Competing predators for a prey in a chemostat model with periodic nutrient input

A system of ordinary differential equations is used to model the interactions of n competing predators on a single prey population in a chemostat environment with a periodic nutrient input. In the case of one or no predators, criteria for the existence of periodic solutions are given. In the general case, conditions for all populations to persist are derived.Research is in part based on a Ph.D. thesis submitted to the Faculty of Graduate Studies, University of AlbertaResearch is partly supported by the Natural Sciences and Engineering Research Council of Canada, Grant No. NSERC A4823     

Geographic variation in prey preference in bark beetle predators

Abstract 1. Bark beetles and their predators are useful systems for addressing questions concerning diet breadth and prey preference in arthropod natural enemies. These predators use bark beetle pheromones to locate their prey, and the response to different pheromones is a measure of prey preference. 2. Trapping experiments were conducted to examine geographic variation in the response to prey pheromones by two bark beetle predators, Thanasimus dubius and Temnochila virescens. The experiments used pheromones for several Dendroctonus and Ips prey species (frontalin, ipsdienol, and ipsenol) and manipulated visual cues involved in prey location (black vs. white traps). The study sites included regions where the frontalin‐emitter Dendroctonus frontalis was in outbreak vs. endemic or absent. 3. There was significant geographic variation in pheromone preference for T. dubius. This predator strongly preferred a pheromone (frontalin) associated with D. frontalis at outbreak sites, while preference was more even at endemic and absent sites. No geographic variation was found in the response by T. virescens. White traps caught fewer insects than black traps for both predators, suggesting that visual cues are also important in prey location. 4. The overall pattern for T. dubius is consistent with switching or optimal foraging theory, assuming D. frontalis is a higher quality prey than Ips. The two predator species partition the prey pheromones in areas where D. frontalis is abundant, possibly to minimise competition and intraguild predation.     

Non-lethal effects of predators on prey growth rates depend on prey density and nutrient additions

A number of studies show that predators can depress prey growth rates by inducing reductions in foraging activity, but the size of this non-lethal effect is quite variable. Here I investigate how prey density and resource productivity may alter the extent to which predators depress the growth rates of their prey. Theory predicts that when resources are overgrazed, an increase in predation risk will have little net effect on individual food intake because the decline in foraging effort will be offset by an increase in resource level. Thus, the non-lethal effects of predators on prey growth rates should depend upon prey density and resource productivity in a predictable manner, with the growth penalty imposed by predators being strongest when resources are undergrazed and weakest when resources are overgrazed. I tested this hypothesis by manipulating predation risk, prey density, and nutrient additions in a mesocosm experiment with the pulmonate snail Helisoma trivolvis . Refuge use by snails was 45% higher in the presence of caged crayfish than in their absence. Snail growth rates were reduced, on average, by 24% in the presence of caged crayfish. However, the magnitude of the growth penalty exacted by crayfish depended on snail density and nutrient additions. When snails were stocked at high density and nutrient additions were low, growth suppression was just 2.6%. At the other extreme, when snails were at low density and nutrient additions were high, growth suppression was 44.6%. Thus, the non-lethal effects of predators on prey growth depend on environmental context, illustrating an important link between individual traits and system-level properties.     

Slight phenotypic variation in predators and prey causes complex predator-prey oscillations

Predator-prey oscillations are expected to show a 1/4-phase lag between predator and prey. However, observed dynamics of natural or experimental predator-prey systems are often more complex. A striking but hardly studied example are sudden interruptions of classic 1/4-lag cycles with periods of antiphase oscillations, or periods without any regular predator-prey oscillations. These interruptions occur for a limited time before the system reverts to regular 1/4-lag oscillations, thus yielding intermittent cycles. Reasons for this behaviour are often difficult to reveal in experimental systems. Here we test the hypothesis that such complex dynamical behaviour may result from minor trait variation and trait adaptation in both the prey and predator, causing recurrent small changes in attack rates that may be hard to capture by empirical measurements. Using a model structure where the degree of trait variation in the predator can be explicitly controlled, we show that a very limited amount of adaptation resulting in 10–15% temporal variation in attack rates is already sufficient to generate these intermittent dynamics. Such minor variation may be present in experimental predator-prey systems, and may explain disruptions in regular 1/4-lag oscillations.     

Dangerous prey and daring predators: a review

How foragers balance risks during foraging is a central focus of optimal foraging studies. While diverse theoretical and empirical work has revealed how foragers should and do manage food and safety from predators, little attention has been given to the risks posed by dangerous prey. This is a potentially important oversight because risk of injury can give rise to foraging costs similar to those arising from the risk of predation, and with similar consequences. Here, we synthesize the literature on how foragers manage risks associated with dangerous prey and adapt previous theory to make the first steps towards a framework for future studies. Though rarely documented, it appears that in some systems predators are frequently injured while hunting and risk of injury can be an important foraging cost. Fitness costs of foraging injuries, which can be fatal, likely vary widely but have rarely been studied and should be the subject of future research. Like other types of risk‐taking behaviour, it appears that there is individual variation in the willingness to take risks, which can be driven by social factors, experience and foraging abilities, or differences in body condition. Because of ongoing modifications to natural communities, including changes in prey availability and relative abundance as well as the introduction of potentially dangerous prey to numerous ecosystems, understanding the prevalence and consequences of hunting dangerous prey should be a priority for behavioural ecologists.     

Detection of prey flocks by predators

The model presented by Vine (1971) to demonstrate a possible advantage of flocking behaviour in reducing predation risk made the unrealistic assumption that flocking would not affect the maximum distance at which a predator could just detect that an animal or animals were present. If flocking does reduce this distance it might yield a compensating disadvantage to prey animals, or an advantage to the predator. While quantification of these possibilities remains problematic in general, empirical data suggests that detection range depends on a simple power function of the number of individuals in a tight circular flock and their physical size. It is concluded that small flocks are likely to be advantageous to the predator and quite possibly disadvantageous to the prey, although the “selfish” advantage of flocking is likely to remain high for prey animals in medium or large sized flocks.     

Intraspecific variation in responses to aposematic prey in a jumping spider (Phidippus regius)

Aposematic signals often allow chemically defended prey to avoid attack from generalist predators, including jumping spiders. However, not all individual predators in a population behave in the same way. Here, in laboratory trials, we document that most individual Phidippus regius jumping spiders attack and reject chemically defended milkweed bugs (Oncopeltus fasciatus), immediately releasing them unharmed. However, a small number of individuals within the population kill and completely consume these presumably toxic prey items. This phenomenon was infrequent with only 14% of our sample (17/122) consuming the milkweed bugs over the course of the study. Individuals that killed and consumed bugs often did so repeatedly; specifically, individuals that consumed a bug in their first test were more likely to kill a bug in their second test and also tended to consume them again. We explored what might drive some (but not all) individuals to consume these bugs and found that neither sex, sexual maturity, body size, laboratory housing type, nor being wild-caught or being laboratory-reared, predicted milkweed bug consumption. Consuming bugs had no negative effects on spider mass or body condition; contrary to expectations, individuals that consumed milkweed bugs actually gained more body mass and increased in body condition. We discuss potential behavioural and physiological variation between individuals that may drive these rare behaviours and the implications for the evolution of prey defences.     

Evolutionary ecology of movement by predators and prey

An essential key to explaining the mechanistic basis of ecological patterns lies in understanding the consequences of adaptive behavior for distributions and abundances of organisms. We developed a model that simultaneously incorporates (a) ecological dynamics across three trophic levels and (b) evolution of behaviors via the processes of mutation, selection, and drift in populations of variable, unique individuals. Using this model to study adaptive movements of predators and prey in a spatially explicit environment produced a number of unexpected results. First, even though predators and prey had limited information and sometimes moved in the “wrong” direction, evolved movement mechanisms allowed them to achieve average spatial distributions approximating optimal, ideal free distributions. Second, predators’ demographic parameters had marked, nonlinear effects on the evolution of movement mechanisms in the prey: As the predator mortality rate was increased past a critical point, prey abruptly shifted from making very frequent movements away from predators to making infrequent movements mainly in response to resources. Third, time series analyses revealed that adaptive, conditional movements coupled ecological dynamics across species and space. Our results provide general predictions, heretofore lacking, about how predators and prey should respond to one another on both ecological and evolutionary time scales.     

Better the devil you know: avian predators find variation in prey toxicity aversive

Toxic prey that signal their defences to predators using conspicuous warning signals are called ‘aposematic’. Predators learn about the toxic content of aposematic prey and reduce their attacks on them. However, through regulating their toxin intake, predators will include aposematic prey in their diets when the benefits of gaining the nutrients they contain outweigh the costs of ingesting the prey''s toxins. Predators face a problem when managing their toxin intake: prey sharing the same warning signal often vary in their toxicities. Given that predators should avoid uncertainty when managing their toxin intake, we tested whether European starlings (Sturnus vulgaris) preferred to eat fixed-defence prey (where all prey contained a 2% quinine solution) to mixed-defence prey (where half the prey contained a 4% quinine solution and the other half contained only water). Our results support the idea that predators should be more ‘risk-averse’ when foraging on variably defended prey and suggest that variation in toxicity levels could be a form of defence.     

Predators buffer the effects of variation in prey nutrient content for nutrient deposition

Predator feeding behavior and digestion regulate the flow of nutrients through ecosystems by determining the fate of prey nutrients. Most predators feed on a diversity of prey items, which differ widely in traits including their nutrient content. Yet, relatively little is known of the mechanisms through which variation in prey nutrient content affects the form by which nutrients are deposited into the environment. The overall goal of this study was to test how variation in the nutrient content of prey affected the fate of nutrients following predation by an arthropod carnivore, the Carolina wolf spider Hogna carolinensis. We manipulated the macronutrient content of prey by varying the diet on which crickets were fed to produce prey treatments that differed in lipid and protein content. Nutrients were measured as both macronutrients and elements in prey and elements in excreta. We found that there was no effect of diet treatment on the amounts of elements or macronutrients in prey carcasses and excreta despite significant variation in the nutrient content of those prey. This is in contrast to studies of some aquatic systems where mass balance by consumers results in variation in excreta content depending on the nutrient content of food. Wolf spiders assimilated the majority of prey nutrients and deposited relatively small and similar amounts of nutrients following feeding. Hence, while prey can vary widely in nutrient content, our findings suggest that this variation has little effect on the amounts of nutrients deposited by predators.     

Intraspecific alkaloid variation in ladybird eggs and its effects on con- and hetero-specific intraguild predators

Sexual size dimorphism (SSD) is a common phenomenon in animals. In many species females are substantially larger than males. Because body size plays a central role in modulating the body temperature (T _b) of ectotherms, intersexual differences in body size may lead to important intersexual differences in thermoregulation. In addition, because SSD is realized by differences in growth rate and because growth rate is strongly temperature dependent in ectotherms, a conflict between male reproductive behaviour and thermoregulation may affect the expression of SSD. In this study, we investigated the thermal implications of SSD in a reptile exhibiting spectacular female-biased SSD: the northern map turtle (Graptemys geographica). Over three seasons, we collected >150,000 measurements of T _b in free-ranging adult and juvenile northern map turtles using surgically implanted miniature temperature loggers. Northern map turtles exhibited seasonal patterns of thermoregulation typical of reptiles in northern latitudes, but we found that large adult females experienced a lower daily maximum T _b and a narrower daily range of T _b than adult males and small juvenile females. In addition, despite more time spent basking, large adult females were not able to thermoregulate as accurately as small turtles. Our findings strongly suggest that body size limits the ability to thermoregulate accurately in large females. By comparing thermoregulatory patterns between adult males and juvenile females of similar body size, we found no evidence that male reproductive behaviours are an impediment to thermoregulation. We also quantified the thermal significance of basking behaviour. We found, contrary to previous findings, that aerial basking allows northern map turtles to raise their T _b substantially above water temperature, indicating that basking behaviour likely plays an important role in thermoregulation.     

Control of aphids on wheat by generalist predators: effects of predator density and the presence of alternative prey

There is evidence for both positive and negative effects of generalist predators on pest populations and the various reasons for these contrasting observations are under debate. We studied the influence of a generalist predator, Pardosa lugubris (Walckenaer) (Araneae: Lycosidae), on an aphid pest species, Rhopalosiphum padi (L.) (Hemiptera: Aphididae; low food quality for the spider), and its host plant wheat, Triticum spec. (Poaceae). We focused on the role of spider density and the availability of alternative prey, Drosophila melanogaster Meigen (Diptera: Drosophilidae; high food quality). The presence of spiders significantly affected plant performance and aphid biomass. Alternative prey and spider density strongly interacted in affecting aphids and plants. High spider density significantly improved plant performance but also at low spider density plants benefited from spiders especially in the presence of alternative prey. The results suggest that generalist arthropod predators may successfully reduce plant damage by herbivores. However, their ability to control prey populations varies with predator nutrition, the control of low-quality prey being enhanced if alternative higher-quality prey is available.     

Temporally fluctuating prey and interfering predators: a positive feedback

Prey densities often show fluctuating patterns over various timescales. Focusing on short-term, within-generation fluctuating patterns of local prey availability, we suggest that prey that show synchronized and high-amplitude fluctuations in availability experience decreased risks of predation, but also enhance the maintenance of predator interference hierarchies by affecting the relative foraging success of unequal conspecific interferers. When predators interfere with each other, they forage less intensely on prey, which benefits prey in terms of decreased predation risk. The system hence involves a positive feedback. We thus argue that short-term temporal fluctuations in local prey availability could be an important mechanism behind how interference-structured social predator systems are developed and sustained. The temporal fluctuations also have implications for the phenotypic diversity of predators, and may be involved in speciation processes.     

Prey attack and predators defend: counterattacking prey trigger parental care in predators

That predators attack and prey defend is an oversimplified view. When size changes during development, large prey may be invulnerable to predators, and small juvenile predators vulnerable to attack by prey. This in turn may trigger a defensive response in adult predators to protect their offspring. Indeed, when sizes overlap, one may wonder "who is the predator and who is the prey"! Experiments with "predatory" mites and thrips "prey" showed that young, vulnerable prey counterattack by killing young predators and adult predators respond by protective parental care, killing young prey that attack their offspring. Thus, young individuals form the Achilles' heel of prey and predators alike, creating a cascade of predator attack, prey counterattack and predator defence. Therefore, size structure and relatedness induce multiple ecological role reversals.     

Dodging the moon: The moon effect on activity allocation of prey in the presence of predators

Animals can adapt their activity patterns throughout the circadian cycle. Prey may use moonlight as a predation risk cue and allocate their activity to lower risk periods. Here, we assessed moon transit influence on the activity allocation of nocturnal mammalian prey, in the presence of a predator (pumas, Puma concolor), during different moon phases, through camera trapping in Central Amazon. Thirty camera traps were installed 2 km apart from each other in each of our three study sites. Prey record distributions were assessed across the moon cycle, and their daily activity patterns were described in each moon phase. The record distributions of pacas (Cuniculus paca) (N = 262) and armadillos (Dasypus sp.) (N = 244) were concentrated in darker nights, contrasting with red brocket deer (Mazama americana) (N = 123) and pumas (N = 31), whose records were evenly distributed through the moon cycle. Yet, every prey was found to avoid the brighter times of the night regardless of the moon phase. These findings suggest prey can shift the temporal distribution of their activities under different moon phases when predators are present, perhaps in response to predation risk variations.     

Well-informed foraging: damage-released chemical cues of injured prey signal quality and size to predators

Predators use a variety of information sources to locate potential prey, and likewise prey animals use numerous sources of information to detect and avoid becoming the meal of a potential predator. In freshwater environments, chemosensory cues often play a crucial role in such predator/prey interactions. The importance of chemosensory information to teleost fish in marine environments is not well understood. Here, we tested whether coral reef fish predators are attracted to damage-released chemical cues from already wounded prey in order to find patches of prey and minimize their own costs of obtaining food. Furthermore, we tested if these chemical cues would convey information about status of the prey. Using y-maze experiments, we found that predatory dottybacks, Pseudochromis fuscus, were more attracted to skin extracts of damselfish, Pomacentrus amboinensis, prey that were in good condition compared to prey in poor body condition. Moreover, in both the laboratory and field, we found that predators could differentiate between skin extracts from prey based on prey size, showing a greater attraction to extracts made from prey that were the appropriate size to consume. This suggests that predators are not attracted to any general substance released from an injured prey fish instead being capable of detecting and distinguishing relatively small differences in the chemical composition of the skin of their prey. These results have implications for understanding predator foraging strategies and highlights that chemical cues play a complex role in predator–prey interactions in marine fish.     

Signal conflict in spider webs driven by predators and prey

Variation in the sensory physiologies of organisms can bias the receptions of signals, driving the direction of signal evolution. Sensory drive in the evolution of signals may be particularly important for organisms that confront trade-offs in signal design between the need for conspicuousness to allow effective transfer of information and the need for crypsis of the signal to unintended receivers. Several genera of orb-weaving spiders include conspicuous silk designs, stabilimenta, in the centre of their webs. Stabilimenta can be highly visible signals to predators, warning them of the presence of a noxious, sticky silk web. However, stabilimenta can also be used by prey as a signal in avoidance of webs, creating a trade-off in signal visibility. I argue that the derived spectral properties of stabilimentum silk have resulted in part from this conflict. The innate colour preferences of insects, their ability to learn colours, and the spectral properties of flowers all suggest that the reflectance spectra of stabilimenta renders them relatively cryptic to many insect prey, while maintaining their visibility to vertebrate predators.     

State-dependent risk-taking by predators in systems with defended prey

Even defended prey items may contain nutrients that can sustain predators in times of energetic need. Conversely, a well-fed predator might be expected to avoid attacking prey items that have a chance of being defended, particularly if there is an abundance of familiar palatable prey to support it. To further understand the implications of optimal state-dependent foraging behaviour by predators in systems that contain defended prey, I developed a stochastic dynamic programming model. This state-dependent approach formally accounts for the trade-off between avoiding starvation and minimising harm from attacking defended prey. It predicts that the mean attack probability of predators on defended models and their undefended mimics should decline in a sigmoidal fashion with increasing availability of alternative undefended prey, and that the foraging decisions of predators should in general be relatively insensitive to the probability that a potentially defended prey item is indeed defended. Some implications of these predictions are that conspicuous warning signals are more likely to evolve in systems that contain an abundance of alternative undefended prey, and that imperfect mimicry will provide almost complete protection to the mimic when predators are readily supported by alternative food sources. Somewhat surprisingly, increasing the density of nutritious undefended mimics while keeping the densities of all other prey types constant tended to decrease the attack rates of predators on encounter with mimics and their defended models. This increase in dietary conservatism arose because in these cases there would be more prey available to sustain the predator if it ever found itself critically low in energy.     

Microcrustacean prey and macroinvertebrate predators in a stream food web

1. The consumption of microcrustacea by two polyphagous predators, larvae of the caddisfly Plectrocnemia conspersa (Curtis) and the alderfly Sialis fuliginosa Pictet, was investigated in an English stream with a well-known macro- and microinvertebrate fauna. Benthic samples were collected in August, November, December and April, and the gut contents of all individuals of both predators were examined. 2. All the microcrustacean groups (Cyclopoida, Harpacticoida, Chydoridae and Ostracoda) were identified in gut contents. Of the ten taxa present in the benthos, all occurred in the diet of P. conspersa; nine were found in S. fuliginosa. 3. Ontogenetic shifts in the diets of both predators were found, and microcrustacea were consumed more frequently by small than large instars. 4. There was little evidence of selective feeding by P. conspersa, whereas ostracods were over-represented in the diet of S. fuliginosa, compared with benthic relative densities. The Chydoridae were under-represented in the diet of both predators. 5. The food web of Broadstone Stream is perhaps the most detailed web available for any running water habitat. Increased taxonomic resolution produced marked changes in values of connectance and predator-prey ratios. Linkage density remained fairly constant at different levels of resolution and were high, indicative of a web of generalist species. Omnivory was pronounced and may be characteristic of donor-controlled systems where organic detritus is the primary energy base.