Signalling conflict between prey and predator attraction

Predators may utilize signals to exploit the sensory biases of their prey or their predators. The inclusion of conspicuous silk structures called decorations or stabilimenta in the webs of some orb‐web spiders (Araneae: Araneidae, Tetragnathidae, Uloboridae) appears to be an example of a sensory exploitation system. The function of these structures is controversial but they may signal to attract prey and/or deter predators. Here, we test these predictions, using a combination of field manipulations and laboratory experiments. In the field, decorations influenced the foraging success of adult female St. Andrew’s Cross spiders, Argiope keyserlingi: inclusion of decorations increased prey capture rates as the available prey also increased. In contrast, when decorations were removed, prey capture rates were low and unrelated to the amount of available prey. Laboratory choice experiments showed that significantly more flies (Chrysomya varipes; Diptera: Calliphoridae) were attracted to decorated webs. However, decorations also attracted predators (adult and juvenile praying mantids, Archimantis latistylus; Mantodea: Mantidae) to the web. St. Andrew’s Cross spiders apparently resolve the conflicting nature of a prey‐ and predator‐attracting signal by varying their decorating behaviour according to the risk of predation: spiders spun fewer decorations if their webs were located in dense vegetation where predators had greater access, than if the webs were located in sparse vegetation.     

Behavioral interactions between a cyclopoid copepod predator and its prey

Behavioral observations on the predatory interactions betweenMesocyclops edax and several different types and sizes of preyrevealed that prey size alone was less important than otherspecific morphological and behavioral characteristics of theprey in deterring successful predation by the copepod. The behavioralresponses of Bosmina and Asplanchna to an attacking copepodwere passive and consisted of a simple retraction of vulnerableswimming appendages which made the prey more difficult to grasp.Daphnia and Diaphanosoma on the other hand exhibited very activeswimming escape responses. Tropocyclops usually avoided M. edaxby fleeing before the larger predator could detect them. Thehard carapaces of Daphnia, Bosmina and Keratella were effectiveat reducing ingestion following capture by M. edax. The resultsof these behavioral observations were supported by enclosureexperiments in which the predator was offered a choice betweentwo prey simultaneously. Cyclopoid copepods are capable of successfullyattacking, capturing and ingesting prey organisms several timestheir own body length. Although size alone may influence thepreference of cyclopoid copepods on large and small individualsof the same or similar prey species, it is not a dependabledeterminant of the preference of cyclopoids on multispecificprey assemblages. ¹Present address: Department of Biology, Williams Hall No. 31,Lehigh University, Bethlehem, PA 18015, USA     

Pursuit-deterrent signals: communication between prey and predator

When encountering predators, prey animals often expose themselves by loud vocalization, by repeated movements or by revealing conspicuous colors. The more elaborate displays were often considered to be warning signals directed to other prey, and the less obvious displays to be intention movements. During the last decade, there has been increasing evidence that, in fact, prey display is aimed at the predator, apparently to deter further pursuit. This communication between two seemingly unlikely partners, prey and predator, appears to be based upon a common interest - satisfying the predator's need for further information.     

Selectivity underlies the dissociation between seasonal prey availability and prey consumption in a generalist predator

Generalist predators are capable of selective foraging, but are predicted to feed in close proportion to prey availability to maximize energetic intake especially when overall prey availability is low. By extension, they are also expected to feed in a more frequency‐dependent manner during winter compared to the more favourable foraging conditions during spring, summer and fall seasons. For 18 months, we observed the foraging patterns of forest‐dwelling wolf spiders from the genus Schizocosa (Araneae: Lycosidae) using PCR‐based gut‐content analysis and simultaneously monitored the activity densities of two common prey: springtails (Collembola) and flies (Diptera). Rates of prey detection within spider guts relative to rates of prey collected in traps were estimated using Roualdes’ c_st model and compared using various linear contrasts to make inferences pertaining to seasonal prey selectivity. Results indicated spiders foraged selectively over the course of the study, contrary to predictions derived from optimal foraging theory. Even during winter, with overall low prey densities, the relative rates of predation compared to available prey differed significantly over time and by prey group. Moreover, these spiders appeared to diversify their diets; the least abundant prey group was consistently overrepresented in the diet within a given season. We suggest that foraging in generalist predators is not necessarily restricted to frequency dependency during winter. In fact, foraging motives other than energy maximization, such as a more nutrient‐focused strategy, may also be optimal for generalist predators during prey‐scarce winters.     

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.     

Dynamics between a predator and a prey switching two kinds of escape motions

In nature, animals are classified into two large groups. Those that form the prey and that form the predator. A prey animal runs for its life when chased by a predatory animal. When prey animals escape from the chasing enemy, they generally use two types of evasive motion. Those are a straight-line escape motion and a zigzag-line escape motion. A fleeing prey switches between two types of evasive behavior in a manner depending on the predator's performance.I propose a mathematical model that expresses behaviors between a prey and a predator. This model brings that a straight-line escape motion is a better solution for an escape from a slow and far predator. On the other hand, an evasive motion for a near or fast enemy is a zigzag-line escape motion. This model suggests that animals have the best evasive strategy.     

Food supplementation affects interactions between a phytoseiid predator and its omnivorous prey

The beneficial effect of food supplements in supporting populations of generalist arthropod predators in agricultural systems has been shown to enhance pest control. When providing additional foods in a crop that is attacked by an omnivorous pest, food supplements may be available to both pest and predator populations resulting in more complex interactions. We assessed the consequences of adding extra food sources to a tritrophic system in the laboratory, consisting of leaf discs of kidney bean plants (Phaseolus vulgaris), western flower thrips Frankliniella occidentalis (Thysanoptera: Thripidae) and the predatory mite Amblydromalus limonicus (Acari: Phytoseiidae). The supplemental food sources tested were cattail pollen, Typha latifolia, dry decapsulated cysts of the brine shrimp, Artemia franciscana (Branchiopoda: Artemiidae) and eggs of the Mediterranean flour moth, Ephestia kuehniella (Lepidoptera: Pyralidae). Larvae of F. occidentalis were observed to feed on all three food sources when applied to bean leaves. The immature development time of F. occidentalis was significantly shorter when T. latifolia pollen was provided compared to bean leaves only and bean leaves supplemented with A. franciscana or E. kuehniella. The predation rate of A. limonicus females on first instars of F. occidentalis decreased with about 30% irrespective of food type supplemented to the leaf discs. The presence of additional foods reduced antipredator behavior of F. occidentalis larvae killing predator eggs. Thrips larvae did not attack eggs of A. limonicus when cattail pollen was added to the leaf discs, whereas 2.5 predator eggs per day were consumed without food supplementation. Leaf damage decreased substantially when Typha pollen was present on the leaf discs. The findings of this study indicate that food supplementation shifts tritrophic interactions both top-down and bottom-up and may affect the outcome of an augmentative biological control program.     

Scale-dependent indirect interactions between two prey species through a shared predator

We investigated the potential for indirect interactions between two prey species, pea aphids ( Acyrthosiphon pisum ) and potato leafhoppers ( Empoasca fabae ), through a shared predator (Nabis spp.), and how these interactions may change across three spatial scales. In greenhouse experiments using small clusters of plants containing pea aphids and/or potato leafhoppers, the predation rates on both pea aphids and potato leafhoppers were independent of the presence of the other species, indicating no indirect interactions. In greenhouse experiments using cages containing 48 plants, when aphids and leafhoppers were confined to separate plants among which nabids could move, pea aphids had a positive effect on the survival of potato leafhoppers from predation. The positive effect of aphids on leafhoppers occurred because nabids spent more time on plants harboring aphids, thereby drawing nabids away from plants containing leafhoppers. Finally, we measured the abundance of nabids in a large-scale experiment designed to manipulate the abundances of pea aphids and potato leafhoppers in alfalfa fields. Fields with high aphid density contained more nabids, thereby suggesting that pea aphids will have a negative indirect effect on potato leafhoppers by increasing the density of nabids within fields. Potato leafhoppers had no indirect effects on pea aphids at any scale. This study shows that indirect interactions between prey species may depend upon spatial scale, because the factors affecting a predator's diet choice on a small scale may differ from those factors affecting a predator's distribution at larger scales.     

Resource dynamics influence the strength of non-consumptive predator effects on prey

Predators influence prey populations both by consuming individual prey, and by inducing changes in prey behaviour that limit reproduction and survival. Because prey trade-off predation risk for forageing gains, the magnitude of predators' non-consumptive effects should depend on resource availability. Studies of non-consumptive effects generally adopt either of two strategies: (i) maintaining a static ration of the prey's resources; and (ii) using resource populations that vary dynamically in response to prey behaviour. Contrasting these experimental designs using meta-analysis, we evaluated whether resource dynamics influence the magnitude of non-consumptive effects on prey growth, survival, fecundity, population density, forageing rate and habitat use. Predators had a more negative effect on prey demography in dynamic- vs. static-resource experiments. Our results highlight the importance of resource dynamics in mediating the magnitude of non-consumptive effects of predators on prey, and illustrate the often-unintended impacts of experimental design on estimates of effect size in ecological interactions.     

捕食关系的Meta分析

应用Ｍｅｔａ分析方法,探讨捕食与被捕食者种群数量的关系,结果捕食效应随地带性而变化,热带效应值最大,亚热带,温带和寒带也有效应,亚热带最小,按分类划分成两个级别时,脊椎动物级别属于大效应,而无脊椎动物级别为小效应,级别间差异不显著,按所在生态系统划分级别分析,陆生生态系统级别的中效应,淡水生态系统为小儿应,总体上表明捕食导致被捕食者种群数量的降低,但不同标准划分的级别里几个级别的效应大小有差别,通     

Spatial dynamics in breeding performance of a predator: the connection to prey availability

Using nationwide long-term data on goshawk and grouse populations in Finland we study the spatial dynamics of the numbers of breeding northern goshawk ( Accipiter gentilis ) pairs, goshawk brood size and offspring sex ratio and their connection to the abundance of grouse. Our first large-scale data comprise of observations on goshawk nests during 1986–2001 pooled to 21 different regions. The second set are annual (1989–1998) observations of brood size and offspring sex ratio (females over the sum of females and males) in goshawk nests all over the country, aggregated to 50 km grid level (n=28 grid units). The third set comprises counts (1989–2001) of four species of woodland grouse, split to adults and juveniles, also given in the same 50 km grid units. Using these data, we show that the annual numbers of northern goshawk nests in the different regions fluctuate in synchrony. Synchrony is also found in long-term fluctuations of northern goshawk brood size and offspring sex ratio. Moreover, synchrony is found in annual numbers of grouse juveniles and adults, the main prey for the northern goshawk. In the brood size and offspring sex ratio of the goshawk, as well as in the annual numbers of grouse juveniles and adults the degree of synchrony falls off with increasing distance. However, only in sex ratios and in grouse dynamics are the slopes of synchrony vs distance roughly matching. We also found that sex ratio either vs grouse juveniles or grouse adults has a more matching spatial dimension (50 km radius) that sex ratio vs brood size. These observation lend support to the hypothesis that goshawk offspring sex ratio and grouse abundance are interconnected. Despite the reason, consequences of spatial coupling in sex ratio could have repercussions on other life history events.     

Population dynamics of thrips prey and their mite predators in a refuge

Prey refuges are expected to affect population dynamics, but direct experimental tests of this hypothesis are scarce. Larvae of western flower thrips Frankliniella occidentalis use the web produced by spider mites as a refuge from predation by the predatory mite Neoseiulus cucumeris. Thrips incur a cost of using the refuge through reduced food quality within the web due to spider mite herbivory, resulting in a reduction of thrips developmental rate. These individual costs and benefits of refuge use were incorporated in a stage-structured predator-prey model developed for this system. The model predicted higher thrips numbers in presence than in absence of the refuge during the initial phase. A greenhouse experiment was carried out to test this prediction: the dynamics of thrips and their predators was followed on plants damaged by spider mites, either with or without web. Thrips densities in presence of predators were higher on plants with web than on unwebbed plants after 3 weeks. Experimental data fitted model predictions, indicating that individual-level measurements of refuge costs and benefits can be extrapolated to the level of interacting populations. Model-derived calculations of thrips population growth rate enable the estimation of the minimum predator density at which thrips benefit from using the web as a refuge. The model also predicted a minor effect of the refuge on the prey density at equilibrium, indicating that the effect of refuges on population dynamics hinges on the temporal scale considered.     

Antagonistic indirect interactions between large and small conspecific prey via a heterospecific predator

Intrapopulation size variation strongly influences ecological interactions because individuals belonging to different size groups have distinct functions. Most demonstrations of the impacts of size variation in trophic systems have focused on size variation in predator species, and the consequences of size variation in prey species are less well understood. We investigated how prey size structure shapes intra‐ and interspecific interactions in experiments with a gape‐limited predator (larvae of the salamander Hynobius retardatus) and its heterospecific prey (frog tadpoles, Rana pirica). We found that large and small tadpole size groups each increased mortality in the other group by intensifying salamander predation; this type of indirect interactions is called apparent competition. The antagonistic impacts on the prey size groups were caused by different size‐specific mechanisms. By consuming small tadpoles, the salamanders grew large enough to consume large tadpoles. The activity of large tadpoles, by increasing the activity of the small tadpoles, may increase the number of encounters with the predator and thus small tadpole mortality. These results suggest that the magnitude of a predator's ecological role, such as whether a top–down trophic cascade is initiated, depends on size variation in its heterospecific prey.     

Population dynamical consequences of reduced predator switching at low total prey densities

Several models of rapid switching by a predator in a two-prey environment are analyzed. The goal is to determine how the dynamics of the system and the potential indirect effects between prey are affected by the dependence of switching on total prey density. In exploring this question, the difference between the population-level consequences of switching in stable and cycling predator-prey systems is also examined. We concentrate on reduced switching at low densities, a feature that is likely because of the difficulty of distinguishing between two very low densities. The main findings are: (1) switching in unstable systems can produce positive indirect effects of one prey species on the other; and (2) reduced switching at low densities can greatly alter the dynamics of the system and the indirect effects between prey. Both of the possibilities are only evident in cycling systems. Reduced switching at low total prey densities leads to heavier predation on the slower-growing prey when both prey species are rare. As a consequence, there is a lag in the recovery of the slower-growing prey species after predator densities fall, and the dynamics of the two prey become desynchronized. The net result is increased indirect interactions between prey, and a greater likelihood of exclusion of the slower growing prey. The analysis of these models suggests a need for more empirical work to determine whether switching is reduced by very low total prey densities, and to study the long-term dynamics that occur in systems with switching predators.     

Population dynamics of cereal aphids: influence of a shared predator and weather

1 Aphid populations may show strong year-to-year fluctuations, but questions remain regarding the dominance of factors that cause this variation, especially the role of natural enemies. To better understand the dynamics of aphid species that occur as pests in cereals, we investigated the relative influence of top-down control by a predator and weather (temperature and precipitation) on population fluctuations of three cereal aphid species.
2 From 1987 to 2005, populations of Metopolophium dirhodum , Sitobion avenae and Rhopalosiphum padi in insecticide-free stands of winter wheat were monitored in the Praha-Ruzyné region of the Czech Republic. Densities of an aphidophagous predator, the ladybeetle Coccinella septempunctata , were recorded from an overwintering site in the landscape. Weather was quantified using historical records.
3 A significant bottom-up effect of densities of aphids on those of C.   septempunctata was found, but evidence of direct top-down regulation of aphids by C.   septempunctata was only significant in the case of R.   padi . There was no significant periodicity in the dynamics of the aphid or C.   septempunctata , suggesting that there was no clear predator-prey cycle. Combinations of C.   septempunctata and weather variables could be used to explain M.   dirhodum and R.   padi per capita rate of change. There were also indications that weather directly affected peak density of M.   dirhodum .
4 We conclude that regional estimates of C.   septempunctata densities are not sufficient to determine whether aphid population dynamics are driven by predator–prey interactions. Feasibility of time series analysis as an investigative tool in aphid population dynamics studies is discussed.     

Time-related predator/prey interactions between birds and fish in a northern Swedish river

Summary Seasonal and diel activity patterns of mergansers, gulls, and terns along a river in northern Sweden were documented, as were those of their fish prey. The seasonal and diel activity patterns of goosandersMergus merganser and gulls (Larus canus, L. argentatus, andL. fuscus) were closely related to that of the river lampreyLampetra fluviatilis. During the peak spawning of the river lamprey, birds showed a nocturnal peak in fishing activity. During the summer solstice, birds were active for 24 h. The activity patterns of red-breasted merganserMergus serrator, ternsSterna spp., and three-spined sticklebacksGasterosteus aculeatus were also similar. Activity pattern of the prey apparently influenced breeding time, diel activity and foraging area of the twoMergus species. Social relations between gulls probably corrdinated their peak in fishing, which coincided with the time lampreys were most efficiently exploited.     

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.