Spatial variability in prey phenology determines predator movement patterns and prey survival

Species have phenological variation among local habitats that are located at relatively small spatial scales. However, less studies have tested how this spatial variability in phenology can mediate intra-/inter-specific interactions. When predators track phenological variation of prey among local habitats, survival of prey within a local habitat strongly influenced by phenological synchrony with their conspecifics in adjacent habitats. Theory predicts that phenological synchrony among local habitats increases prey survival in local habitat within spatially structured environments because the predators have to make a habitat choice for foraging. Consequently, total survival of prey at regional scale should be higher. By using a spatially explicit field experiment, we tested above hypothesis using a prey–predator interaction between tadpole (Rhacophorus arboreus) and newt (Cynops pyrrhogaster). We established enclosures (≈regional scale) consisting of two tanks (≈local habitat scale) with different degree of prey phenological synchrony. We found that phenological synchrony of prey between tanks within each enclosure decreased the mean residence time of the predator in each tank, which resulted in higher survival of prey at a local habitat scale, supporting the theoretical prediction. Furthermore, individual-level variation in predator residence time explained the between-tank variation in prey survival in enclosures with phenological synchrony, implying that movement patterns of the predator can mediate variation in local population dynamics of their prey. However, total survival at each enclosure was not higher under phenological synchrony. These results suggest the importance of relative timing of prey phenology, not absolute timing, among local habitats in determining prey–predator interactions.     

Toxin constraint explains diet choice,survival and population dynamics in a molluscivore shorebird

Recent insights suggest that predators should include (mildly) toxic prey when non-toxic food is scarce. However, the assumption that toxic prey is energetically as profitable as non-toxic prey misses the possibility that non-toxic prey have other ways to avoid being eaten, such as the formation of an indigestible armature. In that case, predators face a trade-off between avoiding toxins and minimizing indigestible ballast intake. Here, we report on the trophic interactions between a shorebird (red knot, Calidris canutus canutus) and its two main bivalve prey, one being mildly toxic but easily digestible, and the other being non-toxic but harder to digest. A novel toxin-based optimal diet model is developed and tested against an existing one that ignores toxin constraints on the basis of data on prey abundance, diet choice, local survival and numbers of red knots at Banc d''Arguin (Mauritania) over 8 years. Observed diet and annual survival rates closely fit the predictions of the toxin-based model, with survival and population size being highest in years when the non-toxic prey is abundant. In the 6 of 8 years when the non-toxic prey is not abundant enough to satisfy the energy requirements, red knots must rely on the toxic alternative.     

A surplus no more? Variation in krill availability impacts reproductive rates of Antarctic baleen whales

The krill surplus hypothesis of unlimited prey resources available for Antarctic predators due to commercial whaling in the 20th century has remained largely untested since the 1970s. Rapid warming of the Western Antarctic Peninsula (WAP) over the past 50 years has resulted in decreased seasonal ice cover and a reduction of krill. The latter is being exacerbated by a commercial krill fishery in the region. Despite this, humpback whale populations have increased but may be at a threshold for growth based on these human-induced changes. Understanding how climate-mediated variation in prey availability influences humpback whale population dynamics is critical for focused management and conservation actions. Using an 8-year dataset (2013–2020), we show that inter-annual humpback whale pregnancy rates, as determined from skin-blubber biopsy samples (n = 616), are positively correlated with krill availability and fluctuations in ice cover in the previous year. Pregnancy rates showed significant inter-annual variability, between 29% and 86%. Our results indicate that krill availability is in fact limiting and affecting reproductive rates, in contrast to the krill surplus hypothesis. This suggests that this population of humpback whales may be at a threshold for population growth due to prey limitations. As a result, continued warming and increased fishing along the WAP, which continue to reduce krill stocks, will likely impact this humpback whale population and other krill predators in the region. Humpback whales are sentinel species of ecosystem health, and changes in pregnancy rates can provide quantifiable signals of the impact of environmental change at the population level. Our findings must be considered paramount in developing new and more restrictive conservation and management plans for the Antarctic marine ecosystem and minimizing the negative impacts of human activities in the region.     

Exploitation and recovery of a sea urchin predator has implications for the resilience of southern California kelp forests

Size-structured predator–prey interactions can be altered by the history of exploitation, if that exploitation is itself size-selective. For example, selective harvesting of larger sized predators can release prey populations in cases where only large individuals are capable of consuming a particular prey species. In this study, we examined how the history of exploitation and recovery (inside marine reserves and due to fisheries management) of California sheephead (Semicossyphus pulcher) has affected size-structured interactions with sea urchin prey in southern California. We show that fishing changes size structure by reducing sizes and alters life histories of sheephead, while management measures that lessen or remove fishing impacts (e.g. marine reserves, effort restrictions) reverse these effects and result in increases in density, size and biomass. We show that predation on sea urchins is size-dependent, such that the diet of larger sheephead is composed of more and larger sized urchins than the diet of smaller fish. These results have implications for kelp forest resilience, because urchins can overgraze kelp in the absence of top-down control. From surveys in a network of marine reserves, we report negative relationships between the abundance of sheephead and urchins and the abundance of urchins and fleshy macroalgae (including giant kelp), indicating the potential for cascading indirect positive effects of top predators on the abundance of primary producers. Management measures such as increased minimum size limits and marine reserves may serve to restore historical trophic roles of key predators and thereby enhance the resilience of marine ecosystems.     

Density dependence,prey accessibility and prey depletion by fisheries drive Peruvian seabird population dynamics

In marine ecosystems top predator populations are shaped by environmental factors affecting their prey abundance. Coupling top predators’ population studies with independent records of prey abundance suggests that prey fluctuations affect fecundity parameters and abundance of their predators. However, prey may be abundant but inaccessible to their predators and a major challenge is to determine the relative importance of prey accessibility in shaping seabird populations. In addition, disentangling the effects of prey abundance and accessibility from the effects of prey removal by fisheries, while accounting for density dependence, remains challenging for marine top predators. Here, we investigate how climate, population density, and the accessibility and removal of prey (the Peruvian anchovy Engraulis ringens) by fisheries influence the population dynamics of the largest sedentary seabird community (≈ 4 million individuals belonging to guanay cormorant Phalacrocorax bougainvillii, Peruvian booby Sula variegata and Peruvian pelican Pelecanus thagus) of the northern Humboldt Current System over the past half‐century. Using Gompertz state–space models we found strong evidence for density dependence in abundance for the three seabird species. After accounting for density dependence, sea surface temperature, prey accessibility (defined by the depth of the upper limit of the subsurface oxygen minimum zone) and prey removal by fisheries were retained as the best predictors of annual population size across species. These factors affected seabird abundance the current year and with year lags, suggesting effects on several demographic parameters including breeding propensity and adult survival. These findings highlight the effects of prey accessibility and fishery removals on seabird populations in marine ecosystems. This will help refine management objectives of marine ecosystems in order to ensure sufficient biomass of forage fish to avoid constraining seabird population dynamics, while taking into account of the effects of environmental variability.     

Match and mismatch: Integrating consumptive effects of predators,prey traits,and habitat selection in colonizing aquatic insects

Predators are a particularly critical component of habitat quality, as they affect survival, morphology, behavior, population size, and community structure through both consumptive and non‐consumptive effects. Non‐consumptive effects can often exceed consumptive effects, but their relative importance is undetermined in many systems. Our objective was to determine the consumptive and non‐consumptive effects of a predaceous aquatic insect, Notonecta irrorata, on colonizing aquatic beetles. We tested how N. irrorata affected survival and habitat selection of colonizing aquatic beetles, how beetle traits contributed to their vulnerability to predation by N. irrorata, and how combined consumptive and non‐consumptive effects affected populations and community structure. Predation vulnerabilities ranged from 0% to 95% mortality, with size, swimming, and exoskeleton traits generating species‐specific vulnerabilities. Habitat selection ranged from predator avoidance to preferentially colonizing predator patches. Attraction of Dytiscidae to N. irrorata may be a natural ecological trap given similar cues produced by these taxa. Hence, species‐specific habitat selection by prey can be either predator‐avoidance responses that reduce consumptive effects, or responses that magnify predator effects. Notonecta irrorata had both strong consumptive and non‐consumptive effects on populations and communities, while combined effects predicted even more distinct communities and populations across patches with or without predators. Our results illustrate that an aquatic invertebrate predator can have functionally unique consumptive effects on prey, attracting and repelling prey, while prey have functionally unique responses to predators. Determining species‐specific consumptive and non‐consumptive effects is important to understand patterns of species diversity across landscapes.     

Linking killer whale survival and prey abundance: food limitation in the oceans' apex predator?

Killer whales (Orcinus orca) are large predators that occupy the top trophic position in the world''s oceans and as such may have important roles in marine ecosystem dynamics. Although the possible top-down effects of killer whale predation on populations of their prey have received much recent attention, little is known of how the abundance of these predators may be limited by bottom-up processes. Here we show, using 25 years of demographic data from two populations of fish-eating killer whales in the northeastern Pacific Ocean, that population trends are driven largely by changes in survival, and that survival rates are strongly correlated with the availability of their principal prey species, Chinook salmon (Oncorhynchus tshawytscha). Our results suggest that, although these killer whales may consume a variety of fish species, they are highly specialized and dependent on this single salmonid species to an extent that it is a limiting factor in their population dynamics. Other ecologically specialized killer whale populations may be similarly constrained to a narrow range of prey species by culturally inherited foraging strategies, and thus are limited in their ability to adapt rapidly to changing prey availability.     

Predator density and the functional responses of coral reef fish

Predation is a key process driving coral reef fish population dynamics, with higher per capita prey mortality rates on reefs with more predators. Reef predators often forage together, and at high densities, they may either cooperate or antagonize one another, thereby causing prey mortality rates to be substantially higher or lower than one would expect if predators did not interact. However, we have a limited mechanistic understanding of how prey mortality rates change with predator densities. We re-analyzed a previously published observational dataset to investigate how the foraging response of the coney grouper (Cephalopholis fulva) feeding on the bluehead wrasse (Thalassoma bifasciatum) changed with shifts in predator and prey densities. Using a model-selection approach, we found that per-predator feeding rates were most consistent with a functional response that declines as predator density increases, suggesting either antagonistic interactions among predators or a shared antipredator behavioral response by the prey. Our findings suggest that variation in predator density (natural or anthropogenic) may have substantial consequences for coral reef fish population dynamics.     

Ongoing collapse of coral-reef shark populations

Marine ecosystems are suffering severe depletion of apex predators worldwide; shark declines are principally due to conservative life-histories and fisheries overexploitation. On coral reefs, sharks are strongly interacting apex predators and play a key role in maintaining healthy reef ecosystems. Despite increasing fishing pressure, reef shark catches are rarely subject to specific limits, with management approaches typically depending upon no-take marine reserves to maintain populations. Here, we reveal that this approach is failing by documenting an ongoing collapse in two of the most abundant reef shark species on the Great Barrier Reef (Australia). We find an order of magnitude fewer sharks on fished reefs compared to no-entry management zones that encompass only 1% of reefs. No-take zones, which are more difficult to enforce than no-entry zones, offer almost no protection for shark populations. Population viability models of whitetip and gray reef sharks project ongoing steep declines in abundance of 7% and 17% per annum, respectively. These findings indicate that current management of no-take areas is inadequate for protecting reef sharks, even in one of the world's most-well-managed reef ecosystems. Further steps are urgently required for protecting this critical functional group from ecological extinction.     

Applying Fishers' ecological knowledge to construct past and future lobster stocks in the Juan Fernández Archipelago, Chile

Over-exploited fisheries are a common feature of the modern world and a range of solutions including area closures (marine reserves; MRs), effort reduction, gear changes, ecosystem-based management, incentives and co-management have been suggested as techniques to rebuild over-fished populations. Historic accounts of lobster (Jasus frontalis) on the Chilean Juan Fernández Archipelago indicate a high abundance at all depths (intertidal to approximately 165 m), but presently lobsters are found almost exclusively in deeper regions of their natural distribution. Fishers' ecological knowledge (FEK) tells a story of serial depletion in lobster abundance at fishing grounds located closest to the fishing port with an associated decline in catch per unit effort (CPUE) throughout recent history. We have re-constructed baselines of lobster biomass throughout human history on the archipelago using historic data, the fishery catch record and FEK to permit examination of the potential effects of MRs, effort reduction and co-management (stewardship of catch) to restore stocks. We employed a bioeconomic model using FEK, fishery catch and effort data, underwater survey information, predicted population growth and response to MR protection (no-take) to explore different management strategies and their trade-offs to restore stocks and improve catches. Our findings indicate that increased stewardship of catch coupled with 30% area closure (MR) provides the best option to reconstruct historic baselines. Based on model predictions, continued exploitation under the current management scheme is highly influenced by annual fluctuations and unsustainable. We propose a community-based co-management program to implement a MR in order to rebuild the lobster population while also providing conservation protection for marine species endemic to the Archipelago.     

Molecular identification of predation by carabid beetles on exotic and native slugs in a strawberry agroecosystem

Generalist predators have the capacity to exert significant pressure on prey populations. However, integrating them into biological control programs relies on a detailed understanding of their foraging behavior and the levels of trophic connectedness with pest species. Carabid beetles are important predators of slugs, pests of agricultural, floricultural and horticultural crops worldwide, but these interactions have been rarely studied outside the Western Palearctic ecozone. Diagnostic molecular gut-content analysis was used to examine the strength of trophic pathways between a community of carabid beetles and two slug species, the exotic Deroceras reticulatum and native Deroceras laeve, in strawberry agroecosystems. Strawberries were grown according to standard horticultural practices for central Kentucky, following traditional bare ground planting or with the addition of detrital subsidies, to quantify the impact of habitat management on the abundance of pests and the strength of these trophic pathways. Following laboratory characterization of species-specific molecular markers targeting both Deroceras species, carabid beetles collected from a strawberry agroecosystem were screened for slug DNA. Field collections revealed important food web pathways existed between Harpalus pensylvanicus and D. reticulatum, with 7.2% screening positive for these prey yet none screening positive for D. laeve. In contrast, Chlaenius tricolor was found to feed on both slug species in the field, with 16% screening positive for both Deroceras. Despite below normal rainfall limiting slug densities in the field, the results presented here reveal the potential importance of carabid beetles in slug population dynamics in the Nearctic.     

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.     

A reinforcement learning-based predator-prey model

Classic population models can often predict the dynamics of biological populations in nature. However, the adaptation process and learning mechanism of species are rarely considered in the study of population dynamics, due to the complex interaction of species, seasonal variation, spatial distribution or other factors. We use reinforcement learning algorithms to improve the existing individual-based ecosystem simulation algorithms, which allows species to spontaneously adjust their strategies according to a short period of experience and then feed back to improve their abilities to make action decisions. Our results show that the reinforcement learning of predators is beneficial to the stability of the ecosystem, and predators can learn to spontaneously form hunting patterns that surround their prey. The learning of prey makes the ecosystem oscillate and meanwhile leads to a higher risk of extinction for predators. When individuals are more likely to die, these herbivores rely on reproductive behavior to maintain their populations; when individuals live longer, herbivores spend more time eating to maintain their own survival. The co-reinforcement learning of predators and prey helps predators to find a more suitable way to survive with their prey, that is, the number of predators is more stable and larger than when only predator or only prey learns.     

Prey bacteria shape the community structure of their predators

Although predator–prey interactions among higher organisms have been studied extensively, only few examples are known for microbes other than protists and viruses. Among the bacteria, the most studied obligate predators are the Bdellovibrio and like organisms (BALOs) that prey on many other bacteria. In the macroscopical world, both predator and prey influence the population size of the other''s community, and may have a role in selection. However, selective pressures among prey and predatory bacteria have been rarely investigated. In this study, Bacteriovorax, a predator within the group of BALOs, in environmental waters were fed two prey bacteria, Vibrio vulnificus and Vibrio parahaemolyticus. The two prey species yielded distinct Bacteriovorax populations, evidence that selective pressures shaped the predator community and diversity. The results of laboratory experiments confirmed the differential predation of Bacteriovorax phylotypes on the two bacteria species. Not only did Bacteriovorax Cluster IX exhibit the versatility to be the exclusive efficient predator on Vibrio vulnificus, thereby, behaving as a specialist, but was also able to prey with similar efficiency on Vibrio parahaemolyticus, indicative of a generalist. Therefore, we proposed a designation of versatilist for this predator. This initiative should provide a basis for further efforts to characterize the predatory patterns of bacterial predators. The results of this study have revealed impacts of the prey on Bacteriovorax predation and in structuring the predator community, and advanced understanding of predation behavior in the microbial world.     

Predator avoidance behavior in the pea aphid: costs,frequency, and population consequences

Induced prey defenses can be costly. These costs have the potential to reduce prey survival or reproduction and, therefore, prey population growth. I estimated the potential for predators to suppress populations of pea aphids (Acyrthosiphon pisum) in alfalfa fields through the induction of pea aphid predator avoidance behavior. I quantified (1) the period of non-feeding activity that follows a disturbance event, (2) the effect of frequent disturbance on aphid reproduction, and (3) the frequency at which aphids are disturbed by predators. In combination, these three values predict that the disturbances induced by predators can substantially reduce aphid population growth. This result stems from the high frequency of predator-induced disturbance, and the observation that even brief disturbances reduce aphid reproduction. The potential for predators to suppress prey populations through induction of prey defenses may be strongest in systems where (1) predators frequently induce prey defensive responses, and (2) prey defenses incur acute survival or reproductive costs. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Critical science gaps impede use of no-take fishery reserves

As well as serving valuable biodiversity conservation roles, functioning no-take fishery reserves protect a portion of the fishery stock as insurance against future over-fishing. So long as there is adequate compliance by the fishing community, it is likely that they will also sustain and even enhance fishery yields in the surrounding area. However, there are significant gaps in scientific knowledge that must be filled if no-take reserves are to be used effectively as fishery management tools. Unfortunately, these gaps are being glossed over by some uncritical advocacy. Here, we review the science, identify the most crucial gaps, and suggest ways to fill them, so that a promising management tool can help meet the growing challenges faced by coastal marine fisheries.     

Evaluating the summer landscapes of predation risk and forage quality for elk (Cervus canadensis)

The recovery of carnivore populations in North American has consequences for trophic interactions and population dynamics of prey. In addition to direct effects on prey populations through killing, predators can influence prey behavior by imposing the risk of predation. The mechanisms through which patterns of space use by predators are linked to behavioral response by prey and nonconsumptive effects on prey population dynamics are poorly understood. Our goal was to characterize population‐ and individual‐level patterns of resource selection by elk (Cervus canadensis) in response to risk of wolves (Canis lupus) and mountain lions (Puma concolor) and evaluate potential nonconsumptive effects of these behavioral patterns. We tested the hypothesis that individual elk risk‐avoidance behavior during summer would result in exposure to lower‐quality forage and reduced body fat and pregnancy rates. First, we evaluated individuals'' second‐order and third‐order resource selection with a used‐available sampling design. At the population level, we found evidence for a positive relationship between second‐ and third‐order selection and forage, and an interaction between forage quality and mountain lion risk such that the relative probability of use at low mountain lion risk increased with forage quality but decreased at high risk at both orders of selection. We found no evidence of a population‐level trade‐off between forage quality and wolf risk. However, we found substantial among‐individual heterogeneity in resource selection patterns such that population‐level patterns were potentially misleading. We found no evidence that the diversity of individual resource selection patterns varied predictably with available resources, or that patterns of individual risk‐related resource selection translated into biologically meaningful changes in body fat or pregnancy rates. Our work highlights the importance of evaluating individual responses to predation risk and predator hunting technique when assessing responses to predators and suggests nonconsumptive effects are not operating at a population scale in this system.     

Optimal predator management for mountain sheep conservation depends on the strength of mesopredator release

Large predators often suppress ungulate population growth, but they may also suppress the abundance of smaller predators that prey on neonatal ungulates. Antagonistic interactions among predators may therefore need to be integrated into predator–prey models to effectively manage ungulate–predator systems. We present a modeling framework that examines the net impact of interacting predators on the population growth rate of shared prey, using interactions among wolves Canis lupus, coyotes Canis latrans and Dall sheep Ovis dalli dalli as a case study. Wolf control is currently employed on approximately 16 million ha in Alaska to increase the abundance of ungulates for human harvest. We hypothesized that the positive effects of wolf control on Dall sheep population growth could be counteracted by increased levels of predation by coyotes. Coyotes and Dall sheep adult females (ewes) and lambs were radiocollared in the Alaska Range from 1999–2005 to estimate fecundity, age‐specific survival rates, and causes of mortality in an area without wolf control. We used stage‐structured population models to simulate the net effect of wolf control on Dall sheep population growth (λ). Our models accounted for stage‐specific predation rates by wolves and coyotes, compensatory mortality, and the potential release of coyote populations due to wolf control. Wolves were the main predators of ewes, coyotes were the main predators of lambs, and wolves were the main source of mortality for coyotes. Population models predicted that wolf control could increase sheep λ by 4% per year in the absence of mesopredator release. However, if wolf control released coyote populations, our models predicted that sheep λ could decrease by up to 3% per year. These results highlight the importance of integrating antagonistic interactions among predators into predator–prey models, because the net effect of predator management on shared prey can depend critically on the strength of mesopredator release.     

Modeling the trophic effects of marine protected area zoning policies: A case study

Marine protected areas (MPAs) are increasingly being recognized as an alternative management tool for conserving marine resources and ecosystems. By integrating organism dispersal rates, ecosystem interactions and fishing effort dynamics, ECOSPACE, a spatially explicit ecosystem-based modeling tool, allowed us to compare the ecological consequences of alternative MPA zoning policies within the proposed Gwaii Haanas National Marine Conservation Area, located off the west coast of British Columbia, Canada. The desired effects of MPAs include higher fishery yields, the conservation of biodiversity, and/or the preservation of intact ecosystems. However, ECOSPACE predicts that when MPAs are small, species interactions and movements may make these objectives difficult to achieve. ECOSPACE suggests that the effects of MPAs are reduced at their boundaries where fishing effort is predicted to concentrate. Furthermore, top predators may become more abundant within MPAs, which could lead to a depression of their prey species and a subsequent increase of species at even lower trophic levels. Trophic cascade patterns and density gradients across boundaries are nontrivial departures from our simple expectations of how MPAs protect areas and will force us to reconsider what constitutes effective conservation. Our ECOSPACE model indicates that the establishment of multi-use buffer zones may help alleviate these realistic but worrisome ecological predictions. When coupled with an overall reduction in harvest pressure, ECOSPACE suggests that a MPA with a large core `no-take' zone and large buffer will result in the greatest increase in organism biomass. The use of marine zoning may be an effective management tactic to reduce social conflict and conserve marine ecosystems.     

Continuing influences of introduced hedgehogs <Emphasis Type="Italic">Erinaceus europaeus</Emphasis> as a predator of wader (Charadrii) eggs four decades after their release on the Outer Hebrides,Scotland

Non-native predators can cause major declines or even localised extinctions in prey populations across the globe, especially on islands. The removal of non-native predators can, therefore, be a crucial conservation management tool but there can be challenges when they are viewed as charismatic in their own right. Four decades after their introduction to islands in the Outer Hebrides, Scotland, European hedgehogs Erinaceus europaeus continue to be an important nest predator for a declining population of breeding waders. Where hedgehogs were rare, clutch survival rates (assessed using nest temperature loggers) of five species of waders (dunlin Calidris alpina, lapwing Vanellus vanellus, redshank Tringa totanus, snipe Gallinago gallinago and ringed plover Charadrius hiaticula) were higher than where hedgehogs were relatively more abundant. Hedgehogs were the most frequent nest predator identified using cameras. However, factors influencing population sizes of breeding waders are complex and unlikely to be attributable to a single species of predator. The interactions between predation, land use, habitat and the changes in each deserve further attention.