Optimal strategies for predator avoidance: the relative importance of speed and manoeuvrability

The relative importance of speed and manoeuvrability in predator-prey chases was assessed by investigating whether or not a pursuing predator could catch its prey in a simple turning gambit initiated by the prey animal. The turning radius and velocity of the prey were normalized by dividing them by those of the predator. With the use of numerical methods to solve implicit equations it was determined for what values of relative radii and velocity the prey could escape. When escape was possible the optimal time of initiation of the prey's turn and the minimum closure distance of the predator were computed. It was found initially by numerical and subsequently by analytic techniques that, in order for the prey to escape the predator in the turning gambit, its normalized velocity must be related to its normalized radius by the inequality: $\text{v>r}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ over the interval 0 ? r ? 1. Situations in which the results of the turning gambit may be expected to give realistic predictions of capture or escape are discussed together with physical factors governing the relationship between turning radius and velocity, and the conditions under which the prey might profitably trade higher velocity for a smaller turning radius. Lastly, a number of specific predator-prey combinations are treated with a view towards illustrating the application of the theory and promoting the collection of simultaneous turning radii and velocity data.     

Predation,apparent competition,and the structure of prey communities

It is argued that alternate prey species in the diet of a food-limited generalist predator should reduce each other's equilibrial abundances, whether or not they directly compete. Such indirect, interspecific interactions are labeled apparent competition. Two examples are discussed in which an observed pattern of habitat segregation was at first interpreted as evidence for direct competition, but later interpreted as apparent competition resulting from shared predation. In order to study the consequences of predator-mediated apparent competition in isolation from other complicating factors, a model community is analyzed in which there is no direct interspecific competition among the prey. An explicit necessary condition for prey species coexistence is derived for the case of one predator feeding on many prey species. This model community has several interesting properties: (1) Prey species with high relative values for a parameter $\text{r}\text{a}$ are “keystone” species in the community; (2) prey species can be excluded from the community by “diffuse” apparent competition; (3) large changes in the niche breadth of the predator need not correspond to large changes in predator density; (4) the prey trophic level as a whole is regulated by the predator, yet each of its constituent species is regulated by both the predator and available resources; (5) increased productivity may either increase, decrease, or leave unchanged the number of species in the community; (6) a decrease in density-independent mortality may decrease species diversity. These conclusions seem to be robust to changes in the prey growth equations and to the incorporation of predator satiation. By contrast, adding prey refugia or predator switching to the model weakens these conclusions. If the predator can be satiated or switched, the elements a_ij comprising the community matrix may have signs opposite the long-term effect of j upon i. The effect of natural selection upon prey species coexistence is discussed. Unless r_i, K_i, and a_i are tightly coupled, natural selection within prey species i will tend to decrease the equilibrial abundance of species j.     

Predatory behavior and preference of a successful invader, the mud crab Dyspanopeus sayi (Panopeidae), on its bivalve prey

Predator-prey relationships between the panopeid crab, Dyspanopeus sayi, and the mytilid, Musculista senhousia, were investigated. Through laboratory experiments, prey-handling behavior, prey size selection, predator foraging behavior and preferences for two types of prey (M. senhousia and the Manila clam Ruditapes philippinarum) were assessed. Handling time differed significantly with respect to the three prey sizes offered (small: 15.0-20.0 mm shell length, SL; medium: 20.1-25.0 mm SL; and large: 25.1-30.0 mm SL); mud crabs were more efficient in predating medium-small than large prey. Although differences in prey profitability were not evident, D. sayi exhibited a marked reluctance to feed on larger-sized prey whilst smaller, more easily predated mussels were available. Size selection may be the result of a mechanical process in which encountered prey are attacked but rejected if they remain unbroken after a certain number of opening attempts. D. sayi exhibited inverse density-dependent foraging. A significant higher mortality of prey was evident at low prey density. Thus, at low predator density, the D. sayi-M. senhousia interaction was a destabilizing type II functional response. Interference responses affected the magnitude of predation intensity by D. sayi on M. senhousia, since as the density of foraging crabs increased, their foraging success fell. At high density (4 crabs tank⁻¹), crabs engaged in a high amount of agonistic activity when encountering a conspecific specimen, greatly diminished prey mortality. Finally, presenting two types of prey, Manila clam juveniles were poorly predated by mud crabs, which focused their predation mostly on M. senhousia. It is hypothesized that, when more accessible prey is available, mud crabs will have a minimal predatory impact on commercial R. philippinarum juvenile stocks.     

Effects of substrate on interactions between juvenile sea scallops (Placopecten magellanicus Gmelin) and predatory sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say)

We investigated the effect of substrate (glass bottom, sand, granule, pebble) on predation of juvenile sea scallops (Placopecten magellanicus) by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) at two prey sizes (11-15 mm and 24-28 mm shell height), and two prey densities (10 and 30 scallops per aquarium) in laboratory experiments. Specifically, we quantified predation rate and underlying behaviours (proportion of time a predator spent searching for and handling prey, encounter rate between predators and prey, and various outcomes of encounters). We detected a significant gradual effect of particle size of natural substrates on sea star predation: specifically, predation rate on and encounter rate with small scallops tended to decrease with increasing particle size (being highest for sand, intermediate for granule, and lowest for pebble). Substrate type did not significantly affect predation rates or behaviours of sea stars preying on large scallops or of rock crabs preying on either scallop size classes. Other factors, such as prey size and density, were important in the scallop-sea star and scallop-rock crab systems. For example, predation rate by sea stars and crabs and certain sea star behaviours (e.g. probability of consuming scallops upon capture) were significantly higher with small scallops than with large scallops. As well, in interactions between small scallops and sea stars, predation rate and encounter rate increased with prey density, and the proportion of time sea stars spent searching was higher at low prey density than high prey density. Thus, substrate type may be a minor factor determining predation risk of seeded scallops during enhancement operations; prey size and prey density may play a more important role. However, substrate type still needs to be considered when choosing a site for scallop enhancement, as it may affect other scallop behaviours (such as movement).     

Type and nature of cues used by Nucella lapillus to evaluate predation risk

The ability of prey to detect and adequately respond to predation risk influences immediate survival and overall fitness. Chemical cues are commonly used by prey to evaluate risk, and the purpose of this study was to elicit the nature of cues used by prey hunted by generalist predators. Nucella lapillus are common, predatory, intertidal snails that evaluate predatory risk using chemical cues. Using Nucella and a suite of its potential predators as a model system, we explored how (1) predator type, (2) predator diet, and (3) injured conspecifics and heterospecifics influence Nucella behavior. Using laboratory flumes, we determined that Nucella responded only to the invasive green crab (Carcinus maenas), the predator it most frequently encounters. Nucella did not respond to rock crabs (Cancer irroratus) or Jonah crabs (Cancer borealis), which are sympatric predators but do not frequently encounter Nucella because these crabs are primarily subtidal. Predator diet did not affect Nucella responses to risk, although starved predator response was not significantly different from controls. Since green crabs are generalist predators, diet cues do not reflect predation risk, and thus altering behavior as a function of predator diet would not likely benefit Nucella. Nucella did, however, react to injured conspecifics, a strategy that may allow them to recognize threats when predators are difficult to detect. Nucella did not react to injured heterospecifics including mussels (Mytilus edulis) and herbivorous snails Littorina littorea, suggesting that they are responding to chemical cues unique to their species. The nature of cues used by Nucella allows them to minimize costs associated with predator avoidance.     

Grouped sequential exploitation of food patches in a flock feeder,the feral pigeon

Feral and laboratory flocks of rock doves ($\text{Columba}$$\text{livia}$) show a pattern of grouped sequential exploitation when simultaneously presented with two dispersed, depleting patches of seed. This behavior contrasts with the ideal free distribution pattern shown when patches are small and concentrated. Grouped sequential exploitation consists of two phases: all pigeons first land together and feed at one patch, then leave one by one for the other patch. Departure times of individuals for the second patch are correlated with feeding rate at patch 1, which is in turn correlated with position in the dominance hierarchy. The decision to switch from patch 1 to patch 2 improves individual feeding rates in all cases, but is done slightly later than it should according to optimal foraging theory.     

Restricting Prey Dispersal Can Overestimate the Importance of Predation in Trophic Cascades

Predators can affect prey populations and, via trophic cascades, predators can indirectly impact resource populations (2 trophic levels below the predator) through consumption of prey (density-mediated indirect effects; DMIEs) and by inducing predator-avoidance behavior in prey (trait-mediated indirect effects; TMIEs). Prey often employ multiple predator-avoidance behaviors, such as dispersal or reduced foraging activity, but estimates of TMIEs are usually on individual behaviors. We assessed direct and indirect predator effects in a mesocosm experiment using a marine food chain consisting of a predator (toadfish – Opsanus tau), prey (mud crab - Panopeus herbstii) and resource (ribbed mussel – Geukensia demissa). We measured dispersal and foraging activity of prey separately by manipulating both the presence and absence of the predator, and whether prey could or could not disperse into a predator-free area. Consumption of prey was 9 times greater when prey could not disperse, probably because mesocosm boundaries increased predator capture success. Although predator presence did not significantly affect the number of crabs that emigrated, the presence of a predator decreased resource consumption by prey, which resulted in fewer resources consumed for each prey that emigrated in the presence of a predator, and reduced the overall TMIE. When prey were unable to disperse, TMIEs on mussel survival were 3 times higher than the DMIEs. When prey were allowed to disperse, the TMIEs on resource survival increased to 11-times the DMIEs. We found that restricting the ability of prey to disperse, or focusing on only one predator-avoidance behavior, may be underestimating TMIEs. Our results indicate that the relative contribution of behavior and consumption in food chain dynamics will depend on which predator-avoidance behaviors are allowed to occur and measured.     

Oxidative phosphorylation by membrane vesicles from Bacillus alcalophilus

ADP and P_i-loaded membrane vesicles from l-malate-grown Bacillus alcalophilus synthesized ATP upon energization with $\text{ascorbate}\text{N,N,N′,N′-}\text{tetramethyl}\text{-p-}\text{phenylenediamine}$. ATP synthesis occurred over a range of external pH from 6.0 to 11.0, under conditions in which the total protonmotive force was as low as ?30 mV. The phosphate potentials (ΔG_p) were calculated to be 11 and 12 kcal/mol at pH 10.5 and 9.0, respectively, whereas the values in vesicles at these two pH values were quite different (?40 ± 20 mV at pH 10.5 and ?125 ± 20 mV at pH 9.0). ATP synthesis was inhibited by KCN, gramicidin, and by N,N′-dicyclohexylcarbodiimide. Inward translocation of protons, concomitant with ATP synthesis, was demonstrated using direct pH monitoring and fluorescence methods. No dependence upon the presence of Na⁺ or K⁺ was found. Thus, ATP synthesis in B. alcalophilus appears to involve a proton-translocating ATPase which functions at low .     

Prey selection and the functional response of sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say) preying on juvenile sea scallops (Placopecten magellanicus (Gmelin)) and blue mussels (Mytilus edulis Linnaeus)

Predators in nature include an array of prey types in their diet, and often select certain types over others. We examined (i) prey selection by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) when offered two prey types, juvenile sea scallops (Placopecten magellanicus) and blue mussels (Mytilus edulis), and (ii) the effect of prey density on predation, prey selection, and component behaviours. We quantified predation rates, behavioural components (proportion of time spent searching for prey, encounter probabilities) and various prey characteristics (shell strength, energy content per prey, handling time per prey) to identify mechanisms underlying predation patterns and to assess the contribution of active and passive prey selection to observed selection of prey. Sea stars strongly selected mussels over scallops, resulting from both active and passive selection. Active selection was associated with the probability of attack upon encounter; it was higher on mussels than on scallops. The probability of capture upon attack, associated with passive selection, was higher for mussels than for scallops, since mussels can not swim to escape predators. Sea stars consumed few scallops when mussels were present, and so did not have a functional response on scallops (the target prey). Rock crabs exhibited prey switching: they selected mussels when scallop density was very low, did not select a certain prey type when scallop density was intermediate, and selected scallops when scallop density was high relative to mussel density. The interplay between encounter rate (associated with passive selection) and probability of consumption upon capture (associated with both active and passive selection) explained observed selection by crabs. Scallops were encountered by crabs relatively more often and/or mussels less often than expected from random movements of animals at all scallop densities. However, the probability of consumption varied with scallop density: it was lower for scallops than mussels at low and intermediate scallop densities, but tended to be higher for scallops than mussels at high scallop densities. When mussels were absent, crabs did not have a functional response on scallops, but rather were at the plateau of the response. When mussels were present with scallops at relatively low density, crabs exhibited a type II functional response on scallops. Our results have implications for the provision of protective refuges for species of interest (i.e., scallops) released onto the sea bed, such as in population enhancement operations and bottom aquaculture.     

The effect of size and temperature on the predation of cockles Cerastoderma edule (L.) by the shore crab Carcinus maenas (L.)

Laboratory studies demonstrated that shore crabs Carcinus maenas (L.) can consume <40 cockles Cerastoderma (= Cardium) edule (L.) per individual · day⁻¹. Various predation techniques used by the crabs are reported. The time required to open and consume individual cockles increased exponentially with prey size. Small (<l5-mm shell height), easily broken cockles appeared to be the most profitable in terms of energy acquisition per unit of handling time, the optimal size of prey increasing with predator size. With unlimited prey available, however, crabs selected prey of mean size smaller than these predicted optima, and much below the maximum size they were capable of opening. Feeding rates, both in terms of cockles ingested or energy intake per day, rose steeply with increasing temperature, but the size range of prey consumed remained unchanged. These data strongly suggest that Carcinus maenas is a potentially important predator of small cockles, particularly during the wanner summer months.     

Structure of the capsular polysaccharide (K6-antigen) from Escherichia coli LP 1092

The linkage pattern of the K6-antigen was investigated using material from the urinary pathogen, $\text{Escherichia coli}$ LP 1092. The polysaccharide consists of ribose and 3-deoxy-$\text{D}\text{-}\text{manno}$-2-octulosonate (KDO) in a ratio of 2:1. Colorimetric procedures, Smith degradation, methylation analysis, and nuclear magnetic resonance spectroscopy were applied to the whole polysaccharide and to a trisaccharide “repeating unit” obtained by mild-acid catalyzed hydrolysis. Together, the data are compatible only with a branched chain structure …$\text{3Rib}\text{f}\text{β1→7KDO}\text{p}\text{β2→3Rib}\text{f}\text{β}$…

     

Preference Alters Consumptive Effects of Predators: Top-Down Effects of a Native Crab on a System of Native and Introduced Prey

Top-down effects of predators in systems depend on the rate at which predators consume prey, and on predator preferences among available prey. In invaded communities, these parameters might be difficult to predict because ecological relationships are typically evolutionarily novel. We examined feeding rates and preferences of a crab native to the Pacific Northwest, Cancer productus, among four prey items: two invasive species of oyster drill (the marine whelks Urosalpinx cinerea and Ocenebra inornata) and two species of oyster (Crassostrea gigas and Ostrea lurida) that are also consumed by U. cinerea and O. inornata. This system is also characterized by intraguild predation because crabs are predators of drills and compete with them for prey (oysters). When only the oysters were offered, crabs did not express a preference and consumed approximately 9 juvenile oysters crab⁻¹ day⁻¹. We then tested whether crabs preferred adult drills of either U. cinerea or O. inornata, or juvenile oysters (C. gigas). While crabs consumed drills and oysters at approximately the same rate when only one type of prey was offered, they expressed a strong preference for juvenile oysters over drills when they were allowed to choose among the three prey items. This preference for oysters might negate the positive indirect effects that crabs have on oysters by crabs consuming drills (trophic cascade) because crabs have a large negative direct effect on oysters when crabs, oysters, and drills co-occur.     

Hydrodynamic mediation of predator-prey interactions: differential patterns of prey susceptibility and predator success explained by variation in water flow

In most shallow water marine systems, fluid movements vary on scales that may influence local community dynamics both directly, through changes in the abundance of species, and indirectly, by modifying important behaviors of organisms. We examined how differences in current speed affect the outcome of predator-prey interactions for two species of marine benthic predators (knobbed whelks, Busycon carica, and blue crabs, Callinectes sapidus) foraging on two common prey species (bay scallops, Argopecten irradians, and hard clams, Mercenaria mercenaria). The predators differ in their foraging strategies and prey in their potential escape responses. Predation by blue crabs, highly mobile predators/scavengers that rely upon chemical odors transported in the water column to locate prey, could be strongly affected by changes in current speed and turbulent mixing because their foraging strategy relies on a high degree of spatial integration of prey odor plumes. Whelks, slow moving, predatory gastropods that often forage with their bodies buried in the sediment, may be less susceptible to flow-induced distortion of prey odor plumes because their sluggish movements result in a high degree of temporal integration of prey odors. Bay scallops, relatively mobile bivalves capable of rapid short-distance swimming burst, and hard clams, sedentary bivalves, have been shown to respond to varying degrees to predator odors that are dispersed in the water column. Flow regime for the predator-prey experiments was manipulated in situ using large channels. Predation by blue crabs on both juvenile hard clams and bay scallops decreased with increases in water flow (0-12 vs. 0-30 cm s⁻¹). Whelk predation on bay scallops increased with increases in water flow, whereas predation by whelks on hard clams did not differ between flow regimes. For blue crabs movement decreased at periods of high water flow. Because blue crabs locate prey through chemolocation of water-borne cues, which are diluted rapidly at higher flows, decreases in foraging may result from the inability to successfully detect prey at enhanced flows. Differences in predation by whelks could not be explained by a similar mechanism. Visual observations of foraging whelks revealed no differences in whelk behavior between the two flow regimes. The pattern of higher whelk predation on scallops at enhanced flow is likely to be related to a flow-inhibiting ability of scallops to detect predator approach. Thus, flow enhancement interferes with three of the predator-prey systems but the effect on predator success depends on whether the predator or prey is most affected.     

Predator size interacts with habitat structure to determine the allometric scaling of the functional response

While both predator body size and prey refuge provided by habitat structure have been established as major factors influencing the functional response (per capita consumption rate as a function of prey density), potential interactions between these factors have rarely been explored. Using a crab predator (Panopeus herbstii) – mussel prey (Brachidontes exustus) system, we examined the allometric scaling of the functional response in oyster (Crassostrea virginica) reef habitat, where crevices within oyster clusters provide mussels refuge from predation. A field survey of mussel distribution showed that mussels attach closer to the cluster periphery at high mussel density, indicating the potential for saturation of the refuge. In functional response experiments, the consumption rate of large crabs was depressed at low prey density relative to small crabs, while at high prey density the reverse was true. Specifically, the attack rate coefficient and handling time both decreased non‐linearly with crab size. An additional manipulation revealed that at low prey densities, the ability of large crabs to maneuver their claws and bodies to extract mussels from crevices was inhibited relative to small crabs by the structured habitat, reducing their attack rate. At high prey densities, crevices were saturated, forcing mussels to the edge of clusters where crabs were only limited by handling time. Our study illuminates a potentially general mechanism where the quality of the prey refuge provided by habitat structure is dependent on the relative size of the predator. Thus anthropogenic influences that alter the natural crab size distribution or degrade reef habitat structure could threaten the long‐term stability of the crab –mussel interaction in reefs.     

A reaction between the superoxide free radical and lipid hydroperoxide in sodium linoleate micelles

The oxidation of unsaturated fatty acid micelles by the superoxide free radical ($\text{O}{}^{\mathrm{?}}{}_2$), during γ irradiation in the presence of formate, is kinetically distinct from oxidation by hydroxyl free radicals (HO.). The evidence suggests that a direct reaction between ($\text{O}{}^{\mathrm{?}}{}_2$) and lipid hydroperoxide initiates a chain oxidation process in the micelles. While tetranitromethane, which reacts rapidly with ($\text{O}{}^{\mathrm{?}}{}_2$), protects the micelles from oxidation, active superoxide dismutase is no more effective than its apoprotein, due to lack of penetration of the micellar environment. We discuss these findings in the light of recent literature, and with reference to their possible significance for biological systems.     

A rapid procedure to detect in situ DNA/RNA hybrids

We developed a new method for detecting DNA/RNA hybrids formed $\text{in}\text{situ}$ using $\text{anti-}\text{DNA}\text{RNA}$ antibodies and the Peroxidase-antiperoxidase immunohistochemical procedure. Using RNA synthesised $\text{in}\text{vitro}$ from cloned $\text{Drosophila}$ histone genes (pDm 500H), we localized by this procedure, the histone genes to the 39 D-E region of the left arm of the second chromosome. This method has several advantages compared to conventional procedures.     

Habitat context influences predator interference interactions and the strength of resource partitioning

Despite increasing evidence that habitat structure can shape predator–prey interactions, few studies have examined the impact of habitat context on interactions among multiple predators and the consequences for combined foraging rates. We investigated the individual and combined effects of stone crabs (Menippe mercenaria) and knobbed whelks (Busycon carica) when foraging on two common bivalves, the hard clam (Mercenaria mercenaria) and the ribbed mussel (Geukensia demissa) in oyster reef and sand flat habitats. Because these species co-occur across these and other estuarine habitats of varying physical complexity, this system is ideal for examining how habitat context influences foraging rates and the generality of predator interactions. Consistent with results from previous studies, consumption rates of each predator in isolation from the other were higher in the sand flat than in the more structurally complex oyster reef habitat. However, consumption by the two predators when combined surprisingly did not differ between the two habitats. This counterintuitive result probably stems from the influence of habitat structure on predator–predator interactions. In the sand-flat habitat, whelks significantly reduced their consumption of their less preferred prey when crabs were present. However, the structurally more complex oyster reef habitat appeared to reduce interference interactions among predators, such that consumption rates when the predators co-occurred did not differ from predation rates when alone. In addition, both habitat context and predator–predator interactions increased resource partitioning by strengthening predator dietary selectivity. Thus, an understanding of how habitat characteristics such as physical complexity influence interactions among predators may be critical to predicting the effects of modifying predator populations on their shared prey.     

Immunoassay for honey bee cytochrome c in single animals with cytochrome c-coated bacteriophages: A sensitive tool for the study of caste formation in the honey bee, Apis Mellifera

The development of a sensitive viroimmunoassay for honey bee cytochrome $\text{c}$ and its usage for early detection of caste differentiation is described. Pure honey bee cytochrome $\text{c}$ was isolated from workers and used to produce antibodies in rabbits. Bacteriophage T4 was chemically modified by covalent attachment of honey bee cytochrome $\text{c}$ using tolylene-2,4-diisocyanate as a cross-linking agent. The immunospecific inactivation of this bacteriophage-cytochrome c conjugate by anti-cytochrome $\text{c}$ antibodies can be inhibited by free cytochrome $\text{c}$. In quantitative determinations, 50% inhibition is reproducibly achieved at a concentration of 6 ng/ml (5 pmol/ml) and as little as 0.3 ng/ml (0.25 pmol/ml) could be detected by this system. Cytochrome $\text{c}$ concentrations were measured in individual animals and substantial differences between corresponding larval stages of worker and queen bees are reported.