Predation risk perception and food scarcity induce alterations of life-cycle traits of the mosquito Culex pipiens

Abstract.  1. Predators may affect prey populations by direct consumption, and by inducing defensive reactions of prey to the predation risk. Food scarcity frequently has effects on the inducible defences of prey, but no consistent pattern of food–predation risk interaction is known.
2. In this study the combined effect of food shortage and predation-risk perception in larvae of the mosquito Culex pipiens was investigated. Water exposed to the aquatic predator bug Notonecta glauca was used as a source of predation intimidation. Mosquito larvae were reared in three different media containing either no predator cues or the cues of N. glauca that had been fed on either C. pipiens larvae or on Daphnia magna . Food was provided in favourable or limited amount for these set-ups.
3. The results showed that chemical cues from the predators fed with prey's conspecifics caused a decreased survival, delayed pre-imaginal development, and reduction in body size of emerged mosquitoes, whereas chemical cues from predators fed with D. magna caused only delayed development. Food scarcity significantly exacerbates the negative effect of the predator cues on pre-imaginal development of C. pipiens . Effects of the cues on larval development and body size of imagoes are significantly stronger for females than for males.
4. The present study suggests that when food is limited, predators can affect population dynamics of prey not only by direct predation, but also by inducing lethal and sublethal effects due to perception of risk imposed by chemical cues. To understand the effects of predators on mosquito population dynamics, environmental parameters such as food deficiency should be considered.     

Interactions of multiple predators with different foraging modes in an aquatic food web

Top predators can have different foraging modes that may alter their interactions and effects on food webs. Interactions between predators may be non-additive resulting from facilitation or interference, whereas their combined effects on a shared prey may result in emergent effects that are risk enhanced or risk reduced. To test the importance of multiple predators with different foraging modes, we examined the interaction between a cruising predator (largemouth bass, Micropterus salmoides) and an ambush predator (muskellunge, Esox masquinongy) foraging on a shared prey (bluegill sunfish, Lepomis macrochirus) with strong anti-predator defense behaviors. Additive and substitution designs were used to compare individual to combined predator treatments in experimental ponds. The multiple predator interaction facilitated growth of the cruising predator in the combined predator treatments, whereas predator species had substitutable effects on the growth of the ambush predator. The combined predator treatments created an emergent effect on the prey; however, the direction was dependent on the experimental design. The additive design found a risk-reducing effect, whereas the substitution design found a risk-enhancing effect for prey fish. Indirect effects from the predators weakly extended to lower trophic levels (i.e., zooplankton community). Our results highlight the need to consider differences in foraging mode of top predators, interactions between predators, and emergent effects on prey to understand food webs.     

Behavioural and life history effects of predator diet cues during ontogeny in damselfly larvae

A central issue in predator–prey interactions is how predator associated chemical cues affect the behaviour and life history of prey. In this study, we investigated how growth and behaviour during ontogeny of a damselfly larva (Coenagrion hastulatum) in high and low food environments was affected by the diet of a predator (Aeshna juncea). We reared larvae in three different predator treatments; no predator, predator feeding on conspecifics and predator feeding on heterospecifics. We found that, independent of food availability, larvae displayed the strongest anti-predator behaviours where predators consumed prey conspecifics. Interestingly, the effect of predator diet on prey activity was only present early in ontogeny, whereas late in ontogeny no difference in prey activity between treatments could be found. In contrast, the significant effect of predator diet on prey spatial distribution was unaffected by time. Larval size was affected by both food availability and predator diet. Larvae reared in the high food treatment grew larger than larvae in the low food treatment. Mean larval size was smallest in the treatment where predators consumed prey conspecifics, intermediate where predators consumed heterospecifics and largest in the treatment without predators. The difference in mean larval size between treatments is probably an effect of reduced larval feeding, due to behavioural responses to chemical cues associated with predator diet. Our study suggests that anti-predator responses can be specific for certain stages in ontogeny. This finding shows the importance of considering where in its ontogeny a study organism is before results are interpreted and generalisations are made. Furthermore, this finding accentuates the importance of long-term studies and may have implications for how results generated by short-term studies can be used.     

Predator interference alters foraging behavior of a generalist predatory arthropod

Interactions between predators foraging in the same patch may strongly influence patch use and functional response. In particular, there is continued interest in how the magnitude of mutual interference shapes predator–prey interactions. Studies commonly focus on either patch use or the functional response without attempting to link these important components of the foraging puzzle. Predictions from both theoretical frameworks suggest that predators should modify foraging efforts in response to changes in feeding rate, but this prediction has received little empirical attention. We study the linkage between patch departure rates and food consumption by the hunting spider, Pardosa milvina, using field enclosures in which prey and predator densities were manipulated. Additionally, the most appropriate functional response model was identified by fitting alternative functional response models to laboratory foraging data. Our results show that although prey availability was the most important determinant of patch departure rates, a greater proportion of predators left enclosures containing elevated predator abundance. Functional response parameter estimation revealed significant levels of interference among predators leading to lower feeding rates even when the area allocated for each predator was kept constant. These results suggest that feeding rates determine patch movement dynamics, where interference induces predators to search for foraging sites that balance the frequency of agonistic interactions with prey encounter rates.     

Snowshoe hare demography during a cyclic population low

1. Snowshoe hare ( Lepus americanus Erxleben) populations were studied in south-west Yukon during the low phase of the 10-year population cycle. Food availability and predator abundance were manipulated in a factorial design to determine the importance of each factor in hare dynamics during this phase.
2. Food was abundant during the low phase, and snowshoe hares were not food limited.
3. Survival of hares was higher than at any other phase of the cycle, and predators were scarce, but >75% of hare deaths resulted from predation.
4. Food addition resulted in higher hare densities and better body condition than on control sites. There were no observable effects of food addition on population rate of increase, recruitment, survival or age structure.
5. Mammalian predator reduction resulted in higher hare densities, higher survival, better body condition and an older age structure. Relative to control populations, recruitment was lower and population rates of increase similar.
6. The joint manipulation of food addition + predator reduction had greater positive effects on hare density and body condition than either single factor manipulation. Survival was better than on control sites, and the age structure was older than on control sites. Population rates of increase were similar, but recruitment was higher on the control areas.
7. We conclude that snowshoe hare dynamics at the low of the cycle are dominated by the interaction of food and predation. Risk of predation also had indirect effects on snowshoe hare age structure and body condition.     

Predator impacts on stream benthic prey

The impact that predators have on benthic, macroinvertebrate prey density in streams is unclear. While some studies show a strong effect of predators on prey density, others show little or no effect. Two factors appear to influence the detection of predator impact on prey density in streams. First, many field studies have small sample sizes and thus might be unable to detect treatment effects. Second, streams contain two broad classes of predators, invertebrates and vertebrates, which might have different impacts on prey density for a variety of reasons, including availability of refuge for prey and prey emigration responses to the two types of predators. In addition, predatory vertebrates have more complex prey communities than predatory invertebrates; this complexity might reduce the impact that predatory vertebrates have on prey because of indirect effects. I conducted a meta-analysis on the results of field studies that manipulate predator density in enclosures to determine (1) if predators have a significant impact on benthic prey density in streams, (2) if the impacts that predatory invertebrates and vertebrates have differ, and (3) if predatory vertebrates have different impacts on predatory prey versus herbivorous prey. The results of the meta-analysis suggest that on average predators have a significant negative effect on prey density, predatory invertebrates have a significantly stronger impact than predatory vertebrates, and predatory vertebrates do not differ in their impact on predatory versus herbivorous invertebrate prey. Three methodological variables (mesh size of enclosures, size of enclosures, and experimental duration) were examined to determine if cross correlations exist that may explain the differences in impact between predatory invertebrates and vertebrates. No correlation exists between mesh size and predator impact. Over all predators, no correlation exists between experimental duration and predator impact; however, within predatory invertebrates a correlation does exist between these variables. Also, a correlation was found between enclosure size and predator impact. This correlation potentially explains the difference in impact between predatory invertebrates and predatory vertebrates. Results of the meta-analysis suggest two important areas for future research: (1) manipulate both types of predators within the same system, and (2) examine their impacts on the same spatial scale.     

Nitrogen deposition and herbivory affect biomass production and allocation in an annual plant

If environmental conditions vary, plasticity in life-history traits is predicted. A recent model indicates that males and females should differ in life-history traits, because sexes differ in optimal attributes depending on species ecology. In this study we test the impact of two biotic factors in combination (presence/absence of predators and low/high food level) on gender specific life-history traits in the damselfly Coenagrion puella (Odonata). Results show that predator presence and low food density decreased activity in both sexes. Additionally, individuals with less food grew more slowly, emerged later, remained smaller and had a higher mortality. At low food densities, however, and in contrast to former investigations, individuals from treatments with predator presence were the same size or larger than individuals without predators. Gender had a strong impact on larval activity and life-history traits and sexes differed in development. Females were less active and took longer to complete development, but emerged at a larger size, weight and fat content. This study highlights the importance of gender specific approaches in life-history research.     

Increases in disturbance and reductions in habitat size interact to suppress predator body size

Food webs are strongly size‐structured so will be vulnerable to changes in environmental factors that affect large predators. However, mechanistic understanding of environmental controls of top predator size is poorly developed. We used streams to investigate how predator body size is altered by three fundamental climate change stressors: reductions in habitat size, increases in disturbance and warmer temperatures. Using new survey data from 74 streams, we showed that habitat size and disturbance were the most important stressors influencing predator body size. A synergistic interaction between that habitat size and disturbance due to flooding meant the sizes of predatory fishes peaked in large, benign habitats and their body size decreased as habitats became either smaller or harsher. These patterns were supported by experiments indicating that habitat‐size reductions and increased flood disturbance decreased both the abundance and biomass of large predators. This research indicates that interacting climate change stressors can influence predator body size, resulting in smaller predators than would be predicted from examining an environmental factor in isolation. Thus, climate‐induced changes to key interacting environmental factors are likely to have synergistic impacts on predator body size which, because of their influence on the strength of biological interactions, will have far‐reaching effects on food‐web responses to global environmental change.     

Constitutive exposure to the volatile methyl salicylate reduces per‐capita foraging efficiency of a generalist predator to learned prey associations

Understanding the factors that influence the ability of predators to find and kill herbivores is central to enhancing their impact on prey populations, but few studies have tested the impact of these factors on predation rates in realistic foraging environments. Using the tri‐trophic system consisting of tomato, Solanum lycopersicum L. (Solanaceae), hornworm caterpillars, Manduca sexta L. (Lepidoptera: Sphingidae), and the predaceous stink bug Podisus maculiventris (Say) (Hemiptera: Pentatomidae), we measured the effects of associative learning and plant volatile camouflage on predator behavior and foraging efficiency in field enclosures. To do so, we compared experienced vs. naive individuals under varying environmental contexts. Experienced predators were those with prior exposure to induced volatiles from the tomato–caterpillar association, whereas naive predators had not experienced tomato, only prey (caterpillars). We varied their environmental foraging matrix using either (1) tomato surrounded by basil (Ocimum basilicum L.; Lamiaceae) or (2) tomato exposed to the synthetic volatile, methyl salicylate (MeSA). We found that (1) experienced predators were more efficient than naive predators, capturing 28% more prey; (2) the tomato–basil combination did not affect predator–prey interactions; and (3) constitutive emission of synthetic MeSA caused a 22% reduction in P. maculiventris predation rate. These differences corresponded with distinct shifts in predator foraging; for example, experienced individuals were less stationary and exhibited unique behaviors such as stylet extension. Taken together, these results suggest that it is possible to improve the function of generalist predators in suppressing prey by coupling odors with food. However, constitutive emission of volatiles to attract natural enemies may ultimately camouflage neighboring plants, reducing the benefits of orientation to learned stimuli such as induced volatiles.     

Effects of predation and dispersal on Mastomys natalensis population dynamics in Tanzanian maize fields

1. We investigate the effects of different levels of predation pressure and rodent dispersal on the population dynamics of the African pest rodent Mastomys natalensis in maize fields in Tanzania. 2. Three levels of predation risk were used in an experimental set-up: natural level (control), excluding predators by nets and attracting avian predators by nest boxes and perch poles. Because dispersal of the rodents could mask the predation pressure treatment effects, control and predator exclusion treatments were repeated with enclosed rodent populations. 3. Population growth during the annual population rise period was faster in the absence of predators and peak population size was higher, but otherwise dynamics patterns were similar for populations where predators had access or were attracted, indicating that compensatory mechanisms operate when rodents are exposed to high levels of predation risk. Reducing dispersal of rodents removed the effect of predation on population growth and peak size, suggesting that local predators may play a role in driving rodent dispersal, but have otherwise little direct effect on population dynamics.     

The interaction between predation risk and food ration on behavior and morphology of Eurasian perch

The risk of both predation and food level has been shown to affect phenotypic development of organisms. However, these two factors also influence animal behavior that in turn may influence phenotypic development. Hence, it might be difficult to disentangle the behavioral effect from the predator or resource‐level effects. This is because the presence of predators and high resource levels usually results in a lower activity, which in turn affects energy expenditure that is used for development and growth. It is therefore necessary to study how behavior interacts with changes in body shape with regard to resource density and predators. Here, we use the classic predator‐induced morphological defense in fish to study the interaction between predator cues, resource availability, and behavioral activity with the aim to determine their relative contribution to changes in body shape. We show that all three variables, the presence of a predator, food level, and activity, both additively and interactively, affected the body shape of perch. In general, the presence of predators, lower swimming activity, and higher food levels induced a deep body shape, with predation and behavior having similar effect and food treatment the smallest effect. The shape changes seemed to be mediated by changes in growth rate as body condition showed a similar effect as shape with regard to food‐level and predator treatments. Our results suggests that shape changes in animals to one environmental factor, for example, predation risk, can be context dependent, and depend on food levels or behavioral responses. Theoretical and empirical studies should further explore how this context dependence affects fitness components such as resource gain and mortality and their implications for population dynamics.     

Balancing food and predator pressure induces chronic stress in songbirds

The never-ending tension between finding food and avoiding predators may be the most universal natural stressor wild animals experience. The 'chronic stress' hypothesis predicts: (i) an animal's stress profile will be a simultaneous function of food and predator pressures given the aforesaid tension; and (ii) these inseparable effects on physiology will produce inseparable effects on demography because of the resulting adverse health effects. This hypothesis was originally proposed to explain synergistic (inseparable) food and predator effects on demography in snowshoe hares (Lepus americanus). We conducted a 2 x 2, manipulative food addition plus natural predator reduction experiment on song sparrows (Melospiza melodia) that was, to our knowledge, the first to demonstrate comparable synergistic effects in a bird: added food and lower predator pressure in combination produced an increase in annual reproductive success almost double that expected from an additive model. Here we report the predicted simultaneous food and predator effects on measures of chronic stress in the context of the same experiment: birds at unfed, high predator pressure (HPP) sites had the highest stress levels; those at either unfed or HPP sites showed intermediate levels; and fed birds at low predator pressure sites had the lowest stress levels.     

Predator-induced plasticity in guppy (<Emphasis Type="Italic">Poecilia reticulata</Emphasis>) life history traits

A number of invertebrates show predator-induced plasticity in life-history and morphological traits that are considered adaptive. Evidence is accumulating that vertebrates may also adjust their life-history traits in response to predators; however, some of the patterns of plasticity, which appear to be an adaptive response specifically to the risk of size-selective predation, may instead result from reduced foraging in response to predator presence. Here, we describe a study of predator-induced plasticity in guppies (Poecilia reticulata). We have predicted that the plastic response to cues from a small, gape-limited, natural predator of guppies, the killlifish (Rivulus hartii), would be the opposite of that caused by reduced food intake. We have found that male guppies increased their size at maturity, both length and mass, in response to the non-lethal presence of this predator. This pattern of plasticity is the opposite of that observed in response to reduced food intake, where male guppies reduce size at maturity. The increase in size at maturity that we observed would likely reduce predation on adult male guppies by this native predator because it is gape-limited and can only eat juvenile and small adult guppies. This size advantage would be important especially because male guppies grow very little after maturity. Therefore, the pattern of plasticity that we observed is likely adaptive. In contrast, female guppies showed no significant response in size at first parturition to the experimental manipulation; however, we did find evidence suggesting that females may produce more, smaller offspring in response to cues from this predator.     

Geographical variation in food web structure in Nepenthes pitcher plants

Abstract. 1. Relative to Nepenthes species in West Malaysia near the evolutionary centre of the genus, outlying species of Nepenthes in the Seychelles, Sri Lanka and Madagascar have fewer species of both prey and predator living in them, fewer and smaller guilds of species, much apparently empty niche space, less complex food webs, and a greater connectance. The ratios of prey to predators, and of connectance (C₁) to the total number of trophic types present remain approximately constant.
2. Differences between the food webs appear to be related in a complex way to the size of the country and its degree of spatial and temporal isolation, the size of the local species pool capable of colonizing the pitchers, and the number of Nepenthes species present. However, the maximal length of food chains in the richest and most complex food webs is probably limited by energetic constraints or environmental predictability.
3. The data may illustrate how food webs change to become more complex, both by the addition of new guilds of species and the addition of species to existing guilds, while at the same time certain properties of the food web are kept approximately constant.     

Host tree architecture mediates the effect of predators on herbivore survival

Abstract.  1. Vegetation structural complexity is an important factor influencing ecological interactions between different trophic levels. In order to investigate relationships between the architecture of trees, the presence of arthropod predators, and survival and parasitism of the autumnal moth Epirrita autumnata Borkhausen, two sets of experiments were conducted.
2. In one experiment, the architectural complexity of mountain birch was manipulated to separate the effects of plant structure and age. In the other experiment the trees were left intact, but chosen to represent varying degrees of natural complexity. Young autumnal moth larvae were placed on the trees and their survival was monitored during the larval period.
3. The larvae survived longer in more complex trees if predation by ants was prevented with a glue ring, whereas in control trees smaller canopy size improved survival times in one experiment. The density of ants observed in the trees was not affected by canopy size but spider density was higher on smaller trees. The effect of canopy structure on larval parasitism was weak; larger canopy size decreased parasitism only in one year. Until the fourth instar the larvae travelled shorter distances in trees with reduced branchiness than in trees with reduced foliage or control treatments. Canopy structure manipulation by pruning did not alter the quality of leaves as food for larvae.
4. The effect of canopy structure on herbivore survival may depend on natural enemy abundance and foraging strategy. In complex canopies herbivores are probably better able to escape predation by ambushing spiders but not by actively searching ants.     

Mammalian predator scent, vegetation cover and tree seedling predation by meadow voles

Herbivores are thought to respond to the increased risk of attack by predators during foraging activities by concentrating feeding in safe habitats and by reducing feeding in the presence of predators. We tested these hypotheses by comparing tree seedling predation by meadow voles within large outdoor enclosures treated either with scent of large mammalian predators (red fox, bobcat, coyote) or a control scent (vinegar). In addition, we compared the distribution of voles in relation to naturally occurring variation in vegetation cover and the tendency of voles to attack tree seedlings planted in small patches with cover manipulation (intact, reduced or removed cover). Predator scent did not affect the rate or spatial distribution of tree seedling predation by voles, nor did it affect giving up densities (a surrogate of patch quitting harvest rate), survival rates, body size or habitat distribution of voles. In both predator scent and vinegar treatments voles preferred abundant vegetation providing good cover, which was also the site of almost all tree seedling predation. We conclude that large mammalian predator scent does not influence the perception by voles of the general safety of habitat, which is more strongly affected by the presence of cover.     

Effects of prey-size and predator-instar on the predation of Daphnia by Notonecta

Abstract. 1. Attack rates and handling times are measured by a series of separate functional response experiments for each instar of Notonecta glauca attacking four size classes of Daphnia magna as prey. The resulting attack rate and handling time surfaces are complex, with maximum attack rates for small predators attacking small prey, and large predators attacking large prey. Adult Notonecta have lower attack rates than the two previous juvenile instars (4 and 5).
2. The literature on attack rates and handling times in other predator—prey interactions that involve a series of different predator and prey size or age classes is reviewed in the context of the Notonecta-Daphnia results. The data suggest that small predator instars will usually compete with large instars for food, unless there is spatial or temporal separation between them.
3. Complex attack rate and handling time surfaces are to be expected wherever a wide range of prey and predator sizes is involved.
4. Size related changes in attack rates and handling times can introduce very complex dynamics into predator-prey interactions.     

Stonefly predation along a hydraulic gradient: a field test of the harsh—benign hypothesis

SUMMARY. 1. Microhabitat preferences of predatory stoneflies and four prey taxa were assessed by taking benthic samples along a hydraulic gradient in a Black Forest stream in West Germany. Densities of predator and prey species were estimated at twenty-one hydraulic regimes.
2. Enclosures containing the stonefly, Dinocras cephalotes , and control cages with no predators were placed in the substrate at hydraulic regimes favourable and unfavourable to predators. Cages received initial prey communities that were obtained from benthic samples taken at hydraulic regimes matching those intended for each cage.
3. Population densities of the two most numerically important prey taxa, the mayfly. Baetis rhodani , and the Chironomidae, were reduced in the presence of Dinocras , but only when enclosures were placed in the hydraulic regimes favourable to the predator. Thus, predation effects increased as the hydraulic regime became more benign to the predators.
4. Densities of two other prey species rare in the diets of Dinocras ( Hydropsyche instabilis and Gammarus fossarum ) were generally unaffected by predators regardless of the hydraulic regime.
5. These data provide support for the hypothesis that perception of the abiotic regime as harsh or benign to predators is a good predictor of predator impact on densities of preferred prey species. In harsher abiotic regimes, impact will be low, while impact will be high in benign abiotic regimes.     

Interacting effects of predation risk and resource level on escape speed of amphibian larvae along a latitudinal gradient

Fast‐growing genotypes living in time‐constrained environments are often more prone to predation, suggesting that growth‐predation risk trade‐offs are important factors maintaining variation in growth along climatic gradients. However, the mechanisms underlying how fast growth increases predation‐mediated mortality are not well understood. Here, we investigated if slow‐growing, low‐latitude individuals have faster escape swimming speed than fast‐growing high‐latitude individuals using common frog (Rana temporaria) tadpoles from eight populations collected along a 1500 km latitudinal gradient. We measured escape speed in terms of burst and endurance speeds in tadpoles raised in the laboratory at two food levels and in the presence and absence of a predator (Aeshna dragonfly larvae). We did not find any latitudinal trend in escape speed performance. In low food treatments, burst speed was higher in tadpoles reared with predators but did not differ between high‐food treatments. Endurance speed, on the contrary, was lower in high‐food tadpoles reared with predators and did not differ between treatments at low food levels. Tadpoles reared with predators showed inducible morphology (increased relative body size and tail depth), which had positive effects on speed endurance at low but not at high food levels. Burst speed was positively affected by tail length and tail muscle size in the absence of predators. Our results suggest that escape speed does not trade‐off with fast growth along the latitudinal gradient in R. temporaria tadpoles. Instead, escape speed is a plastic trait and strongly influenced by the interaction between resource level and predation risk.     

Lethal and non-lethal effects of multiple indigenous predators on the invasive golden apple snail (Pomacea canaliculata)

1. We investigated the individual and combined effects of two predators (the climbing perch, Anabas testudineus, and the wetland crab, Esanthelphusa nimoafi) indigenous to wetlands in Laos, on the behaviour and survival of the invasive South American golden apple snail (Pomacea canaliculata). The snail is considered a pest, consuming large amounts of rice and other aquatic vegetation in the region. 2. Snail avoidance reactions to released predator chemical cues were investigated in aquaria while the effects of predators on a mixed snail population were studied in field enclosures that contained native aquatic plants (Salvinia cucullata, Ludwigia adscendens and Ipomoea aquatica). 3. In the aquaria experiment, neonate (2–3 mm) and medium‐sized snails (8–10 mm) responded to fish chemical cues by going to the surface, whereas adult snails (35–40 mm) went to the bottom. In contrast, no size class of snails reacted to chemical cues released by crabs. 4. In the field experiment, fish reduced the abundance of neonate snails, and crabs reduced the abundance of all size classes. The effect of the combined predators could not be predicted from the mortality rate observed in single predator treatments. The survival of neonate and medium‐sized snails was greater and of adults less than expected. The presence of predators did not affect egg production. Snails consumed significant amounts of plants despite the presence of predators. 5. Our findings suggest that some indigenous Asian predators have lethal and sublethal effects on P. canaliculata that depend on snail size and predator type. When in the presence of several predators the response of snails to one predator may either increase or decrease the vulnerability of snails to the others.