No evidence of nonlinear effects of predator density,refuge availability,or body size of prey on prey mortality rates

Predator density, refuge availability, and body size of prey can all affect the mortality rate of prey. We assume that more predators will lead to an increase in prey mortality rate, but behavioral interactions between predators and prey, and availability of refuge, may lead to nonlinear effects of increased number of predators on prey mortality rates. We tested for nonlinear effects in prey mortality rates in a mesocosm experiment with different size classes of western mosquitofish (Gambusia affinis) as the prey, different numbers of green sunfish (Lepomis cyanellus) as the predators, and different levels of refuge. Predator number and size class of prey, but not refuge availability, had significant effects on the mortality rate of prey. Change in mortality rate of prey was linear and equal across the range of predator numbers. Each new predator increased the mortality rate by about 10% overall, and mortality rates were higher for smaller size classes. Predator–prey interactions at the individual level may not scale up to create nonlinearity in prey mortality rates with increasing predator density at the population level.     

Feeding while evading predators by a lotic mayfly: linking short-term foraging behaviours to long-term fitness consequences

Laboratory experiments were performed to quantify the combined effects of food abundance (low, high) and predation threat, imposed using a model fish (safe, risky) on fitness correlates (i.e. growth, time to emergence, adult body mass, fecundity, egg size) of the mayfly Baetis tricaudatus. These effects were determined by rearing larvae under different combinations of food abundance and predator threat. Fitness correlates were significantly affected by food abundance, predation threat or the interaction of these factors. High food abundance and the absence of predation threat significantly increased larval growth rates, adult body mass, fecundity, egg size, and decreased time to emergence. Long-term effects of predator threat and food abundance on fitness correlates of B. tricaudatus were compared to previously conducted short-term patch choice trials to test for concordance between short-term measures of patch choice and their potential longer-term fitness consequences. This comparison indicated that patches that were utilized the most were those that yielded the highest fitness benefits. These results suggest that behavioural costs of balancing mortality risk due to predation against food acquisition by B. tricaudatus can have strong fitness consequences.     

Effect of resource gradient on age and size at maturity and their influence on early‐life fecundity in the predatory Asian lady beetle,Harmonia axyridis

Variation in food availability impacts the performance of insects in terms of their size and age to maturity and fecundity. Age at maturity determines how quickly individuals in a population can start to reproduce and how much they can reproduce. Results from studies on various insect species show that food availability influences the size and fecundity of adult females. It is predicted that under poor growth conditions, variation in size is low, but variation in age at maturity is considerable. This prediction was examined in a widely distributed lady beetle species, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), a predator of aphids and coccids. Using a food gradient from low to high aphid prey density, performance of females that were reared on excess food was recorded for pre‐reproductive duration, size at reproductive maturity, number of aphids consumed, and fecundity in the first 10 days of their reproductive period. Results suggested that female H. axyridis that were reared on surplus food when kept at low prey density (poor growth condition) took, on average, three times longer to attain maturity and produced, on average, 14 times fewer eggs than females that were also reared on surplus food, but kept at high prey density (good growth condition). Females performed best at a prey density of 30 aphids per female per 150 cm². Results suggested that the current food availability significantly influenced the age and size of females at maturity and their fecundity. Age and size at maturity of female lady beetles showed non‐linear responses to prey density as well as the occurrence of a minimum size of females, below which H. axyridis females fail to mature. The steep slope recorded at lower prey densities suggests relatively high variation in age at maturity but low variation in size.     

Phosphorus availability mediates plasticity in life-history traits and predator–prey interactions in Daphnia

We analysed growth plasticity of two Daphnia pulex clones under low‐phosphorus (LP) and high phosphorus (HP) conditions, in the presence of Chaoborus kairomones to examine how food quality (P‐availability) might impact life‐history responses and vulnerability to predation. Overall, clone 1 grew faster, and was larger at maturity. Under HP, both clones responded to kairomones by increasing growth, age and size at maturity, and decreasing fecundity. Under LP, both clones suffered reduced growth, and fecundity. However, the magnitude of response to kairomones depended on a clone by P‐availability interaction. Chaoborus presented a 1 : 1 clonal mixture under HP or LP, consumed more individuals under LP. Moreover, fewer clone 1 individuals were consumed. Studying the effects of P‐availability on life histories, and predator–prey interactions may help us understand the mechanisms generating and maintaining plasticity, as well as influencing genotypic diversity and microevolutionary processes in natural populations.     

Food availability and parasite infection influence the correlated responses of life history traits to selection for age at pupation in the mosquito Aedes aegypti

We selected six lines of mosquito Aedes aegypti for earlier or later pupation and measured the correlated responses of several life history traits: adult size, two fecundity measures and pre-adult survival. We further examined the influence of two environmental parameters – larval food availability and infection by the microsporidian parasite Edhazardia aedis– on the correlated responses. Pre-adult survival did not respond to selection for age at pupation in any environment. For all of the other traits, the environment influenced the correlated response, though the contribution of the different environmental aspects differed among traits. While the correlated response of adult size was influenced only by larval food availability, the likelihood that a female laid eggs was influenced by parasite infection, and the correlated response of the number of eggs was influenced by the interaction of the two environmental parameters. Generally, a deteriorating environment moved the correlated response from one favouring later pupation to one favouring earlier pupation. Larval food availability and parasite infection also influenced the association between the mean wing length and fecundity of the selected lines. At high food availability, there was a positive relationship between adult size and fecundity, while infected mosquitoes reared at low food availability showed the opposite trend. We discuss these results in light of the coevolutionary potential of the host–parasite interaction.     

Antipredator responses in Tetranychus urticae differ with predator specialization

The behavioural response of Tetranychus urticae to chemical cues from specialist predatory mites, Phytoseiulus persimilis, or generalist predatory bugs, Orius majusculus, on either bean or strawberry was studied in experimental arenas. Predators were placed on the leaf disc for 24 h and removed before T. urticae females were introduced. After 24 h, prey fecundity (number of eggs laid) and dispersal (number of prey drowned in the water barrier) were assessed. Chemical cues from the specialist predator resulted in reduced prey fecundity, significantly different from the generalist predator and control treatments. No interaction effect was found between plant species and prey fecundity, while significantly more eggs were laid on bean than on strawberry. Predator cues irrespective of predator specialization resulted in more prey dispersal than in the control. Findings emphasize the importance of specialization in the predator species complex for the degree and type of antipredator responses and resulting biological control.     

Resource availability modulates the effect of body size on reproductive development

Within-species variation in animal body size predicts major differences in life history, for example, in reproductive development, fecundity, and even longevity. Purely from an energetic perspective, large size could entail larger energy reserves, fuelling different life functions, such as reproduction and survival (the “energy reserve” hypothesis). Conversely, larger body size could demand more energy for maintenance, and larger individuals might do worse in reproduction and survival under resource shortage (the “energy demand” hypothesis). Disentangling these alternative hypotheses is difficult because large size often correlates with better resource availability during growth, which could mask direct effects of body size on fitness traits. Here, we used experimental body size manipulation in the freshwater cnidarian Hydra oligactis, coupled with manipulation of resource (food) availability to separate direct effects of body size from resource availability on fitness traits (sexual development time, fecundity, and survival). We found significant interaction between body size and food availability in sexual development time in both males and females, such that large individuals responded less strongly to variation in resource availability. These results are consistent with an energy reserve effect of large size in Hydra. Surprisingly, the response was different in males and females: small and starved females delayed their reproduction, while small and starved males developed reproductive organs faster. In case of fecundity and survival, both size and food availability had significant effects, but we detected no interaction between them. Our observations suggest that in Hydra, small individuals are sensitive to fluctuations in resource availability, but these small individuals are able to adjust their reproductive development to maintain fitness.     

Contributions of juvenile and adult diet to the lifetime reproductive success and lifespan of a spider

Food availability can vary widely for animals in nature and can have large effects on growth, reproduction and survival. While the consequences of food limitation for animals have been extensively studied, significant questions still remain including how ontogenetic variation in food availability contributes to lifetime reproductive success. We tested the effects of juvenile and adult food limitation on the lifetime reproductive success and lifespan of bridge spiders, Larinioides sclopetarius. Food availability was manipulated (low or high) over the entire juvenile and adult stage in a full‐factorial design and reproductive output and lifespan were measured. Juvenile and adult food limitation both reduced lifetime egg and hatchling production with effect sizes that were not significantly different from each other. Unlike some other arthropods, where juvenile food limitation reduces fecundity by reducing adult body size, body size was not affected by juvenile diet in bridge spiders. Clutch size was also significantly reduced by both juvenile and adult food limitation. The effect of adult diet on clutch size was stronger than that of juvenile diet. Juvenile and adult food limitation both extended total lifespan, and adult food limitation extended adult longevity (i.e. time from maturation to death). However, juvenile food limitation decreased adult longevity, in contrast to what would be predicted by dietary or caloric restriction. Compensatory feeding and growth are widely recognized mechanisms through which animals can ameliorate some of the negative effects of periods of food limitation. Yet our results combined with studies of a range of other species suggest that there may be lasting consequences of juvenile food limitation on lifetime reproductive success that cannot be compensated for by adult feeding in some species.     

Differential effects of native vs. invasive predators on a common Caribbean reef fish

Predators may have consumptive (lethal) and non-consumptive (sub-lethal) effects on prey. Non-consumptive effects include altered behavior and reduced growth and fecundity. Native prey may not recognize non-native predators as a threat, and therefore may suffer pronounced effects. Additionally, non-native predators may elicit different behavioral responses from prey compared to native predators. Theory predicts that consumptive effects should be greater for non-native predators (due to prey naiveté), and non-consumptive effects should be greater for native predators (due to predator recognition). To test these hypotheses, I monitored bicolor damselfish (Stegastes partitus) in the presence of invasive predatory Pacific lionfish (Pterois spp.), a native predator (graysby, Cephalopholis cruentata), and an egg predator (bluehead wrasse, Thalassoma bifasciatum). Body size and location of lionfish and graysby were monitored on reefs in the Bahamas. Bicolor fecundity was measured as the number and size of egg-masses that individual fish laid. Bicolor fecundity was negatively correlated with lionfish density but not graysby or bluehead density. Neither predator had a detectable effect on bicolor body size, but lionfish density was negatively correlated with the size of mature adult damselfish. I observed behavioral responses of bicolors to the two piscivores, to bluehead wrasse, and to two herbivorous fishes (Acanthurus coeruleus, Scarus spp.) as non-aggressive controls. Bicolors changed behavior (feeding and aggression) in the presence of all native fishes, but not in the presence of lionfish. Thus, differential effects exist between native and non-native predators, and invasive lionfish pose a non-consumptive threat to bicolor damselfish via reduced growth and fecundity.     

Effects of scarcity and excess of larval food on life history traits of Aedes aegypti (Diptera: Culicidae)

Few studies have assessed the effects of food scarcity or excess on the life history traits of Aedes aegypti (L.) (Diptera: Culicidae) independently from larval density. We assessed immature survival, development time, and adult size in relation to food availability. We reared cohorts of 30 Ae. aegypti larvae from newly hatched to adult emergence with different food availability. Food conditions were kept constant by transferring larvae each day to a new food solution. Immature development was completed by some individuals in all treatments. The shortest development time, the largest adults, and the highest survival were observed at intermediate food levels. The most important effects of food scarcity were an extension in development time, a decrease in the size of adults, and a slight decrease in survival, while the most important effects of food excess were an important decrease in survival and a slight decrease in the size of adults. The variability in development time and adult size within sex and treatment increased at decreasing food availability. The results suggest that although the studied population has adapted to a wide range of food availabilities, both scarcity and excess of food have important negative impacts on fitness.     

Impact of Chaoborus predation upon the structure and dynamics of a crustacean zooplankton community

Summary During ice-free seasons of 1975, 1977 and 1978, replicated experimental alteration of spring densities of predatory Chaoborus larvae inside 20–27 m³ enclosures in a fishless oligotrophic lake had relatively small, but significant, short-term effects upon prey species abundances. Enhancement of predator densities generally had greater numerical effects relative to controls than did complete removal of predators. With the exceptions Diaphanosoma and Bosmina under artificially elevated Chaoborus densities, numerical effects on prey species did not persist for more than a few weeks after midsummer in these 3 years. During cooler 1976, however, much larger Chaoborus effects in May and June persisted into September. Low temperatures and small initial population densities slowed population growth and tended to increase the proportion of each species' recruitment lost to Chaoborus predation. As water temperatures increased during 3 of 4 summers, rapid juvenile development and compensating increase in adult fertility generally permitted most prey species to escape regulation by these large, univoltine and semivoltine predators. With growing prey population size, declining food levels suppressed crustacean fecundity in July and August, thereby permitting predatory losses to climb again to substantial fractions of the reduced prey recruitment. Nevertheless, prey densities in predator-free and control or predator-enhanced enclosures differed little from July through September (except in 1976). Thus, summer population growth of most prey species seemed more limited by food shortages than by predators per se. Growth of individual zooplankters was affected by food availability during critical periods in July and August in all years, and Chaoborus predation seemed to influence the timing of this food limitation in at least 2 of the 4 years     

Are there interactive effects of mate availability and predation risk on life history and defence in a simultaneous hermaphrodite?

Encountering mates and avoiding predators are ubiquitous challenges faced by many organisms and they can affect the expression of many traits including growth, timing of maturity and resource allocation to reproduction. However, these two factors are commonly considered in isolation rather than simultaneously. We examined whether predation risk and mate availability interact to affect morphology and life-history traits (including lifetime fecundity) of a hermaphroditic snail (Physa acuta). We found that mate availability reduced juvenile growth rate and final size. Predator cues from crayfish induced delayed reproduction, but there were no reduced fecundity costs associated with predator induction. Although there were interactive effects on longevity, lifetime fecundity was determined by the number of reproductive days. Therefore, our results indicate a resource-allocation trade-off among growth, longevity and reproduction. Future consideration of this interaction will be important for understanding how resource-allocation plasticity affects the integration of defensive, life-history and mating-system traits.     

No overall benefit of predator exclosure cages for the endangered St. Helena Plover Charadrius sanctaehelenae

Predator exclosure cages are designed to increase the clutch survival of ground‐nesting birds. Predator exclosures provided for the endangered St. Helena Plover Charadrius sanctaehelenae, however, did not result in differences in clutch survival between protected and control nests and may have resulted in elevated adult mortality. Exclosures did not exclude all cats, the dominant nest predator, and it is likely that cats caused the adult mortalities observed close to the exclosures. A population model indicates that even if predator exclosures had excluded all cats, the benefits of increased clutch survival would have been more than negated by the estimated decrease in adult survival. The overall effect of predator exclosures needs to be clarified for other species, taking into consideration annual productivity and adult survival, to understand the circumstances in which predator exclosures are beneficial.     

Consumer versus resource control and the importance of habitat heterogeneity for estuarine bivalves

The relative influence of consumers (top down) and resources (bottom up) on the distribution and abundance of organisms remains a key question in ecology. We examined the relationships between consumer and resource variables along a productivity gradient for a dominant predator–prey interaction in a marine soft‐sediment system. We 1) quantified density and size of the clam Macoma balthica (prey species) in six replicate sites at each of four habitat types (shallow mud, deep mud, muddy sand and detrital mud) in the Rhode River, Chesapeake Bay. We selected one habitat type of high food availability and clam density (shallow mud) and another of low food availability and clam density (muddy sand) for manipulative experiments. Then, we 2) measured M. balthica survival and growth through transplants, 3) measured food availability as sedimentary organic carbon content, 4) quantified predator density, and 5) calculated predator foraging efficiency in the two habitat types. Clam density in the four habitat types differed and was related to sedimentary carbon availability and predator density. One of the habitats, detrital mud, appeared to be a population sink because it only held juvenile Macoma that never survived to reproductive age. Macoma size and growth, and predator (mainly blue crab Callinectes sapidus) densities were positively correlated with productivity and were higher in shallow mud than muddy sand. In contrast, Macoma mortality, local ‘interaction strength’, and predator foraging efficiency were lower in the productive habitat (shallow mud). Thus, predation intensity was inversely correlated with productivity (food availability); consumer and resource effects differed by habitat type; and, at a relatively small spatial scale, consumer and resource forces jointly determined population dynamics in this soft‐sediment marine system.     

Application of predator-associated cues to control small brown planthoppers: non-consumptive effects of predators suppress the pest population

Predator non-consumptive effects (NCEs) have been well studied in many ecosystems and NCEs can alter the behavior, morphology and life history of prey, producing strong trait-mediated indirect effects (TMIEs) on host plants. However, studies involving the application of NCEs to control pests in the field, and instances of combined laboratory bioassay and field practice are rare. Here, we examine the development, reproduction and behavior of small brown planthoppers, Laodelphax striatellus (Fallén), when exposed to predator cues from caged predators (Paederus fuscipes Curtis), or predator body extracts (in solvents with different polarities) in the laboratory. Field foliage sprays of these extracts were also used to test their effects on the L. striatellus population and rice plant biomass. Nymph development and egg hatch rate in L. striatellus were not influenced, but adult longevity was shorter, and fecundity and weight gain were lower, when nymphs were exposed to the predator cues. Adults exposed to predator cues also gained less weight and laid fewer eggs. The poorer developmental and reproductive performances might result from lower activity levels observed in threatened L. striatellus. The field foliage sprays of predator cues decreased L. striatellus abundance and increased rice plant biomass, suggesting their possible application for pest control. Predator cues extracted using chloroform increased stronger NCEs and TMIEs, indicating their non-polar characteristics. Our studies advance the understanding of how NCEs shape the life history and behavior of L. striatellus and improve rice growth, laying new foundations for future research on novel pest control materials and methods.

     

Influence of host age and host deprivation on longevity, egg load dynamics and oviposition rate in Microplitis rufiventris

Adult size, longevity, egg load dynamics and oviposition ofMicroplitis rufiventris Kok. which began their development in the first, second, third （preferred hosts） or fourth （non-preferred hosts） instar larvae of Spodoptera littoralis （Boisd.） were studied. The parasitoid size was largely determined by the initial host size at parasitism. Non-ovipositing females derived from older hosts lived for longer periods than those derived from younger ones. However, the ovipositing females, irrespective of their size, lived for almost the same periods. At emergence, the oviducts of adult females contain a significant amount of mature eggs available for oviposition for a few hours on eclosion day. Egg load increases during the early phase of adult life. The amount of additional mature eggs and rate of egg maturation per hour was greater for wasps derived from preferred hosts compared with those in females derived from non-preferred hosts. The pattern of egg production in M. rufiventris females depended on the availability of hosts for parasitization. Host-deprived females depleted the egg complement with aging; the longer the host deprivation, the lower the oviduct egg load. Marked reduction in both realized or potential fecundity of host-deprived females was observed following host availability. Host privation for more than 3 days induced a marked deficit fecundity pattern through the female＇ s life. The realized fecundity was determined by the interaction among host availability, the number of eggs that are matured over the female＇ s life span, oviposition rate and host size from which the female was derived. These results suggest that： （i） M. rufiventris wasp is a weak synovigenic species; （ii） the maturation of additional eggs is inhibited once the maximum oviduct egg load is reached; （iii） the egg load of the newly emerged female is significantly less than the realized fecundity; and （iv） because M. rufiventris females oviposit fewer eggs when they begin depleting their egg supply at 3 days, augmentative releases will require release immediately following emergence to ensure the highest parasitization rate in the field.     

Climate and mammalian life histories

Mammals display considerable geographical variation in life history traits. To understand how climatic factors might influence this variation, we analysed the relationship between life history traits – adult body size, litter size, number of litters per year, gestation length, neonate body mass, weaning age and age at sexual maturity – and several environmental variables quantifying the seasonality and predictability of temperature and precipitation across the distribution range of five terrestrial mammal groups. Environmental factors correlated strongly with each other; therefore, we used principal components analysis to obtain orthogonal climatic predictors that could be used in multivariate models. We found that in bats, primates and even‐toed ungulates adult body size tends to be larger in species inhabiting cold, dry, seasonal environments, whereas in carnivores and rodents a smaller body size is characteristic of warm, dry environments, suggesting that low food availability might limit adult size. Species inhabiting cold, dry, seasonal habitats have fewer, larger litters and shorter gestation periods; however, annual fecundity in these species is not higher, implying that the large litter size of mammals living at high latitudes is probably a consequence of time constraints imposed by strong seasonality. On the other hand, the number of litters per year and annual fecundity were greater in species inhabiting environments with higher seasonality in precipitation. Lastly, we found little evidence for specific effects of environmental variability. Our results highlight the complex effects of environmental factors in the evolution of life history traits in mammals. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 719–736.     

Effect of diet and mating status on ovarian development and oviposition in the polyphagous predator Macrolophus caliginosus (Heteroptera: Miridae)

The mirid bug Macrolophus caliginosus is commercially reared on eggs of Ephestia kuehniella, constituting an effective but expensive factitious food. Artificial diets can decrease the rearing costs of this natural enemy, but developing and evaluating an artificial diet is a very time-consuming activity. In the current study, development and reproduction of M. caliginosus on two artificial diets based on egg yolk were investigated. The artificial diets resulted in longer development and lower adult weights, but survival was comparable with that of control insects fed E. kuehniella eggs. Reproductive potential of the predator reared on factitious and artificial foods was assessed using a dissection method. The influence of nymphal food on fecundity was less important than that of adult food. Adults fed E. kuehniella eggs had a preoviposition period of about 4 days, whereas adults offered only plant material started laying eggs about 7 days after emergence. Ovarian scores at day 7 were higher for females fed E. kuehniella eggs than for those given access only to a tobacco leaf. Ovarian scores were not significantly affected by mating status. In a final test, a parallel comparison of two methods for assessing reproductive response to diet was made. Here, adult couples were offered one of four diets: E. kuehniella eggs, one of two artificial diets or no food. Half of the females were dissected and the other half was held for determining lifetime oviposition. Females fed E. kuehniella eggs had superior ovarian scores and laid more eggs than those fed either artificial diet or those given no extra food. A good correlation (r = 0.97) was obtained between ovarian scores and oviposition data, indicating that dissecting females after 1 week provides a reliable estimate of fecundity as affected by diet quality. Rapid reproductive assessments as used in the current study will help to increase the rate of development of artificial diets and may contribute to more cost effective production methods for augmentative biological control agents.     

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.     

Evidence for carry-over effects of predator exposure on pathogen transmission potential

Accumulating evidence indicates that species interactions such as competition and predation can indirectly alter interactions with other community members, including parasites. For example, presence of predators can induce behavioural defences in the prey, resulting in a change in susceptibility to parasites. Such predator-induced phenotypic changes may be especially pervasive in prey with discrete larval and adult stages, for which exposure to predators during larval development can have strong carry-over effects on adult phenotypes. To the best of our knowledge, no study to date has examined possible carry-over effects of predator exposure on pathogen transmission. We addressed this question using a natural food web consisting of the human malaria parasite Plasmodium falciparum, the mosquito vector Anopheles coluzzii and a backswimmer, an aquatic predator of mosquito larvae. Although predator exposure did not significantly alter mosquito susceptibility to P. falciparum, it incurred strong fitness costs on other key mosquito life-history traits, including larval development, adult size, fecundity and longevity. Using an epidemiological model, we show that larval predator exposure should overall significantly decrease malaria transmission. These results highlight the importance of taking into account the effect of environmental stressors on disease ecology and epidemiology.