Effects of successive predator attacks on prey aggregations

We study the cumulative effect of successive predator attacks on the disturbance of a prey aggregation using a modelling approach. Our model intends to represent fish schools attacked by both aerial and underwater predators. This individual-based model uses long-distance attraction and short-distance repulsion between prey, which leads to prey aggregation and swarming in the absence of predators. When intermediate-distance alignment is added to the model, the prey aggregation displays a cohesive displacement, i.e., schooling, instead of swarming. Including predators, i.e. with repulsion behaviour for prey to predators in the model, leads to flash expansion of the prey aggregation after a predator attack. When several predators attack successively, the prey aggregation dynamics is a succession of expanding-grouping-swarming/schooling phases. We quantify this dynamics by recording the changes in the simulated prey aggregation radius over time. This radius is computed as the longest distance of individual prey to the aggregation centroid, and it is assumed to increase along with prey disturbance. The prey aggregation radius generally increases during flash expansion, then decreases during grouping until reaching a constant lowest level during swarming/schooling. This general dynamics is modulated by several parameters: the frequency, direction (vertical vs. horizontal) and target (centroid of the prey aggregation vs. random prey) of predator attacks; the distance at which prey detect predators; the number of prey and predators. Our results suggest that both aerial and underwater predators are more efficient at disturbing fish schools by increasing their attack frequency at such level that the fish cannot return to swarming/schooling. We find that a mix between aerial and underwater predators is more efficient at disturbing a fish school than a single type of attack, suggesting that aerial and underwater foragers may gain mutual benefits in forming foraging groups.     

Non-linear dynamics of a pelagic ecosystem model with multiple predator and prey types

Using numerical techniques, we explored the dynamics of a one-dimensional,six-component nutrient–phytoplankton–zooplankton(NPZ) model in which zooplankton grazed on a mixed prey field.Five alternative functional forms were implemented to describezooplankton grazing, and the form for predation on mesozooplanktonwas prescribed by a product of a specific predation rate (h)and the mesozooplankton concentration raised to a power (q),which we varied between one and two. With all five grazing functions,Hopf bifurcations, where the form of the solution transitionedbetween steady equilibrium and periodic limit cycles, persistedacross the q–h parameter space. Regardless of the valuesof h and q, with some forms of the grazing function, we wereunable to find steady equilibrium solutions that simultaneouslycomprised non-zero concentrations for all six model components.Extensions of Michaelis–Menten-based single resource grazingformulations to multiple resources resulted in periodic solutionsfor a large portion of the q–h space. Conversely, extensionsof the sigmoidal grazing formulation to multiple resources resultedin steady solutions for a large portion of q–h parameterspace. Our results demonstrate the consequences of the functionalform of biological processes on the form of the model solutions.Both the steady or oscillatory nature of state variable concentrationsand the likelihood of their elimination are important considerationsfor ecosystem-modelling studies, particularly when attemptingto model an ecosystem in which multiple phytoplankton and zooplanktoncomponents are thought to persist simultaneously for at leasta portion of the seasonal cycle.     

Land cover diversity increases predator aggregation and consumption of prey

A lower diversity of land cover types is purported to decrease arthropod diversity in agroecosystems and is dependent on patterns of land use and fragmentation. Ants, important providers of ecosystem services such as biological control, are susceptible to landscape‐level changes. We determined the relationships between land cover diversity and fragmentation on the within‐field spatial associations of ants to pests and resulting predation events by combining mapping and molecular tools. Increased land cover diversity and decreased fragmentation increased ant abundance, spatial association to pests and predation. Land cover diversity and fragmentation were more explanatory than land cover types. Even so, specific land cover types, such as deciduous forest, influenced ant and pest diversity more so than abundance. These results indicate that geospatial techniques and molecular gut content analysis can be combined to determine the role of land use in influencing predator–prey interactions and resulting predation events in agroecosystems.     

Multiple predator effects result in risk reduction for prey across multiple prey densities

Investigating how prey density influences a prey’s combined predation risk from multiple predator species is critical for understanding the widespread importance of multiple predator effects. We conducted experiments that crossed six treatments consisting of zero, one, or two predator species (hellgrammites, greenside darters, and creek chubs) with three treatments in which we varied the density of mayfly prey. None of the multiple predator effects in our system were independent, and instead, the presence of multiple predator species resulted in risk reduction for the prey across both multiple predator combinations and all three levels of prey density. Risk reduction is likely to have population-level consequences for the prey, resulting in larger prey populations than would be predicted if the effects of multiple predator species were independent. For one of the two multiple predator combinations, the magnitude of risk reduction marginally increased with prey density. As a result, models predicting the combined risk from multiple predator species in this system will sometimes need to account for prey density as a factor influencing per-capita prey death rates.     

Viability selection on prey morphology by a generalist predator

Prey use their locomotory capacity to escape predators, and there should thus be strong viability selection on locomotory morphology of prey. We compared feather morphology of wood pigeons Columba palumbus killed by goshawks Accipiter gentilis with that of survivors to quantify directional and quadratic selection on primary and rectrix feathers. The goshawk is mainly a predator attacking by surprise, leaving wood pigeons with an ability to accelerate fast at a selective advantage. There was directional selection for light primary feathers with a narrow calamus. In addition, there was directional selection for increased area of rectrices. These patterns of natural selection were confirmed in multivariate analyses of selection that showed selection for light primary feathers with a large area and narrow calamus and for a large area of rectrix feathers. These results provide evidence of selection on different aspects of feather morphology directly related to flight performance and thus escape ability from predators.     

Adaptive changes in prey vulnerability shape the response of predator populations to mortality

Simple models are used to explore how adaptive changes in prey vulnerability alter the population response of their predator to increased mortality. If the mortality is an imposed harvest, the change in prey vulnerability also influences the relationship between harvest effort and yield of the predator. The models assume that different prey phenotypes share a single resource, but have different vulnerabilities to the predator. Decreased vulnerability is assumed to decrease resource consumption rate. Adaptive change may occur by phenotypic changes in the traits of a single species or by shifts in the abundances of a pair of coexisting species or morphs. The response of the predator population is influenced by the shape of the predator's functional response, the shape of resource density dependence, and the shape of the tradeoff between vulnerability and food intake in the prey. Given a linear predator functional response, adaptive prey defense tends to produce a decelerating decline in predator population size with increased mortality. Prey defense may also greatly increase the range of mortality rates that allow predator persistence. If the predator has a type-2 response with a significant handling time, adaptive prey defense may have a greater variety of effects on the predator's response to mortality, sometimes producing alternative attractors, population cycles, or increased mean predator density. Situations in which there is disruptive selection on prey defense often imply a bimodal change in yield as a function of harvesting effort, with a minimum at intermediate effort. These results argue against using single-species models of density dependent growth to manage predatory species, and illustrate the importance of incorporating anti-predator behavior into models in applied population ecology.     

Local and distant prey-related cues influence when an acarine predator leaves a prey patch

Over relatively long distances, the predatory mite Phytoseiulus persimilis is able to detect volatiles produced by bean plants that are infested by its prey, Tetranychus urticae, the twospotted spider mite. Our investigation examined the separate and combined effects of prey, their products, and prey-induced plant volatiles on when P. persimilis left a potential prey host plant. In wind tunnels, we assessed the relative importance of and interaction among local and distant prey-related cues. The examination of local cues included: (1) all local cues (prey eggs, webbing, and prey-induced plant volatiles), (2) food (prey eggs) and webbing only, (3) plant volatiles only, and (4) no prey-related cues. The examination of distant cues involved the presence or absence of prey-induced plant volatiles from upwind plants. External volatile cues, produced by placing prey-infested plants upwind in the wind tunnel, resulted in more predators leaving downwind plants, and leaving sooner, than when clean plants were upwind, regardless of the availability of prey or prey-related cues on the local plant. However, local cues, especially the presence of food/webbing, had a greater effect than distant cues on timing of predator leaving. Predators remained in larger numbers and for longer times on prey-infested plants. However, the presence of either locally-produced plant volatiles or food/webbing alone still reduced the number of predators leaving a plant in the first hour compared to clean plants. After the first hour, the number of predators leaving was primarily driven by the presence of food/webbing. When no food/webbing was available, predators left plants rapidly; if food/webbing was available, some predatory mites remained on plants at least 24 hours. Even if no food/webbing was available, predators presented with local volatiles remained on plants for several hours longer than on clean plants without local volatiles. These small changes in leaving rates may lead to differences in local population dynamics, and possibly regional persistence, of the predator-prey interaction in patchy environments.     

High intraguild predator density induces thinning effects on and increases temporal overlap with prey populations

Intraguild (IG) predator density can alter its effects on intraguild prey populations through several mechanisms, including density-dependent processes that affect IG predator traits such as size or growth that enhance or limit its predatory abilities. We examined whether intraspecific density-dependence altered IG predator traits, as well as the subsequent interspecific effects among its intraguild prey within a larval salamander guild. Four densities of ringed salamanders (Ambystoma annulatum), the IG predator, were combined with the presence/absence of spotted salamanders (A. maculatum), the IG prey, within experimental mesocosms. We modeled the effects of A. annulatum density on both conspecific and heterospecific responses that would be indicative of density-dependent competition and predation, respectively. We also modeled the reciprocal interspecific effects of A. maculatum on A. annulatum. We found that increasing intraspecific density negatively affected morphological traits but not survival of A. annulatum. No interspecific effects of A. maculatum on A. annulatum were observed. Alternatively, traits of A. maculatum showed nonlinear relationships with increasing A. annulatum density. Thinning effects of A. annulatum on A. maculatum were observed, as survival was positively and size negatively related for A. maculatum with IG predator density. The temporal overlap of the IG predator and prey also increased nonlinearly with IG predator density, intensifying the potential encounter rate of the two species. Overall, this study shows that density-dependent processes in IG predators can significantly affect traits of both themselves, as well as IG prey, which could ultimately change whether competition or predation occurs between the two groups.     

Laboratory observations of prey behaviour for prey acquisition by a predator among three larval mosquitoes

The effect of prey behaviour on prey acquisition by a predator was examined in the laboratory, using three larval mosquitoes.Aedes albopictus was acquired by the predator,Toxorhynchites towadensis, at a higher rate thanOrthopodomiya anopheloides. Toxorphynchites towadensis was a sit-and-wait predator.Aedes albopictus was more active thanO. anopheloides. Orthopodomiya anopheloides ran away before the predator attacked, butA. albopictus did not. The escape ratio inO. anopheloides was higher than that inA. albopictus. These results suggest that the difference in the prey acquisition ratio by the predator between prey species is caused by different behavioural patterns of the prey to the predator.     

A predator prey model with age structure

A general predator-prey model is considered in which the predator population is assumed to have an age structure which significantly affects its fecundity. The model equations are derived from the general McKendrick equations for age structured populations. The existence, stability and destabilization of equilibria are studied as they depend on the prey's natural carrying capacity and the maturation periodm of the predator. The main result of the paper is that for a broad class of maturation functions positive equilibria are either unstable for smallm or are destabilized asm decreases to zero. This is in contrast to the usual rule of thumb that increasing (not decreasing) delays in growth rate responses cause instabilities.Research supported by National Science Foundation Grant No. MCS-7901307-01Research supported by National Scholarship for Study Abroad No. EDN/S-59/80 from the government of India     

Lack of prey switching and strong preference for mosquito prey by a temporary pond specialist predator

1. The strengths of trophic interactions within ecosystems can be mediated by complex mechanisms that require elucidation if researchers are to understand and predict population- and community-level stabilities. Where multiple prey types co-occur, prey switching (i.e. frequency-dependent predation) by predators may facilitate low-density prey refuge effects which promote coexistence. On the other hand, lack of switching and strong preferences by predators can strongly suppress prey populations, which is especially important considering vector species such as mosquitoes. 2. The present study quantifies prey switching and preference patterns of the temporary pond specialist copepod Lovenula raynerae towards larvae of the medically important Culex pipiens mosquito complex in the presence of different proportions of alternative Daphnia pulex prey. Further, it examines whether prey switching and preferences are contingent on the sex of the predator. 3. Lovenula raynerae exhibited a lack of prey switching and strong preference for larval mosquito prey overall, irrespective of predator sex. Also, when larval mosquitoes were available in higher proportions over daphniids, the strength of this positive selectivity increased. There was very little low-density refuge for mosquitoes where they were rare. 4. Lack of prey switching and strong preferences towards mosquitoes by predatory paradiaptomid copepods may enhance population-level regulation of disease vector mosquitoes that exploit temporary pond-style habitats. Accordingly, the conservation and promotion of these predators might enable better management of medically important species across landscapes.     

Conspicuous colouration attracts prey to a stationary predator

Abstract 1. Conspicuous body colouration is counter‐intuitive in stationary predators because sit‐and‐wait tactics frequently rely on concealed traps to capture prey. Consequently, bright colours and contrasting patterns should be rare in predators using traps as they may alert potential prey. Yet, some orb‐weaving spiders are brightly coloured and contrastingly patterned. How can conspicuousness of trap‐building sit‐and‐wait predators be favoured by natural selection? 2. Observations of spiny spiders Gasteracantha fornicata in north‐eastern Australia showed that the size of spiders relative to their orb webs correlated positively with relative prey numbers already captured in their webs. A possible explanation is that the relatively larger appearance of the yellow–black striped dorsal surface of this spider attracts more visually oriented prey items. Prey attracted to webs may get trapped, thereby increasing the spiders' foraging success. 3. To test this hypothesis for the function of conspicuous body colouration, a field experiment was conducted that documented the prey capture rates of spiny spiders after manipulating or sham‐manipulating their appearance. 4. As predicted, spiders that were dyed black on their striped dorsal surface caught relatively fewer prey items than did control spiders. Thus, conspicuous dorsal body colouration may be adaptive in spiny spiders because it increases foraging success and, presumably, survival rates and reproductive outputs. Overall, these data support the colour‐as‐prey‐attractant hypothesis in a stationary, trap‐building predator.     

Quantitative prey species detection in predator guts across multiple trophic levels by mapping unassembled shotgun reads

Quantifying species trophic interaction strengths is crucial for understanding community dynamics and has significant implications for pest management and species conservation. DNA-based methods to identify species interactions have revolutionized these efforts, but a significant limitation is the poor ability to quantify the strength of trophic interactions, that is the biomass or number of prey consumed. We present an improved pipeline, called Lazaro, to map unassembled shotgun reads to a comprehensive arthropod mitogenome database and show that the number of prey reads detected is quantitatively predicted from the prey biomass consumed, even for indirect predation. Two feeding bioassays were performed: starved coccinellid larvae consuming different numbers of aphids (Prey Quantity bioassay), and starved coccinellid larvae consuming a chrysopid larvae that had consumed aphids (Direct and Indirect Predation bioassay). Prey taxonomic assignment against a mitochondrial genome database had high accuracy (99.8% positive predictive value) and the number of prey reads was directly related to the number of prey consumed and inversely related to the elapsed time since consumption with high significance (r² = .932, p = 4.92E-6). Aphids were detected up to 6 h after direct predation plus 3 h after indirect predation (9 h in total) and detection was related to the predator-specific decay rates. Lazaro enabled quantitative predictions of prey consumption across multiple trophic levels with high taxonomic resolution while eliminating all false positives, except for a few confirmed contaminants, and may be valuable for characterizing prey consumed by field-sampled predators. Moreover, Lazaro is readily applicable for species diversity determination from any degraded environmental DNA.     

Differential vulnerability of prey to an invading top predator: integrating field surveys and laboratory experiments

Abstract 1. A new top predator, the dragonfly Cordulegaster boltonii, invaded Broadstone Stream (U.K.) in the mid‐1990s. This provided a rare opportunity to assess the impact of a new, large carnivore on a community that has been studied since the 1970s and has one of the most detailed food webs yet published. The vulnerability of the resident species to the invader was assessed by integrating experiments, which examined discrete stages in the predation sequence, with empirical survey data. 2. Although the new predator preyed on nearly every macro‐invertebrate in the food web, vulnerability varied considerably among prey species. Size‐related handling constraints initially set the predator's diet, resulting in strong ontogenetic shifts, with progressively larger prey being added while small prey were retained in the diet, as predators grew. Within the size range of vulnerable prey, encounter rate limited the strength of predation, with mobile, epibenthic species being most at risk. Contrary to most studies of interactions between freshwater predators (usually stoneflies) and prey (usually mayflies), the new predator did not elicit avoidance responses from its prey, probably because it combined a highly cryptic feeding posture with an extremely rapid attack response. 3. The invader exploited its prey heavily in experiments, even at prey densities orders of magnitude above ambient. In the field, electivity reflected prey availability, as determined by mobility and microhabitat use, rather than prey abundance or active predator choice. Consequently, the invader had skewed effects within the prey assemblage, with sedentary, interstitial species being far less vulnerable than more active, epibenthic species, some of which, including a previous top predator, have declined markedly since the invasion. 4. By examining the predation sequence in detail and integrating surveys with experiments, species traits and system characteristics that determine the strength of trophic interactions may be identified, and their potential importance in natural food webs assessed. In so doing, greater insight can be gained into which species (and systems) will be most vulnerable to invading or exotic predators, an imperative in both pure and applied ecology.     

Introduced invertebrates are important prey for a generalist predator

Concern over biological invasions has drawn increased attention to the impacts of introduced predators or competitors, but not to the importance of introduced prey. North American forests are rich in introduced invertebrates, including species that represent relatively novel taxonomic or trophic guilds and show biased distributions among forest types. We analysed the diets of red‐backed salamanders, Plethodon cinereus, from three upland and three lowland forests to determine whether introduced prey are important contributors to geographical or temporal variation in salamander food resources. We found several introduced species were volumetrically important salamander prey, and were responsible for resource differences between forest types and much of the seasonal fluctuation in food resources in both forest types. In lowland forests, rain had a stronger effect on salamander predation on non‐native earthworms than native taxa, creating more dynamic resource fluctuations in resource levels than was observed in upland forests where earthworms were absent. With one exception, predation on non‐native species was positively associated with predation on native species, suggesting non‐native prey have added to salamander resources rather than replaced salamander predation on native taxa. We hypothesize that the novel resource gradients created by non‐native prey introductions are contributing to patterns of geographical and temporal phenotypic variation among salamander populations.     

Stink bug predator kills prey with salivary non-proteinaceous compounds

Podisus nigrispinus Dallas (Hemiptera: Pentatomidae) is a predator insect with potential applications in biological control because both nymphs and adults have been shown to prey on other insect pests by injection of toxic salivary gland contents. This study identified non-proteinaceous compounds with insecticidal activity from the saliva of P. nigrispinus in Anticarsia gemmatalis. In particular, the ether extract from P. nigrispinus saliva led to mortality in A. gemmatalis larvae, with a LC₅₀ = 2.04 μL and LC₉₀ = 3.27 μL. N,N-dimethylaniline and 1,2,5-trithiepane fractions were identified as non-proteinaceous extract components. N,N-dimethylaniline had a LC₅₀ = 136.1 nL and LC₉₀ = 413.8 nL, suggesting that it could be responsible for toxicity in P. nigrispinus saliva.     

Prey-mediated avoidance of an intraguild predator by its intraguild prey

Intraguild (IG) predation is an important factor influencing community structure, yet factors allowing coexistence of IG predator and IG prey are not well understood. The existence of spatial refuges for IG prey has recently been noted for their importance in allowing coexistence. However, reduction in basal prey availability might lead IG prey to leave spatial refuges for greater access to prey, leading to increased IG predation and fewer opportunities for coexistence. We determined how the availability of prey affected space-use patterns of bobcats (Lynx rufus, IG prey) in relation to coyote space-use patterns (Canis latrans, IG predators). We located animals from fall 2007 to spring 2009 and estimated bobcat home ranges and core areas seasonally. For each bobcat relocation, we determined intensity of coyote use, distance to water, small mammal biomass, and mean small mammal biomass of the home range during the season the location was collected. We built generalized linear mixed models and used Akaike Information Criteria to determine which factors best predicted bobcat space use. Coyote intensity was a primary determinant of bobcat core area location. In bobcat home ranges with abundant prey, core areas occurred where coyote use was low, but shifted to areas intensively used by coyotes when prey declined. High spatial variability in basal prey abundance allowed some bobcats to avoid coyotes while at the same time others were forced into more risky areas. Our results suggest that multiple behavioral strategies associated with spatial variation in basal prey abundance likely allow IG prey and IG predators to coexist.