Functional responses modified by predator density

Functional and Numerical Responses of Harmonia axyridis (Coleoptera: Coccinellidae) to Rhopalosiphum nymphaeae (Hemiptera: Aphididae) and Their Potential for Biological Control

The water lily aphid (Rhopalosiphum nymphaeae) is a highly polyphagous herbivore that causes severe damage to many terrestrial and aquatic plants, especially lotus. Due to environmental concerns about water pollution and other issues caused by chemical control methods, there is an urgent need to develop effective and sustainable control methods. The harlequin ladybird (Harmonia axyridis) is a well-known aphid predator and may pose a potential threat to R. nymphaeae. To study the predation ability of H. axyridis at different developmental stages on R. nymphaeae, we assessed the functional response, attack rate, and search effect of H. axyridis larvae and adults preying on R. nymphaeae. The numerical response of this process was also evaluated under a constant ladybird-to-aphid ratio and constant aphid density conditions, respectively. Our results showed that all predator stages exhibited type II functional responses. The predation rate of individual H. axyridis on R. nymphaeae nymphs significantly increased as prey density increased. In contrast, the search effect of H. axyridis gradually decreased with an increase in prey density. Meanwhile, H. axyridis at different developmental stages possess varying predation abilities; fourth instar and adult H. axyridis were found to be highly efficient predators of R. nymphaeae. H. axyridis adults exhibited the highest predation ability and predation rate, while both the adult and fourth-instar larvae exhibited the highest attack rate. Moreover, fourth-instar larvae exhibited the highest search effect value at initially lower prey densities, although adults surpassed them at higher prey densities. Our results also indicated that H. axyridis exhibited varying degrees of intraspecific interference and self-interference influence as predator density increases. These results strongly support H. axyridis as an effective biocontrol agent for R. nymphaeae.

Defense against Paramecium predation via long filament morphology favors the survival of Raphidiopsis raciborskii populations

Raphidiopsis blooms are notorious for cyanotoxin formation and strong invasiveness, threatening the stability of aquatic ecosystems and human health. The protozoa Paramecium can potentially serve as an organism for controlling Raphidiopsis blooms owing to its grazing effect. However, the grazing ability of Paramecium is largely determined by the size of the prey, and the population of Raphidiopsis consists of filaments of varying lengths and sizes. The selective grazing behavior of Paramecium toward short-length or small-sized filaments in the Raphidiopsis population, as opposed to long filaments, remains unclear. Therefore, in this study, we co-cultured the predator Paramecium sp. with different initial abundances and the prey Raphidiopsis raciborskii to explore this knowledge gap. Our results suggested that: (1) the population of R. raciborskii declined under the selective grazing effect of Paramecium sp. on short filaments, whereas R. raciborskii with long filaments survived; (2) the growth of Paramecium sp. feeding on the same abundance of R. raciborskii was reduced at higher initial abundances, whereas its carrying capacity exhibited an opposite trend; (3) under ingestion by Paramecium sp., the morphology of R. raciborskii developed in the direction of becoming larger, and higher initial abundances of Paramecium sp. intensified this process; (4) increasing initial abundance of Paramecium sp. aggravated the decline of R. raciborskii photosynthetic activity. Therefore, the grazing effect of Paramecium sp. on R. raciborskii mainly affects filaments of short length or small size. Collectively, these results clarify the inter-species interaction between the protozoa Paramecium and filamentous cyanobacteria Raphidiopsis, including population dynamics and morphological and physiological changes in the predator and prey. Such insights into the interactions between Paramecium and R. raciborskii may have implications for the biological control of blooms caused by filamentous cyanobacteria.

Fusion dsRNA designs incorporating multiple target sequences can enhance the aphid control capacity of an RNAi-based strategy

BACKGROUND

RNA interference (RNAi) is the sequence-dependent suppression of gene expression by double-stranded RNA (dsRNA). This is a promising strategy for the control of insect pests because dsRNA can be rationally designed to maximize efficacy and biosafety, the latter by using sequences that are found in target pests but are safe for non-target insects. However, this has yet to be optimized in aphids, destructive sap-sucking pests that also transmit plant viruses. We used the green peach aphid (Myzus persicae) as a case study to optimize the efficiency of RNAi by applying a novel fusion dsRNA design.

RESULTS

Comparative transcriptomics revealed a number of genes that are induced in feeding aphids, and eight candidate genes were chosen as RNAi targets. To improve RNAi efficiency, our fusion dsRNA design approach combined optimal gene fragments (highly conserved in several aphid species but with less homology in beneficial insects such as the predator ladybeetle Propylea japonica) from three candidate genes. We compared this RNAi-based biological control approach with conventional chemical control using imidacloprid. We found that the fusion dsRNA strategy inhibited the aphid population to a significantly greater extent than single-target RNAi and did not affect ladybeetle fitness, allowing an additive effect between RNAi and natural predation, whereas imidacloprid was harmful to aphids and ladybeetles.

CONCLUSION

Our fusion dsRNA design approach enhances the ability of RNAi to control aphids without harming natural predators. © 2024 Society of Chemical Industry.

Adaptive experimental design produces superior and more efficient estimates of predator functional response

Ecological dynamics are strongly influenced by the relationship between prey density and predator feeding behavior-that is, the predatory functional response. A useful understanding of this relationship requires us to distinguish between competing models of the functional response, and to robustly estimate the model parameters. Recent advances in this topic have revealed bias in model comparison, as well as in model parameter estimation in functional response studies, mainly attributed to the quality of data. Here, we propose that an adaptive experimental design framework can mitigate these challenges. We then present the first practical demonstration of the improvements it offers over standard experimental design. Our results reveal that adaptive design can efficiently identify the preferred functional response model among the competing models, and can produce much more precise posterior distributions for the estimated functional response parameters. By increasing the efficiency of experimentation, adaptive experimental design will lead to reduced logistical burden.

`Friend or foe' and decision making initiative in complex conflict environments

We present a novel mathematical model of two adversarial forces in the vicinity of a non-combatant population in order to explore the impact of each force pursuing specific decision-making strategies. Each force has the opportunity to draw support by enabling the decision-making initiative of the population, in tension with maintaining tactical and organisational effectiveness over their adversary. Each dynamic model component of force, population and decision-making, is defined by the archetypal Lanchester, Lotka-Volterra and Kuramoto-Sakaguchi models, with feedback between each component adding heterogeneity. Developing a scheme where cultural factors determine decision-making strategies for each force, this work highlights the parametric and topological factors that influence favourable results in a non-linear system where physical outcomes are highly dependent on the non-physical and cognitive nature of each force's intended strategy.

Ectophagous folivores do not profit from rich resources on phylogenetically isolated trees

Resource use by consumers across patches is often proportional to the quantity or quality of the resource within these patches. In folivores, such proportional use of resources is likely to be more efficient when plants are spatially proximate, such as trees forming a forest canopy. However, resources provided by forest-trees are often not used proportionally. We hypothesised that proportional use of resources is reduced when host trees are isolated among phylogenetically distant neighbours that mask olfactory and visual search cues, and reduce folivore movement between trees. Such phylogenetically distant neighbourhoods might sort out species that are specialists, poor dispersers, or have poor access to information about leaf quality. We studied individual oaks, their leaf size and quality, their folivory and abundance of folivores (mostly Lepidopteran ectophages, gallers and miners), and parasitism of folivores. We found that leaf consumption by ectophages hardly increased with increasing leaf size when host trees were phylogenetically isolated. We found a similar effect on host use by parasitoids in 1 year. In contrast, we found no consistent effects in other folivore guilds. Relative abundances of specialists and species with wingless females declined with phylogenetic isolation. However, resource use within each of these groups was inconsistently affected by phylogenetic isolation. We suggest that phylogenetic isolation prevents ectophages from effectively choosing trees with abundant resources, and also sorts out species likely to recruit in situ on their host tree. Trees in phylogenetically distant neighbourhoods may be selected for larger leaves and greater reliance on induced defences.

Food web functional responses

This article reviews the nature of functional responses that have commonly been used to represent feeding relationships in the ecological literature. It compares these with the range of functional response forms that are likely to characterize species in natural communities. The latter set of responses involves many more variables. The article reviews the history of functional response models, and examines previous work that has allowed the functional response of a predator to a single type of prey to depend on additional variables beyond the abundance of that prey type. While a number of more complex functional responses have been discussed over the years, many variables affecting feeding rates are still typically omitted from models of food webs. The influences on functional responses from trophic levels above that of the predator or below that of the prey are particularly likely to be ignored, although models and data have suggested that they can have large effects on the functional response. The influences of adaptive behavior and of the time-scale of response measurement are also too often ignored. Some of the known and unknown consequences of these omissions are discussed.

Rising temperature more strongly promotes low-abundance Paramecium to remove Microcystis and degrade microcystins

Driven by global warming and eutrophication, Microcystis blooms have posed a severe threat to freshwater ecosystems, especially their derived pollutants cause serious harm to aquatic organisms, thus it is urgent to develop an effective strategy to eliminate nuisance Microcystis. Some protozoa can efficiently graze on toxic Microcystis aeruginosa and degrade cyanotoxins, and play a vital role in regulating harmful cyanobacteria. In the process of protozoa feeding on harmful algae, both temperature and protozoa population density are critical factors that affect the consequences of harmful M. aeruginosa population dynamics. In this study, we first found that Paramecium multimicronucleatum has strong ability to feed on M. aeruginosa, and then studied the interactive effects between temperature and initial density of P. multimicronucleatum on controlling M. aeruginosa. Results showed that increasing temperature accelerated the elimination of M. aeruginosa by P. multimicronucleatum, e.g. the time for M. aeruginosa elimination at 32 °C was shortened to 3.5-4 days. The higher temperatures (26, 29, and 32 °C) were more conducive to improve the efficiency of controlling M. aeruginosa by P. multimicronucleatum with low initial density (10 inds mL^-1). Furthermore, P. multimicronucleatum can rapidly degrade microcystins, and the degradation ratio approximately 100% at 32 °C after 6 days. This is the first study to discover that P. multimicronucleatum can high efficiently graze on M. aeruginosa and has a much higher grazing rate (3.5-5.5 × 10⁴Microcystis Paramecium^-1 d^-1) than other protozoa. These findings contribute to the establishment of a new feasible method for the biological control of M. aeruginosa, and provide a theoretical guidance for the practical application of P. multimicronucleatum in the removal of M. aeruginosa.

Impact of a nonnative parasitoid species on intraspecific interference and offspring sex ratio

In an assemblage of multiple predators sharing a single prey species, the combined effects of the component species may scale unpredictably because of emergent interspecific interactions. Prior studies suggest that chaotic but persistent community dynamics are induced by intra-/interspecific interactions between native and nonnative parasitoids competing over a shared host. Here, we test the impact of the nonnative parasitoid Heterospilus prosopidis (Hymenoptera: Braconidae) on the intraspecific interference and offspring sex ratio of the native parasitoid Anisopteromalus calandrae (Hymenoptera: Pteromalidae). We found that the nonnative parasitoid reduced intraspecific interference among native parasitoids and decreased the proportion of female offspring produced by the native parasitoid (predicted under conditions of reduced host availability). At higher host densities, the nonnative parasitoid contributed less to the total proportion of hosts parasitized, as its innate saturating Type II response changed to a dome-shaped Type IV response with increasing density of the native parasitoid, while the native parasitoid retained its increasing Type I response. This inverse host-density-dependent response between the two parasitoids and associated competitive superiority can explain the observed changes in parasitism; at high host densities, the searching efficiency of the native parasitoid increases via host feeding while the nonnative parasitoid experiences egg limitation. These results highlight the importance of the complementary top-down effects of multiple consumers on a single resource.

Geometric Complexity and the Information-Theoretic Comparison of Functional-Response Models

The assessment of relative model performance using information criteria like AIC and BIC has become routine among functional-response studies, reflecting trends in the broader ecological literature. Such information criteria allow comparison across diverse models because they penalize each model's fit by its parametric complexity—in terms of their number of free parameters—which allows simpler models to outperform similarly fitting models of higher parametric complexity. However, criteria like AIC and BIC do not consider an additional form of model complexity, referred to as geometric complexity, which relates specifically to the mathematical form of the model. Models of equivalent parametric complexity can differ in their geometric complexity and thereby in their ability to flexibly fit data. Here we use the Fisher Information Approximation to compare, explain, and contextualize how geometric complexity varies across a large compilation of single-prey functional-response models—including prey-, ratio-, and predator-dependent formulations—reflecting varying apparent degrees and forms of non-linearity. Because a model's geometric complexity varies with the data's underlying experimental design, we also sought to determine which designs are best at leveling the playing field among functional-response models. Our analyses illustrate (1) the large differences in geometric complexity that exist among functional-response models, (2) there is no experimental design that can minimize these differences across all models, and (3) even the qualitative nature by which some models are more or less flexible than others is reversed by changes in experimental design. Failure to appreciate model flexibility in the empirical evaluation of functional-response models may therefore lead to biased inferences for predator–prey ecology, particularly at low experimental sample sizes where its impact is strongest. We conclude by discussing the statistical and epistemological challenges that model flexibility poses for the study of functional responses as it relates to the attainment of biological truth and predictive ability.

Time-delayed and stochastic effects in a predator-prey model with ratio dependence and Holling type III functional response

In this article, we derive and analyze a novel predator-prey model with account for maturation delay in predators, ratio dependence, and Holling type III functional response. The analysis of the system's steady states reveals conditions on predation rate, predator growth rate, and maturation time that can result in a prey-only equilibrium or facilitate simultaneous survival of prey and predators in the form of a stable coexistence steady state, or sustain periodic oscillations around this state. Demographic stochasticity in the model is explored by means of deriving a delayed chemical master equation. Using system size expansion, we study the structure of stochastic oscillations around the deterministically stable coexistence state by analyzing the dependence of variance and coherence of stochastic oscillations on system parameters. Numerical simulations of the stochastic model are performed to illustrate stochastic amplification, where individual stochastic realizations can exhibit sustained oscillations in the case, where deterministically the system approaches a stable steady state. These results provide a framework for studying realistic predator-prey systems with Holling type III functional response in the presence of stochasticity, where an important role is played by non-negligible predator maturation delay.

Predator size affects the intensity of mutual interference in a predatory mirid

Interference competition occurs when access to an available resource is negatively affected by interactions with other individuals, where mutual interference involves individuals of the same species. The interactive phenomena among individuals may be size-dependent, since body size is a major factor that may alter prey consumption rates and ultimately the dynamics and structure of food webs.A study was initiated in order to evaluate the effect of mutual interference in the prey-specific attack rates and handling times of same size class predators, incorporating variation in consumer size. For this purpose, laboratory functional response experiments were conducted using same age predators, that is, newly hatched (first instar) or mature (fifth instar) nymphs of the polyphagous mirid predator Macrolophus pygmaeus preying on Ephestia kuehniella (Lepidoptera: Pyralidae) eggs.The experiments involved four predator density treatments, that is, one, two, three, or four predators of same age, that is, either first- or fifth-instar nymphs, which were exposed to several prey densities. The Crowley-Martin model, which allows for interference competition between foraging predators, was used to fit the data.The results showed that mutual interference between predator's nymphs may occur that affect their foraging efficiency. The values of the attack rate coefficient were dependent on the predator density and for the first-instar nymphs were significantly lower at the highest predator density than the lower predator densities, whereas for the fifth-instar nymphs in all density treatments were significantly lower to that of the individual foragers' ones.These results indicate that mutual interference is more intense for larger predators and is more obvious at low prey densities where the competition level is higher. The wider use of predator-dependent functional response models will help toward a mechanistic understanding of intraspecific interactions and its consequences on the stability and structure of food webs.

Social behaviour and foraging success of little egrets (Egretta garzetta)

Intraspecific interactions among predators can change the game between a predator and its prey. Individuals of different size or sex can differ in their responses to conspecific competitors. We studied intraspecific interactions among pairs of little egrets (Egretta garzetta) while foraging on responsive prey (comet goldfish, Carassius auratus). Testing little egrets in an artificial patchy environment both singly and while engaged in social forging in pairs (male & female) at two prey densities, allowed us to explore individual differences in foraging success. We found sexual dimorphism with males being bigger and more aggressive than females. However, female foraging success was positively affected by the time they spent foraging with a conspecific male, suggesting they might be able to mitigate male aggressiveness with an indirect positive interaction. Despite the presence of direct interactions between individuals in the pair, egret foraging success was not affected by such interactions, nor by prey density. Our results demonstrate the importance of sex and an individual's ability to adjust its social behaviour based on the behaviour of others in this predator-prey foraging game.

Habitat selection patterns are density dependent under the ideal free distribution

Despite being widely used, habitat selection models are rarely reliable and informative when applied across different ecosystems or over time. One possible explanation is that habitat selection is context-dependent due to variation in consumer density and/or resource availability. The goal of this paper is to provide a general theoretical perspective on the contributory mechanisms of consumer and resource density-dependent habitat selection, as well as on our capacity to account for their effects. Towards this goal we revisit the ideal free distribution (IFD), where consumers are assumed to be omniscient, equally competitive and freely moving, and are hence expected to instantaneously distribute themselves across a heterogeneous landscape such that fitness is equalised across the population. Although these assumptions are clearly unrealistic to some degree, the simplicity of the structure in IFD provides a useful theoretical vantage point to help clarify our understanding of more complex spatial processes. Of equal importance, IFD assumptions are compatible with the assumptions underlying common habitat selection models. Here we show how a fitness-maximising space use model, based on IFD, gives rise to resource and consumer density-dependent shifts in consumer distribution, providing a mechanistic explanation for the context-dependent outcomes often reported in habitat selection analysis. Our model suggests that adaptive shifts in consumer distribution patterns would be expected to lead to nonlinear and often non-monotonic patterns of habitat selection. These results indicate that even under the simplest of assumptions about adaptive organismal behaviour, habitat selection strength should critically depend on system-wide characteristics. Clarifying the impact of adaptive behavioural responses may be pivotal in making meaningful ecological inferences about observed patterns of habitat selection and allow reliable transferability of habitat selection predictions across time and space.

Predator type influences the frequency of functional responses to prey in marine habitats

The functional response of a consumer to a gradient of resource density is a widespread and consistent framework used to quantify the importance of consumption to population dynamics and stability. Within benthic marine ecosystems, both crustaceans and fishes can provide strong top-down pressure on prey populations. Taxon-specific differences in biomechanics or habitat use, among other factors, may lead to variable functional response forms or parameter values (attack rate, handling time). Based on a review of 189 individual functional response fits, we find that these predator guilds differ in their frequency distribution of functional response types, with crustaceans exhibiting nearly double the proportion of sigmoidal, density-dependent functional responses (Holling type III) as predatory fishes. The implications of this finding for prey population stability are significant because type III responses allow prey persistence while type II responses are de-stabilizing and can lead to extinction. Comparing per capita predation rates across diverse taxa can provide integrative insights into predatory effects and the ability of predation to drive community structure.

Conspecifics, not pollen, reduce omnivore prey consumption

Pollen can decrease (via reduced consumption) or increase (via numerical response) an omnivores consumption of animal prey. Although pollen can increase predation pressure through numerical responses of omnivores, pollen may also suppress predation by increasing omnivore interactions with conspecifics. Despite this potential, studies of the impacts of pollen on predation by omnivores often overlook the effect of these tissues on intraspecific interactions between omnivores. We designed three studies to examine how Spartina foliosa pollen and conspecific density impact scale insect prey consumption by ladybeetle (Naemia seriata) omnivores. First, we assessed how pollen impacts scale insect consumption by isolated ladybeetles. Second, we measured how pollen influences ladybeetle prey suppression when numerical responses were possible. Third, because initial experiments suggested the consumption rates of individual ladybeetles depended on conspecific density, we compared per capita consumption rates of ladybeetles across ladybeetle density. Pollen did not influence prey consumption by isolated ladybeetles. When numerical responses were possible, pollen did not influence total predation on prey despite increasing ladybeetle density, suggesting that pollen decreased per capita prey consumption by ladybeetles. The discrepancy between these studies is likely a consequence of differences in ladybeetle density-the presence of only two other conspecifics decreased per capita prey consumption by 76%. Our findings suggest that pollen may not alter the population level effects of omnivores on prey when omnivore numerical responses are offset by reductions in per capita predation rate.

Eco-epidemiological interactions with predator interference and infection

Predator interference is a form of competition between predator individuals over access to their prey. There is broad empirical evidence for interference to exist in different strengths in various types of ecological communities. At the same time, parasites are increasingly recognized to alter food web structure and dynamics. In order to investigate the eco-epidemiological interplay between interference and infection, we develop and analyze mathematical models of a predator-prey system, where the predators are subject to both interference and infectious disease. In the absence of infection, equilibrium predator density is known to show a non-monotonic response to interference by first increasing and then decreasing with increasing interference levels. We show that predator infection can change this pattern into a monotonically decreasing predator response to interference, provided the transmissibility is large enough and the pathogenicity is moderate such that the impact of disease on host population density prevails over interference effects. This holds for both types of disease transmission studied here, density-dependent and frequency-dependent. For density-dependent transmission, we find that intermediate values of interference can facilitate disease persistence, whereas the disease would disappear for small or large interference levels. By contrast, for frequency-dependent transmission, disease emergence is independent of interference levels. These dynamic interactions may be important for the understanding of potential biocontrol measures and of spread patterns of zoonotic diseases.

Mutual interference between adult females of Galendromus flumenis (Acari: Phytoseiidae) feeding on eggs of Banks grass mite decreases predation efficiency and increases emigration rate

The Banks grass mite, Oligonychus pratensis (Banks) (Acari: Tetranychidae) causes significant damage to dates in California (USA), if not controlled. Studies are underway to develop biological control strategies against this pest in dates using the predatory mite Galendromus flumenis (Chant) (Acari: Phytoseiidae). In California date gardens, this predator is found in low numbers that are insufficient for the economic suppression of Banks grass mites, and our research aims to understand why it fails to keep up with prey densities. The hypothesis that prey density and predator interference interactively determine the predation efficiency of G. flumenis was tested. In addition, the effect of arena size and prey and predator density manipulations on the emigration rate of the predator was investigated. Our results indicate that the per capita predation rate of G. flumenis decreases steeply with increasing predator density due to mutual interference. Analysis of emigration data considering the arena size and predator numbers showed that the emigration rate of G. flumenis was higher from small arenas, and increased with increasing predator numbers. When emigration data were analyzed using prey and predator densities as independent variables, only the effect of predator density was significant, suggesting that higher predator density increases the emigration rate of G. flumenis. These results contribute to our understanding of the predator-prey interactions, and help in designing strategies for more efficient augmentative releases of G. flumenis.

Evaluation of alternative prey-, predator-, and ratio-dependent functional response models in a zooplankton microcosm

There is strenuous debate among ecologists regarding the inclusion of predator density into the originally prey-dependent functional response. We provided comprehensive empirical comparisons of alternative functional response models for the predatory ostracod Heterocypris incongruens (Ramdohr, 1808) and the rotifer Brachionus calyciflorus (Pallas, 1766) as its prey in small freshwater microcosms. Prey killed was measured at factorial combinations of four predator densities and five prey densities, and was recorded at 3 min intervals over 60 min experiments. To support the potential effect of predator interference on per capita kill rate, we recorded ostracod activity and aggression. Kill rate increased following a saturating function with increasing prey density and decreased with increasing predator density. Model evaluation using an information–theoretic approach indicated that the Arditi–Ginzburg type II ratio-dependent model performed best, followed by the Arditi–Akcakaya and Beddington–DeAngelis type II predator-dependent models, suggesting that predator interference was important in predicting kill rates. Interference among predators increased and their activity decreased with increasing predator density, providing confirmation that interference was responsible for the predator-dependent effect. By combining a microcosm experiment and behavioral observations, our results suggest that predator interference at realistic population densities influences ostracod kill rates and this form of interference was best accommodated by predator-dependent models.

A shift from exploitation to interference competition with increasing density affects population and community dynamics

Intraspecific competition influences population and community dynamics and occurs via two mechanisms. Exploitative competition is an indirect effect that occurs through use of a shared resource and depends on resource availability. Interference competition occurs by obstructing access to a resource and may not depend on resource availability. Our study tested whether the strength of interference competition changes with protozoa population density. We grew experimental microcosms of protozoa and bacteria under different combinations of protozoan density and basal resource availability. We then solved a dynamic predator–prey model for parameters of the functional response using population growth rates measured in our experiment. As population density increased, competition shifted from exploitation to interference, and competition was less dependent on resource levels. Surprisingly, the effect of resources was weakest when competition was the most intense. We found that at low population densities, competition was largely exploitative and resource availability had a large effect on population growth rates, but the effect of resources was much weaker at high densities. This shift in competitive mechanism could have implications for interspecific competition, trophic interactions, community diversity, and natural selection. We also tested whether this shift in the mechanism of competition with protozoa density affected the structure of the bacterial prey community. We found that both resources and protozoa density affected the structure of the bacterial prey community, suggesting that competitive mechanism may also affect trophic interactions.

Spatial arrangement of prey affects the shape of ratio-dependent functional response in strongly antagonistic predators

Predators play a key role in shaping natural ecosystems, and understanding the factors that influence a predator's kill rate is central to predicting predator-prey dynamics. While prey density has a well-established effect on predation, it is increasingly apparent that predator density also can critically influence predator kill rates. The effects of both prey and predator density on the functional response will, however, be determined in part by their distribution on the landscape. To examine this complex relationship we experimentally manipulated prey density, predator density, and prey distribution using a tadpole (prey)-dragonfly nymph (predator) system. Predation was strongly ratio-dependent irrespective of prey distribution, but the shape of the functional response changed from hyperbolic to sigmoidal when prey were clumped in space. This sigmoidal functional response reflected a relatively strong negative effect of predator interference on kill rates at low prey: predator ratios when prey were clumped. Prey aggregation also appeared to promote stabilizing density-dependent intraguild predation in our system. We conclude that systems with highly antagonistic predators and patchily distributed prey are more likely to experience stable dynamics, and that our understanding of the functional response will be improved by research that examines directly the mechanisms generating interference.

Dynamics of a non-autonomous density-dependent predator–prey model with Beddington–DeAngelis type

The goal of this paper is to investigate the dynamics of a non-autonomous density-dependent predator–prey system with Beddington–DeAngelis functional response, where not only the prey density dependence but also the predator density dependence are considered, such that the studied predator–prey system conforms to the realistically biological environment. We firstly introduce a sufficient condition for the permanence of the system and then use a specific set to obtain a weaker sufficient condition. Afterward, we provide corresponding conditions for the extinction of the system and the existence of boundary periodical solutions, respectively. Further, we get a sufficient condition for global attractiveness of the boundary periodic solution by constructing a Lyapunov function, arriving at the uniqueness of boundary periodic solutions since the uniqueness of boundary periodic solutions can be ensured by global attractiveness. Finally, based on the existence of positive periodic solutions, which can be ensured by the Brouwer fixed-point theorem, we provide a sufficient condition for the uniqueness of positive periodic solutions.

Does Mutual Interference Affect the Feeding Rate of Aphidophagous Coccinellids? A Modeling Perspective

Mutual interference involves direct interactions between individuals of the same species that may alter their foraging success. Larvae of aphidophagous coccinellids typically stay within a patch during their lifetime, displaying remarkable aggregation to their prey. Thus, as larvae are exposed to each other, frequent encounters may affect their foraging success. A study was initiated in order to determine the effect of mutual interference in the coccinellids’ feeding rate. One to four 4^th larval instars of the fourteen-spotted ladybird beetle Propylea quatuordecimpunctata were exposed for 6 hours into plastic containers with different densities of the black bean aphid, Aphis fabae, on potted Vicia faba plants. The data were used to fit a purely prey-dependent Holling type II model and its alternatives which account for interference competition and have thus far been underutilized, i.e. the Beddington-DeAngelis, the Crowley-Martin and a modified Hassell-Varley model. The Crowley-Martin mechanistic model appeared to be slightly better among the competing models. The results showed that although the feeding rate became approximately independent of predator density at high prey density, some predator dependence in the coccinellid’s functional response was observed at the low prey—high predator density combination. It appears that at low prey densities, digestion breaks are negligible so that the predators do waste time interfering with each other, whereas at high prey densities time loss during digestion breaks may fully accommodate the cost of interference, so that the time cost may be negligible.

Food web persistence is enhanced by non-trophic interactions

The strength of interspecific interactions is often proposed to affect food web stability, with weaker interactions increasing the persistence of species, and food webs as a whole. However, the mechanisms that modify interaction strengths, and their effects on food web persistence are not fully understood. Using food webs containing different combinations of predator, prey, and nonprey species, we investigated how predation risk of susceptible prey is affected by the presence of species not directly trophically linked to either predators or prey. We predicted that indirect alterations to the strength of trophic interactions translate to changes in persistence time of extinction-prone species. We assembled interaction webs of protist consumers and turbellarian predators with eight different combinations of prey, predators and nonprey species, and recorded abundances for over 130 prey generations. Persistence of predation-susceptible species was increased by the presence of nonprey. Furthermore, multiple nonprey species acted synergistically to increase prey persistence, such that persistence was greater than would be predicted from the dynamics of simpler food webs. We also found evidence suggesting increased food web complexity may weaken interspecific competition, increasing persistence of poorer competitors. Our results demonstrate that persistence times in complex food webs cannot be predicted from the dynamics of simplified systems, and that species not directly involved in consumptive interactions likely play key roles in maintaining persistence. Global species diversity is currently declining at an unprecedented rate and our findings reveal that concurrent loss of species that modify trophic interactions may have unpredictable consequences for food web stability.

Heated relations: temperature-mediated shifts in consumption across trophic levels

A rise in temperature will intensify the feeding links involving ectotherms in food webs. However, it is unclear how the effects will quantitatively differ between the plant-herbivore and herbivore-carnivore interface. To test how warming could differentially affect rates of herbivory and carnivory, we studied trophic interaction strength in a food chain comprised of green algae, herbivorous rotifers and carnivorous rotifers at 10, 15, 20 and 25°C. We found significant warming-induced changes in feeding by both herbivorous and carnivorous rotifers, but these responses occurred at different parts of the entire temperature gradient. The strongest response of the per capita herbivore's ingestion rate occurred due to an increase in temperature from 15 to 20°C (1.9 fold: from 834 to 1611 algal cells per h(-1)) and of the per capita carnivore's ingestion rate from 20 to 25°C (1.6 fold: from 1.5 to 2.5 prey h(-1)). Handling time, an important component of a consumer's functional response, significantly decreased from 15 to 20°C in herbivorous rotifers. In contrast, it decreased from 20 to 25°C in carnivorous rotifers. Attack rates significantly and strongly increased from 10 to 25°C in the herbivorous animals, but not at all in the carnivores. Our results exemplify how the relative forces of top-down control exerted by herbivores and carnivores may strongly shift under global warming. But warming, and its magnitude, are not the only issue: If our results would prove to be representative, shifts in ectotherm interactions will quantitatively differ when a 5°C increase starts out from a low, intermediate or high initial temperature. This would imply that warming could have different effects on the relative forces of carnivory and herbivory in habitats differing in average temperature, as would exist at different altitudes and latitudes.

Dynamical behaviors of a diffusive predator–prey system with Beddington–DeAngelis functional response

In this paper, we present a diffusive predator–prey system with Beddington–DeAngelis functional response, where the prey species can disperse between the two patches, and there is competition between the two predators. Sufficient conditions for the permanence and extinction of system are established based on the upper and lower solution methods and comparison theory of differential equation. Furthermore, the global asymptotic stability of positive solutions is obtained by constructing a suitable Lyapunov function. By using the continuation theorem in coincidence degree theory, we show the periodicity of positive solutions. Finally, we illustrate global asymptotic stability of the model by a simulation figure.

Consequences of variation in foraging success among predators on numerical response

The relationship between foraging success and reproduction is commonly assumed to be linear in theoretical investigations. Although the exact relationship (e.g., linear or nonlinear) does not influence qualitative conclusions of models under some assumptions, an inclusion of individual behavioral variation can make it otherwise due to Jensen's inequality. In particular, a mechanism that stabilizes food web dynamics is generated when two conditions are satisfied: (1) the reproduction of predators experiences diminishing returns from foraging success (i.e., concave down relationship between foraging success and reproduction) and (2) foraging success variation among predator individuals increases with the predator density. However, empirical results that confirm these conditions are scarce. This study describes the mechanism as a hypothesis for stability and discusses some important considerations for empirical verifications of the mechanism.

Predation potential of Chilocorus cacti (Coleoptera: Coccinellidae) to the prickly pear cacti pest Dactylopius opuntiae (Hemiptera: Dactylopiidae)

Functional response of the predator Chilocorus cacti (Linnaeus) (Coleoptera: Coccinellidae) on five densities of Dactylopius opuntiae (Cockerell) (Hemiptera: Dactylopiidae) female adults was assessed under laboratory conditions. The searching efficiency of C. cacti significantly decreased as prey density increased. The logistic regression for the predator had a negative and significant linear parameter indicating a type II functional response. Non-linear regression for Holling predator equation estimated a handling time of 1.79 ± 0.129 h and attack rate coefficient of 0.1003 ± 0.030. Most of this handling time was because the predator spent a lot of time removing the waxy coating that protects adult females of D. opuntiae. Chilocorus cacti consumes females of D. opuntiae in their reproductive stage; therefore, it could be an effective natural enemy to suppress or regulate low density populations of D. opuntiae, preventing them to reach high densities.

Non-linear feeding functional responses in the Greater Flamingo (Phoenicopterus roseus) predict immediate negative impact of wetland degradation on this flagship species

Accurate knowledge of the functional response of predators to prey density is essential for understanding food web dynamics, to parameterize mechanistic models of animal responses to environmental change, and for designing appropriate conservation measures. Greater flamingos (Phoenicopterus roseus), a flagship species of Mediterranean wetlands, primarily feed on Artemias (Artemia spp.) in commercial salt pans, an industry which may collapse for economic reasons. Flamingos also feed on alternative prey such as Chironomid larvae (e.g., Chironomid spp.) and rice seeds (Oryza sativa). However, the profitability of these food items for flamingos remains unknown. We determined the functional responses of flamingos feeding on Artemias, Chironomids, or rice. Experiments were conducted on 11 captive flamingos. For each food item, we offered different ranges of food densities, up to 13 times natural abundance. Video footage allowed estimating intake rates. Contrary to theoretical predictions for filter feeders, intake rates did not increase linearly with increasing food density (type I). Intake rates rather increased asymptotically with increasing food density (type II) or followed a sigmoid shape (type III). Hence, flamingos were not able to ingest food in direct proportion to their abundance, possibly because of unique bill structure resulting in limited filtering capabilities. Overall, flamingos foraged more efficiently on Artemias. When feeding on Chironomids, birds had lower instantaneous rates of food discovery and required more time to extract food from the sediment and ingest it, than when filtering Artemias from the water column. However, feeding on rice was energetically more profitable for flamingos than feeding on Artemias or Chironomids, explaining their attraction for rice fields. Crucially, we found that food densities required for flamingos to reach asymptotic intake rates are rarely met under natural conditions. This allows us to predict an immediate negative effect of any decrease in prey density upon flamingo foraging performance.

Habitat structure alters top-down control in litter communities

The question whether top-down or bottom-up forces dominate trophic relationships, energy flow, and abundances within food webs has fuelled much ecological research with particular focus on soil litter ecosystems. Because litter simultaneously provides habitat structure and a basal resource, disentangling direct trophic and indirect non-trophic effects on different trophic levels remains challenging. Here, we focussed on short-term per capita interaction strengths of generalist predators (centipedes) on their microbi-detritivore prey (springtails) and addressed how the habitat structuring effects of the leaf litter modifies this interaction. We performed a series of laboratory functional response experiments where four levels of habitat structure were constructed by adding different amounts of leaf litter to the experimental arenas. We found that increased leaf litter reduced the consumption rate of the predator. We interpreted this as a dilution effect of the augmented habitat size provided by the increasing leaf litter surface available to the species. Dilution of the prey population decreased encounter rates, whereas the capture success was not affected. Interestingly, our results imply that top-down control by centipedes decreased with increasing resource supply for the microbi-detritivore prey (i.e. the leaf litter that simultaneously provides habitat structure). Therefore, effective top-down control of predators on microbi-detritvore populations seems unlikely in litter-rich ecosystems due to the non-trophic, habitat-structuring effect of the basal litter resource.

Intermittency in processing explains the diversity and shape of functional grazing responses

Central to theoretical studies of trophic interactions is the formulation of the consumer response to varying food availability. Response functions, however, are only rarely derived in mechanistic ways. As a consequence, the uncertainty in the functional representation of feeding remains large, as, e.g., evident from the ongoing debate on the usage of Ivlev, or Holling type I, II, and III functions in aquatic ecosystem models. Here, I refer to the work of Sjöberg in Ecol Model 10:215-225 (1980) who proposed to apply elements of the queuing theory developed in operational research to plankton-plankton interactions. Within this frame, food item processing is subdivided into two major stages which may operate with variable synchronicity. Asynchronous phasing of the two stages enhances the probability of long total processing times. This phenomenon is here termed feeding intermittency. Intermittency is assumed to determine the functional form of grazing kinetics, for which a novel grazing function containing a "shape" parameter is derived. Using this function, I evaluate the hypotheses that intermittency is influenced by (1) patchiness in the prey field (e.g., related to turbulence), and (2) the ratio of actual prey size to optimal prey size. Evidence for the first hypothesis arises from explaining reported variations in clearance rates of Acartia tonsa under different turbulence regimes. Further model applications to ingestion data for rotifers, copepods, and ciliates support the view that an increasing food size enhances intermittency and, this way, affects functional grazing responses. In the application to ciliate grazing, a possible prey density effect appears, possibly due to an intermittent activation of a feeding sub-stage. Queueing theory offers mechanistic explanations for transitions between Holling I-, II-, and Ivlev-type grazing. In doing so for variable prey size ratios, it may also refine size-based ecosystem models which are increasingly emerging in plankton ecology.

Ecosystem functions and densities of contributing functional groups respond in a different way to chemical stress

Understanding whether and to what extent ecosystem functions respond to chemicals is a major challenge in environmental toxicology. The available data gathered by ecosystem-level experiments (micro- and mesocosms) often describe the responses of taxa densities to stress. However, whether these responses are proportional to the responses of associated ecosystem functions to stress is unclear. By combining a carbon budget modeling technique with data from a standardized microcosm experiment with a known community composition, we quantified three ecosystem functions (net primary production [NPP], net mesozooplankton production [NZP], and net bacterial production [NBP]) at three Cu concentrations, with a control. Changes of these ecosystem functions with increasing chemical concentrations were not always proportional to the Cu effects on the densities of the contributing functional groups. For example, Cu treatments decreased mesozooplankton density by 100-fold and increased phytoplankton density 10- to 100-fold while increasing NZP and leaving NPP unaltered. However, in contrast, Cu affected microzooplankton and the associated function (NBP) in a comparable way. We illustrate that differences in the response of phytoplankton/mesozooplankton densities and the associated ecosystem functions to stress occur because functional rates (e.g., photosynthesis rates/ingestion rates) vary among Cu treatments and in time. These variations could be explained by food web ecology but not by direct Cu effects, indicating that ecology may be a useful basis for understanding environmental effects of stressors.