Invasive insect abundance varies across the biogeographic distribution of a native host plant.

Studies of biogeographic variation in species abundances are fundamental to understanding and predicting the impacts of invasive exotic species. We quantified the abundance of the introduced and now invasive biocontrol weevil, Rhinocyllus conicus, on a newly adopted native host plant, Cirsium canescens (Platte thistle), across the plant's distributional range. We used regression and structural equation analyses to examine variation in weevil abundance at 92-108 sites over three years in relation to variation in abiotic and biotic parameters hypothesized to be important in insect or plant dynamics and distribution. We found that R. conicus now occurs throughout the majority of the range of C. canescens, even in the center of the native plant's distribution where its coevolved, targeted weed host (Carduus nutans, musk thistle) is absent. In fact, weevil densities were greater in the center of the native plant's distribution in the Sand Hills formation than in peripheral sand patches closer to areas where the targeted 'exotic thistle is common. None of the macroclimatic variables examined were consistent predictors of the large-scale variation in weevil abundance on C. canescens. In addition to biogeographic position, the only other consistent predictor of weevil densities across sites was the number of flower heads per C. canescens plant. These results exclude the "spillover" hypothesis to explain nontarget feeding on this newly adopted native host species. Instead, the results are consistent with the alternative hypothesis that exotic weevil abundance on C. canescens is related to the local availability of native floral resources. Because C. canescens densities have declined with increases in R. conicus at sites central in the plant's distribution, these results suggest that isolated, peripheral populations of C. canescens are likely to be critical for persistence of Platte thistle. More generally, this study suggests that the persistence of a native species that is impacted by an exotic natural enemy may require preservation of populations in habitats outside the optimal portion of the native species' distribution.     

Evaluating impacts of forage fish abundance on marine predators

Forage fish—small, low trophic level, pelagic fish such as herrings, sardines, and anchovies—are important prey species in marine ecosystems and also support large commercial fisheries. In many parts of the world, forage fish fisheries are managed using precautionary principles that target catch limits below the maximum sustainable yield. However, there are increasing calls to further limit forage fish catch to safeguard their fish, seabird, and marine mammal predators. The effectiveness of these extra-precautionary regulations, which assume that increasing prey abundance increases predator productivity, are under debate. In this study, we used prey-linked population models to measure the influence of forage fish abundance on the population growth rates of 45 marine predator populations representing 32 fish, seabird, and mammal species from 5 regions around the world. We used simulated data to confirm the ability of the statistical model to accurately detect prey influences under varying levels of influence strength and process variability. Our results indicate that predator productivity was rarely influenced by the abundance of their forage fish prey. Only 6 predator populations (13% of the total) were positively influenced by increasing prey abundance and the model exhibited high power to detect prey influences when they existed. These results suggest that additional limitation of forage fish harvest to levels well below sustainable yields would rarely result in detectable increases in marine predator populations.     

Outbreak suppression by predators depends on spatial distribution of prey

The capacity of a predator population to suppress a prey population that varies in abundance and spatial distribution is explored in a lattice simulation model. The model is based on empirically derived parameters for particular species. Within season predation by Pterostichus cupreus (Coleoptera: Carabidae) of varying densities and distributions of the prey Rhopalosiphum padi (Homoptera: Aphididae) in spring cereals was simulated. From these spatially explicit simulations prey population suppression was found to be largely dependent on the spatial distribution of the prey. A possible mechanism was that high degrees of prey aggregation provided refuge for the prey that, when aggregated, escaped detection by P. cupreus. In contrast, P. cupreus was found to efficiently suppress incipient outbreaks for evenly distributed prey populations, even at high prey densities. A higher predator density compensated for the lowered control ability of the predators for highly aggregated prey populations and hastened the decline of the prey population.     

Invasive competitor and native seed predators contribute to rarity of the narrow endemic Astragalus sinuatus Piper

The conservation of rare plant species hinges on our ability to identify the underlying mechanisms that limit rare plant populations. Theory on rarity suggests that both predispersal seed predation and competition can be important mechanisms influencing abundance and/or distribution of rare plant populations. Yet few studies have tested these interactions, and those that have evaluated each mechanism independently. Astragalus sinuatus Piper (Whited's milkvetch) is a narrow endemic plant species restricted to eight populations within a 10-km2 area in eastern Washington. We used experimental and observational methods to test the effects of native insect predispersal seed predators and an invasive grass (Bromus tectorum L. [cheatgrass]) on seed set and population density of A. sinuatus. We quantified per capita seed production and pod predation rates across four sites and among four years. Seed predation rates were high across four sites (66-82%) and all years (65-82%). Experimental reduction of predispersal seed predators significantly increased per capita seed set of A. sinuatus (164-345%) at two experimental sites. Concurrently, two seed addition experiments demonstrated the effect of seed loss and presence of B. tectorum on seedling recruitment and establishment of A. sinuatus over four growing seasons. In the first seed addition experiment, we found no difference in recruitment and establishment between low (40) and high (120) seed addition levels. In the second addition experiment (one level of addition; 40 seeds), we found that recruitment and survivorship increased 200% in plots where B. tectorum was removed compared to plots where B. tectorum was present. Thus, seed addition had no impact in the presence of B. tectorum; conversely, in the absence of B. tectorum, seed addition was highly effective at increasing population numbers. Results suggest that, in areas where B. tectorum is present, recruitment is site limited, and it is seed limited when B. tectorum is absent. We recommend that managers reduce B. tectorum in an effort to increase population growth of A. sinuatus; in areas where B. tectorum is absent, short-term reduction of insect predators should be considered as a strategy to increase population growth of this rare species.     

Savviness of prey to introduced predators

The prey naivety hypothesis posits that prey are vulnerable to introduced predators because many generations in slow gradual coevolution are needed for appropriate avoidance responses to develop. It predicts that prey will be more responsive to native than introduced predators and less responsive to introduced predators that differ substantially from native predators and from those newly established. To test these predictions, we conducted a global meta-analysis of studies that measured the wariness responses of small mammals to the scent of sympatric mammalian mesopredators. We identified 26 studies that met our selection criteria. These studies comprised 134 experiments reporting on the responses of 36 small mammal species to the scent of six introduced mesopredators and 12 native mesopredators. For each introduced mesopredator, we measured their phylogenetic and functional distance to local native mesopredators and the number of years sympatric with their prey. We used predator and prey body mass as a measure of predation risk. Globally, small mammals were similarly wary of the scent of native and introduced mesopredators; phylogenetic and functional distance between introduced mesopredators and closest native mesopredators had no effect on wariness; and wariness was unrelated to the number of prey generations, or years, since first contact with introduced mesopredators. Small mammal wariness was associated with predator-prey body mass ratio, regardless of the nativity. The one thing animals do not seem to recognize is whether their predators are native.     

The Scotia Sea krill fishery and its possible impacts on dependent predators: modeling localized depletion of prey

The nature and impact of fishing on predators that share a fished resource is an important consideration in ecosystem-based fisheries management. Krill (Euphausia superba) is a keystone species in the Antarctic, serving as a fundamental forage source for predators and simultaneously being subject to fishing. We developed a spatial multispecies operating model (SMOM) of krill-predator fishery dynamics to help advise on allocation of the total krill catch among 15 small-scale management units (SSMUs) in the Scotia Sea, with a goal to reduce the potential impact of fishing on krill predators. The operating model describes the underlying population dynamics and is used in simulations to compare different management options for adjusting fishing activities (e.g., a different spatial distribution of catches). The numerous uncertainties regarding the choice of parameter values pose a major impediment to constructing reliable ecosystem models. The pragmatic solution proposed here involves the use of operating models that are composed of alternative combinations of parameters that essentially try to bound the uncertainty in, for example, the choice of survival rate estimates as well as the functional relationships between predators and prey. Despite the large uncertainties, it is possible to discriminate the ecosystem impacts of different spatial fishing allocations. The spatial structure of the model is fundamental to addressing concerns of localized depletion of prey in the vicinity of land-based predator breeding colonies. Results of the model have been considered in recent management deliberations for spatial allocations of krill catches in the Scotia Sea and their associated impacts on dependent predator species.     

Taste-rejection by predators and the evolution of unpalatability in prey

Aposematic species advertise their unpalatability to potential predators using conspicuous warning colouration. The initial evolution of aposematism is thought to occur by warningly coloured mutants emerging in an already unpalatable cryptic species. However, possessing defence chemicals is often costly, and it is difficult to understand what the selective benefits might be for a mutation causing its bearer to be defended in a population of otherwise palatable cryptic prey. One solution to this problem is that chemically defended individuals are tasted and rejected by predators, and are, therefore, more likely to survive predatory attacks than undefended individuals. Using naïve domestic chicks Gallus gallus domesticus as predators and cryptic green chick crumbs as prey, we asked whether the accuracy with which birds discriminated between palatable and unpalatable prey was affected by the palatability of the unpalatable prey (moderately or highly defended), or their frequency in the population (10 or 25%). Birds could discriminate between green prey on the basis of their defences, and showed better discrimination between palatable and unpalatable prey when defended crumbs were highly unpalatable, compared to when they were moderately unpalatable. Although there was no detectable effect of the frequency of unpalatable prey in the population on predator taste-rejection behaviour in our main analysis, frequency did appear to affect the strategies that birds used in their foraging decisions when prey were only moderately unpalatable. How birds used taste to reject prey also suggests that birds may be able to monitor and regulate their chemical intake according to the frequency and defence levels of the unpalatable prey. Taken together, these results show that avian predators can generate selection for unpalatability in cryptic prey by sampling and taste-rejecting prey, but that a relatively large chemical difference between palatable and unpalatable prey may be necessary before unpalatable prey can enjoy a selective advantage. The exact nature of this evolutionary dynamic will depend on other environmental factors, such as defence costs and prey availability, but it provides a mechanism by which defences can evolve in a cryptic population.     

Coexistence of intraguild predators and prey in resource-rich environments

The prevalence of intraguild predation (IGP) in productive environments has long puzzled ecologists. Theory predicts the exclusion of intraguild prey from such environments, but data consistently defy this expectation. This suggests that coexistence mechanisms at high resource productivity may differ from those at lower productivity. Here I present a mathematical model that investigates multiple coexistence mechanisms. I incorporate two biological features widely observed in IGP communities: intraspecific interference via cannibalism or superparasitism, and temporal refuges arising from differential sensitivities to abiotic variation. I develop predictions based on three aspects of the IG prey-IG predator interaction: mutual invasibility, transient dynamics, and long-term abundances. These predictions specify the conditions under which coexistence mechanisms reinforce vs. deter one another: when a competition-IGP trade-off allows coexistence at intermediate productivity a temporal refuge for the intraguild prey always allows coexistence at high productivity, but intraspecific interference does so only at a net fitness cost to the intraguild predator. Intraspecific interference that benefits the intraguild predator not only reduces tradeoff-mediated coexistence at intermediate productivity, but also undermines the refuge's coexistence-enhancing effect at high productivity. Different mechanism combinations yield characteristic signatures in time series data during transient dynamics. By judicious measurement of parameters and examining time series for critical signatures, one can elucidate the mechanisms that allow IGP to prevail in resource-rich environments.     

Consequences for predators of rescue and Allee effects on prey

The size of a population can be augmented by enriching the carrying capacity of its limiting resource, or by subsidising the renewal of the resource. The well known ‘paradox of enrichment’ models the first case, in which enrichment can force consumers and their limiting resource into destabilising limit cycles, whereas impoverishment stabilises the dynamics. In this paper we model the case of resource subsidy, where the resource is a limiting prey to predators. In contrast to enrichment, the system is stabilised by an influx of prey in the form of a rescue effect, and destabilised by an outflux of prey in the form of an Allee effect. Limit cycles are not sustained by the Allee effect; instead both populations collapse to zero over a large region of the predator-prey phase plane. The catastrophic extinction of prey requires the presence of both an Allee effect on prey and a predator with a type II functional response, though neither needs to contribute a large impact to prey dynamics. The novel implication is that consumers exaggerate the impact of Allee effects on a renewing resource. Conversely, an Allee effect in the form of a cull of resource, even of small value, can trigger local extinction of resource-dependent consumers.     

The introduction of Littorina littorea to British Columbia, Canada: potential impacts and the importance of biotic resistance by native predators

Although the establishment and spread of non-indigenous species depends upon survival in the face of novel environmental conditions and novel biological interactions, relatively little attention has been focused on the specific role of native predators in limiting invasion success. The European common periwinkle, Littorina littorea, was recently introduced to the Pacific coast of Canada and provides a case study of an introduction into an area with an important predator guild (sea stars) that is functionally minor in the invader’s native habitat. Here, we assess the likelihood of establishment, spread, and negative ecological impact of this introduced gastropod, with an emphasis on the role of native sea stars as agents of biotic resistance. Size frequency distributions and local market availability suggest that L. littorea was most likely introduced via the live seafood trade. Non-native hitchhikers (e.g., the trematode Cryptocotyle lingua) were found on/in both market and field specimens. Laboratory studies and field observations confirmed that L. littorea can survive seasonal low salinity in Vancouver, British Columbia. Periwinkles also readily consumed native Ulva, suggesting that periwinkles could impact native communities via herbivory or resource competition. Unlike native gastropods, however, L. littorea lacked behavioural avoidance responses to Northeast Pacific predatory sea stars (Pisaster ochraceus and Pycnopodia helianthoides), and sea star predation rates on L. littorea were much higher than predation rates on native turban snails (Chlorostoma funebralis) in common garden experiments. We therefore expect periwinkle establishment in British Columbia to be limited to areas with low predator density, as is seen in its field distribution to date. We caution that this conclusion may understate the importance of the L. littorea introduction if it also serves as a vector for additional non-indigenous species such as C. lingua.     

The effects of opportunistic and intentional predators on the herding behavior of prey

In this article, we study how predator behavior influences the aggregation of prey into herds. Game-theoretic models of herd formation are developed based on different survival probabilities of solitary prey and prey that join the herd and on the predator's preference of what type of prey to search for. For an intentional predator that will only pursue its preferred type of prey, a single herd with no solitaries cannot emerge unless the herd acts as a prey refuge. If neither prey choice provides a refuge, it is shown that an equilibrium always exists where there are both types of prey and the predator does not always search for the same type of prey (i.e., a mixed equilibrium exists). On the other hand, if the predator is opportunistic in that it sometimes shifts to pursue the type of prey that is observed first, there may be a single herd equilibrium that does not act as a prey refuge when there is a high level of opportunistic behavior. For low opportunistic levels, a mixed equilibrium is again the only outcome. The evolutionary stability of each equilibrium is tested to see if it predicts the eventual herding behavior of prey in its corresponding model. Our analysis confirms that both predator and prey preferences (for herd or solitary) have strong effects on why prey aggregate. In particular, in our models, only the opportunistic predator can maintain all prey in a single herd that is under predation risk.     

Consumptive and nonconsumptive effects of predators on metacommunities of competing prey

Although predators affect prey both via consumption and by changing prey migration behavior, the interplay between these two effects is rarely incorporated into spatial models of predator-prey dynamics and competition among prey. We develop a model where generalist predators have consumptive effects (i.e., altering the likelihood of local prey extinction) as well as nonconsumptive effects (altering the likelihood of colonization) on spatially separated prey populations (metapopulations). We then extend this model to explore the effects of predators on competition among prey. We find that generalist predators can promote persistence of prey metapopulations by promoting prey colonization, but predators can also hasten system-wide extinction by either increasing local extinction or reducing prey migration. By altering rates of prey migration, predators in one location can exert remote control over prey dynamics in another location via predator-mediated changes in prey flux. Thus, the effect of predators may extend well beyond the proportion of patches they visit. In the context of prey metacommunities, predator-mediated shifts in prey migration and mortality can shift the competition-colonization trade-off among competing prey, leading to changes in the prey community as well as changes in the susceptibility of prey species to habitat loss. Consequently, native prey communities may be susceptible to invasion not only by exotic prey species that experience reduced amounts of mortality from resident predators, but also by exotic prey species that exhibit strong dispersal in response to generalist native predators. Ultimately, our work suggests that the consumptive and nonconsumptive effects of generalist predators may have strong, yet potentially cryptic, effects on competing prey capable of mediating coexistence, fostering invasion, and interacting with anthropogenic habitat alteration.     

Behavioral response to predators reverses the outcome of competition between prey species

Summary In a laboratory experiment it was shown that piscivorous predators reversed the outcome of competitive interactions between two fish prey species, juveniles of roach (Rutilus rutilus) and perch (Perca fluviatilis), by behaviorally affecting their use of two available habitats, an open water habitat and a structurally complex refuge. The shift in the competitive relationship was the result of predators forcing the juvenile fishes into a prey refuge with high structural complexity. While roach was competitively superior in the unstructured habitat, perch was superior in the structurally complex prey refuge. The reversal in competitive relationship was demonstrated both with respect to foraging rate and growth rate and resulted from the high structural complexity in the prey refuge interfering with the roach's swimming performance. Because survival and growth patterns through the juvenile stages have profound effects on the population/community dynamics of size-structured populations such as those of fish, behaviorally induced changes in competitive ability should have significant implications also at the population and community levels.     

Testing a new model of aphid abundance with sedentary and non-sedentary predators

Aphid population dynamics has been thoroughly investigated, especially in tree-dwelling aphids. Among the controls of the aphid rate of increase are the negative effects of antagonists, the positive effects of mutualists, the density-dependence of the aphid dynamics, and the non-stationary quality of plant tissues. Here we present a mechanistic model of aphid growth that considers most of these governing factors using a simple formulation. What is new in this model is that it considers two kinds of antagonists. The first kind is a guild of aphid predator specialists that includes ladybirds (Coleoptera: Coccinellidae), but also species of some families of Hemiptera, Diptera, and Neuroptera. The second kind of antagonists consists of omnivores or generalist predators and in this particular setting is exemplified by the European earwig Forficula auricularia (Dermaptera: Forficulidae). The model developed here compared the effects of these two different kinds of aphid predators, the second one always at the site (sedentary predators) and the first one that arrives in important numbers only once the aphid population has already developed to some degree (non-sedentary predators). Multiple model parameter sets, representing different hypotheses about controls on aphid populations, were evaluated within the Generalised Likelihood Uncertainty Estimation (GLUE) methodology. The model correctly reproduced the experimental data obtained in an organic citrus grove showing the important effect that sedentary predators as earwigs can have on the aphid populations. Low densities of sedentary predators or even low predation rates can have a disproportionate effect on the final aphid density, as they prey on small populations, when the per capita effect on the aphid population is higher. During the main spring peak of aphids the role of non-sedentary predators is secondary, as they track the aphid density rather than control it. However, these non-sedentary predators are important within the proposed model to keep the second autumn peak of aphids at low values.     

Local interactions between predators and prey call into question commonly used functional responses

Commonly used functional response models (Holling’s type I and type II models) assume that the encounter rate of a predator increases linearly with prey density, provided that the predator is searching for prey. In other other words, aN (a is the baseline encounter rate and N is prey density) describes the encounter rate. This study examined whether the models are adequate when predators and prey interact locally by using a spatially explicit individual based model because local interactions affect the spatial distribution of predators and prey, which also affects the encounter rate. Predators were assumed to possess a spatial perception range that influenced their foraging behavior (e.g., if a prey is in the perception range, the predator moves towards the prey). The effect of antipredator behavior by prey was also examined. The results suggest that prey and predator densities as well as handling time affect the baseline rate (i.e., parameter a) as opposed to the common assumption that the parameter is constant. The nature of model deviations depended on both the antipredator behavior and the predators’ perception range. Understanding these deviations is important as they qualitatively affect community dynamics.     

Overestimations of food abundance: predator responses to prey aggregation

Understanding and predicting the consequences of trophic interactions for community processes requires knowledge of the role of food availability, which is often wrongly conflated with prey abundance. For prey animals in groups, this is not fully understood. Previous work has shown that oystercatchers more frequently attack solitary rather than aggregated limpets and are more successful in predation attempts on singletons. It has also been demonstrated that an attack on one limpet in a group alerts the entire group, all of which then clamp down and become unavailable. I show that Eurasian Oystercatchers (Haematopus ostralegus L.) attack only one limpet in a group and then move on to attack another individual limpet, and I also demonstrate that the distance they move is greater than the distance at which groups of limpets have been known to detect attacks. Thus in the oystercatcher-limpet predator-prey system on rocky shores, groups of limpets are actually one prey item independently of the number of limpets in the group. This has implications for assessment of food supply for avian predators on rocky shores, with consequences for our understanding of previously documented trophic cascades.     

Response of predators to loss and fragmentation of prey habitat: a review of theory

Despite extensive empirical research and previous reviews, no clear patterns regarding the effects of habitat loss and fragmentation on predator-prey interactions have emerged. We suggest that this is because empirical researchers do not design their studies to test specific hypotheses arising from the theoretical literature. In fact, theoretical work is almost completely ignored by empirical researchers, perhaps because it may be inaccessible to them. The purpose of this paper is to review theoretical work on the effects of habitat loss and fragmentation on predator-prey interactions. We provide a summary of clear, testable theoretical predictions for empirical researchers. To test one or more of these predictions, an empiricist will need certain information on the predator and prey species of interest. This includes: (1) whether the predator is a specialist on one prey species or feeds on many kinds of prey (omnivore and generalist); (2) whether the predator is restricted to the same habitat type as the focal prey (specialist), can use a variety of habitats but has higher survival in the prey habitat (omnivore), or lives primarily outside of the focal prey's habitat (generalist); (3) whether prey-only patches have lower prey extinction rates than predator-prey patches; and (4) whether the prey emigrate at higher rates from predator-prey patches than from prey-only patches. Empiricists also need to be clear on whether they are testing a prediction about habitat loss or habitat fragmentation and need to conduct empirical studies at spatial scales appropriate for testing the theoretical prediction(s). We suggest that appropriate use of the theoretical predictions in future empirical research will resolve the apparent inconsistencies in the empirical literature on this topic.     

Risk vs. reward: how predators and prey respond to aging olfactory cues

Many animals use olfaction to find food and avoid predators, and must negotiate environments containing odors of varying compositions, strengths, and ages to distinguish useful cues from background noise. Temporal variation in odor cues (i.e., “freshness”) seems an obvious way that animals could distinguish cues, yet there is little experimental evidence for this phenomenon. Fresh cues provide a more reliable indicator of donor presence than aged cues, but we hypothesize that the benefits of responding to aged cues depend on whether the cue indicates the proximity of a predator or a potential meal. As prey cannot remain eternally risk averse in response to predator odor, we predict that antipredator responses should diminish as predator cues age. In contrast, animals searching for food should investigate aged prey cues if investigation costs are sufficiently low and the potential benefit (a meal) sufficiently high; thus, we predict that predators will maintain interest in aged prey cues. We tested these ideas using free-ranging rats (Rattus spp.) in two separate experiments; firstly assessing giving-up densities in the presence of predator odor, and secondly examining investigation rates of prey odors. As predicted, giving-up densities dropped once predator odor had aged, but investigation rates remained similar for aged and fresh prey odor. Thus, rats used temporal variation in odor cues to evaluate the cost–benefit relationship of responding to predator and prey odors. We suggest that the ecological significance of variable cue age needs more research and should be considered when interpreting behavioral responses to olfactory information.     

Antipredator responses of koomal (Trichosurus vulpecula hypoleucus) against introduced and native predators

Antipredator behavior studies generally assess prey responses to single predator species although most real systems contain multiple species. In multi-predator environments prey ideally use antipredator responses that are effective against all predator species, although responses may only be effective against one predator and counterproductive for another. Multi-predator systems may also include introduced predators that the prey did not co-evolve with, so the prey may either fail to recognize their threat (level 1 naiveté), use ineffective responses (level 2 naiveté) or succumb to their superior hunting ability (level 3 naiveté). We analyzed microhabitat selection of an Australian marsupial (koomal, Trichosurus vulpecula hypoleucus) when faced with spatiotemporal differences in the activity/density levels of one native (chuditch, Dasyurus geoffroii) and two introduced predators (red fox, Vulpes vulpes; feral cat, Felis catus). From this, we inferred whether koomal recognized introduced predators as a threat, and whether they minimized predation risk by either staying close to trees and/or using open or dense microhabitats. Koomal remained close to escape trees regardless of the predator species present, or activity/density levels, suggesting koomal employ this behavior as a first line of defense. Koomal shifted to dense cover only under high risk scenarios (i.e., with multiple predator species present at high densities). When predation risk was low, koomal used open microhabitats, which likely provided benefits not associated with predator avoidance. Koomal did not exhibit level 1 naiveté, although further studies are required to determine if they exhibit higher levels of naiveté (2–3) against foxes and cats.