards face a trade-off between using a warm but snake-scented refuge or a cold but odorless one. Results did not show differences in refuge use in relation to refuge temperature, because common wall lizards only entered to investigate it, but they were not forced to hide. So, common wall lizards did not have to be at suboptimal temperatures for longer times. Common wall lizards avoided the use of predator-scented refuges, regardless of thermal conditions, and they also increased their movement rate, trying to escape from the terrarium. Because snakes are inconspicuous inside refuges, an avoidance response to their chemicals may enhance the survival possibilities of common wall lizards. We conclude that in common wall lizards, predation-risk costs are more important than thermal costs in determining refuge use.