Metabolism of epipelagic tropical ctenophores

Measurements of respiration and excretion at 25°C were made for five species of ctenophores collected during five cruises to the Bahamas (1982–1984). The mean element-specific respiration and ammonium excretion rates of freshly collected specimens of all species ranged from 4 to 16% d^-1, the mean atomic O:N ratios were 10 to 16, and ammonium averaged 60 to 90% of the total dissolved nitrogen excreted. For adult ctenophores, the carbon content ranged from 0.6% carbon (as percent of dry weight) for Bolinopsis vitrea to 3.7% carbon for Beroë ovata. There was a marked increase in the organic content (% carbon of dry weight) of small Bolinopsis vitrea with tentacles compared to fully lobate adults. B. vitrea had increasingly higher metabolic rates when held at food concentrations up to 100 copepods 1^-1 (about 250 g C 1^-1). The overall range between starved and well-fed B. vitrea was about two times for respiration and a factor of three for ammonium excretion. B. vitrea decreased from well-fed to a starved metabolic rate in about a day after removal from food. The metabolic rate of Eurhamphaea vexilligera was not measurably affected by short-term starvation or feeding (maximum 25 copepods 1^-1). In feeding experiments, E. vexilligera of 20 to 56 mm length fed at rates equivalent to clearance rates of 250 to 1 800 ml h^-1.     

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.     

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.     

Recent distribution and size structure of gelatinous organisms in the southern Black Sea and their interactions with fish catches

The spatial distributions of gelatinous organism were studied during three cruises in 2006–2007. These were the first such studies conducted in the southern Black Sea for last decade. Additionally, the different methods of estimating gelatinous organism biomass presently in use were compared and recent situation of pelagic fishery interacting with zooplankton in the Black Sea were overviewed. Biomasses and abundances of both invading Mnemiopsis leidyi, and the resident Aurelia aurita and Pleurobrachia pileus were remarkably reduced (total gelatinous biomass: 417–537 g m⁻² in May, June, and 150 g m⁻² in October) as compared with those before the 1997 with invasion Beroe ovata in the southern Black Sea. Specimens of B. ovata were detected only in October when individuals with lengths of 10–50 mm comprised 93% of its population. Length–weight based biomasses were significantly variable for all species depending on the size structure in time. Larger sized individuals (>30 mm) of M. leidyi appeared as compared with those before year 1997. The ecosystem of the Black Sea has been suppressed both bottom–up through decreased eutrophication and top–down through reduced grazing on mesozooplankton by M. leidyi that are in turn controlled by grazing by B. ovata. The catch of anchovy was abruptly reduced to 120,000 tons due presumably to the considerably increased catch of its predator, the bonito (64,000 tons) in the year 2005.     

Turning the game around: toxicity in a nudibranch-sponge predator–prey association

Escalation theory proposes enemy-related selection as the most relevant factor of natural selection among individual organisms. When hazardous to predators, prey might be considered enemies that influence predator evolution. Opisthobranch molluscs that prey on chemically defended prey are an interesting study case on this subject. Predation on chemically defended species paved the way for opisthobranchs to enter in an arms race, developing means to detoxify and/or excrete harmful compounds, which led to the sequestration of those compounds and their self-defensive use, an escalation of defenses. Here we aim to understand whether the opisthobranch predator is better protected than its chemically defended prey, using as predator–prey model, a nudibranch (Hypselodoris cantabrica) and the sponge it preys upon (Dysidea fragilis), and from which it obtains deterrent chemical compounds. Specimens of both species were collected on the Portuguese coast, and their crude extracts were analyzed and used in palatability tests. Nudibranchs revealed a higher natural concentration of crude extract, probably due to a progressive accumulation of the compounds. Both predator and prey extracts revealed similar mixtures of deterrent metabolites (furanosesquiterpenes). Palatability tests revealed a more effective deterrence in the nudibranch extracts because significant rejection rates were observed at lower concentrations than those necessary for the sponge extracts to have the same effect. We concluded that the predator is chemically better protected than its prey, which suggests that its acquisition of chemical defenses reveals a defensive escalation.     

Timid spider uses odor and visual cues to actively select protected nesting sites near ants

Associations in which a more vulnerable species gains protection by seeking out the company of a pugnacious “protector” species capable of deterring predators are documented among mixed-species groups from various taxa, but experimental studies are rare. We consider an unusual arthropod-based example in which the associate species, Phintella piantensis, is a jumping spider (Salticidae) that associates with the territorial weaver ant Oecophylla smaragdina, which in turn is a potential predator of Phintella. However, the predator we consider in this mixed-species association is Scytodes sp., a spitting spider (Scytodae) that often targets salticids as prey. Scytodes adopts a strategy of building its web over salticid nests and then preying on resident salticids when they leave or return to their nests. Our experiments show that, on the basis of olfactory cues, Scytodes is deterred from the vicinity of O. smaragdina. Phintella builds dense ant-proof nests to minimize the risk of being killed by Oecophylla, and we show that olfactory as well as visual cues of ants elicit nest building by Phintella. We propose that Phintella actively chooses to situate nests in the vicinity of weaver ants as defense against a specific ant-averse predator that singles out salticids as preferred prey.     

Tetrodotoxin affects survival probability of rough-skinned newts (Taricha granulosa) faced with TTX-resistant garter snake predators (Thamnophis sirtalis)

Lethal chemical defenses in prey species can have profound effects on interactions with predators. The presence of lethal defenses in prey can correct the selective imbalance suggested by the life-dinner principle in which the fitness consequences of an encounter between predator and prey should be much greater for the prey species than the predator. Despite the apparent adaptive advantages of lethality the evolution of deadly prey presents a fundamental dilemma. How might lethal defenses confer an individual fitness advantage if both predators and prey die during interactions? We examined the interaction between the rough-skinned newt (Taricha granulosa), which contains a powerful neurotoxin called tetrodotoxin (TTX), and the common garter snake (Thamnophis sirtalis). In some sympatric populations, Th. sirtalis have evolved physiological resistance to TTX. Whether the newts’ toxin confers protection from snake predators or has been disarmed by the snakes’ physiological resistance has not yet been directly tested. In predator–prey trials, newts that were rejected by snakes had greater concentrations of TTX in their skin (4.52 ± 3.49 mg TTX/g skin) than those that were eaten (1.72 ± 1.53 mg TTX/g skin). Despite the plethora of taxa that appear to use TTX defensively, this is the first direct and quantitative demonstration of the antipredator efficacy of TTX. Because the survival probability of a newt (and thus fitness) is affected by individual TTX concentration, selection can drive the escalation of toxin levels in newts. The variable fitness consequences associated with both TTX levels of newts and resistance to TTX in snakes that may promote a strong and symmetrical coevolutionary relationship have now been demonstrated.     

Population development of the invader ctenophore Mnemiopsis leidyi, in the Black Sea and in other seas of the Mediterranean basin

In the last two decades of the twentieth century, the ctenophore Mnemiopsis leidyi (A. Agassiz) has invaded the Black, Azov, Marmara and Aegean Seas, and, recently, the Caspian Sea. Here, we compare its spatial and temporal distribution, seasonal dynamics and the time and duration of reproduction. We also discuss factors that control its abundance throughout its invasive range and its effect on ecosystems. Observations are based on the long-term field data collected by three research institutes. An analysis of the effects of temperature, salinity, prey (zoo- and ichthyoplankton) availability and predation (by ctenophores of the genus Beroe) on M. leidyi population size, and the effects of M. leidyi on zoo- and ichthyoplankton, and on fish populations in the Black and Azov Seas is also provided. With the Black Sea current, M. leidyi spreads to the upper layers of the Sea of Marmara, where it now occurs around the year. At regular intervals, the Black Sea current also takes it to the northern Aegean Sea. In contrast, it has to re-invade the Sea of Azov every spring or summer, dying out during winter when the temperature drops below 4°C. The warm summer and mild winter temperatures, relatively low salinity and abundance of prey in the Black Sea are close to optimal for M. leidyi, while they are suboptimal in the northern Aegean Sea, where salinity and temperature are often too high. In the Black Sea the absence of gelatinous and other predators led to an enormous ctenophore abundance for a decade, but with the appearance of Beroe ovata in 1999, M. leidyi abundance greatly decreased. Analysis of seasonal dynamics of M. leidyi in the Black Sea and in other seas of the Mediterranean basin indicates similarities in the timing of maximum abundance and biomass, in spite of some differences in the initiation and duration of reproduction. A peak biomass and density occurred in 1989 in the Black and Azov Seas and in 1990 in the other seas. The M. leidyi invasion negatively affected the ecosystems of the Black Sea and the Sea of Azov. The zooplankton, ichthyoplankton and zooplanktivorous fish stocks all underwent profound changes. Similar effects, but less pronounced, were recorded in the Sea of Marmara. Effects on Mediterranean food chains have, so far, remained insignificant. Salinity is probably supraoptimal here, and several predators prevent M. leidyi from reaching outbreak levels.     

Predation-affected spatial pattern changes in a prey population

Predation-affected spatial pattern changes in a prey population were studied. The spatial pattern of a population of Galleria mellonella is changed by its predator, Podisus maculiventris. The pattern is affected by the frequency of attack by the predator (attack ability), the homogeneity of the attack ability within a predator population and the mobility of the predator. A mathematical model incorporating these three factors was constructed, and several computer simulations were conducted by changing the parameter values. A natural enemy population having high and homogeneous attack ability and high mobility effectively kills prey individuals.     

Antipredator responses to invasive lionfish, Pterois volitans: interspecific differences in cue utilization by two coral reef gobies

Understanding prey response to predators and their utilization of sensory cues to assess local predation risk is crucial in determining how predator avoidance strategies affect population demographics. This study examined the antipredator behaviors of two ecologically similar species of Caribbean coral reef fish, Coryphopterus glaucofraenum and Gnatholepis thompsoni, and characterized their responses to different reef predators. In laboratory assays, the two species of gobies were exposed to predator visual cues (native Nassau grouper predator vs. invasive lionfish predator), damage-released chemical cues from gobies, and combinations of these, along with appropriate controls. Behavioral responses indicate that the two prey species differ in their utilization of visual and chemical cues. Visual cues from predators were decisive for both species’ responses, demonstrating their relative importance in the sensory hierarchy, whereas damage-released cues were a source of information only for C. glaucofraenum. Both prey species could distinguish between native and invasive predators and subsequently altered their antipredator responses.     

Effects of prey concentration,prey size,predator life stage,predator starvation,and season on predation rates of the carnivorous copepod Euchaeta elongata

Adult females of the large carnivorous copepod Euchaeta elongata Esterly were collected from 1977 to 1980 in Port Susan, Washington, USA. Predation rates of the adult females increased with increasing prey abundance when fed the following 4 sizes of copepods: adult females of Calanus pacificus (average prosome length [PL] of 2 650 μm), adults of Aetideus divergens (PL of 1 560 μm), adult females of Pseudocalanus spp. (PL of 1 060 μm), and nauplii of C. pacificus (PL of 410 μm). Saturation feeding levels were reached when adult females of the predator were fed the small adult copepod, Pseudocalanus spp. Maximum biomass ingested of this small copepod was more than the maximum amount ingested of the larger copepods. Predation rates of the predatory copepodids at Stages IV and V also increased with increasing concentration of the 1 060 μm (PL) prey. High feeding rates exhibited by both adults and copepodids at Stage V of the predator indicate their importance as sources of mortality on populations of small copepods. Ingestion efficiency E _i (prey wholly consumed [prey attacked]^-1) varied as follows: adults of E. elongata were more efficient than copepodids of E. elongata; adults were more efficient than copepodids when ingesting smaller prey; starved adults were more efficient than fed ones; and both adults and copepodids were more efficient at low food concentrations. For adults of E. elongata, there were no marked seasonal variations in predation or respiratory rates that would represent acclimatory responses; however, small adults obtained during winter were more efficient at ingesting prey than were the larger adults gathered in summer. This seasonal variation in the efficiency of ingestion may be a useful indicator of physiological state: high E _i values could indicate that predators are starving in winter, and low E _i values could indicate that predators are satiated in summer.     

Population and seasonal variation in response to prey calls by an eavesdropping bat

The fringe-lipped bat, Trachops cirrhosus, is an eavesdropping predator that hunts frogs and katydids by approaching these preys' sexual advertisement calls. In captivity, bats can rapidly learn to associate novel acoustic stimuli with food rewards. It is unknown how this learning ability is related to foraging behavior in the wild where prey and the calls that identify them vary over space and time. In two bat populations that differ in available prey species (Soberanía, Panama, and La Selva, Costa Rica), we presented wild-caught bats with frog calls, katydid calls, and control stimuli. Bats in Soberanía were significantly more responsive to complex calls and choruses of the túngara frog, Physalaemus pustulosus, than were bats in La Selva. La Selva bats were significantly more responsive to katydid calls (Steirodon sp.) than Soberanía bats. We also examined seasonal variation in bat response to prey cues. Bats were captured in Soberanía in dry and wet seasons and presented with the calls of a dry season breeding frog (Smilisca sila), a wet season breeding frog (P. pustulosus), and four katydid species. Bats captured in the dry season were significantly more responsive to the calls of S. sila than bats captured in the wet season, but there were no seasonal differences in response to the calls of P. pustulosus or the katydid calls. We demonstrate plasticity in the foraging behavior of this eavesdropping predator but also show that response to prey cues is not predicted solely by prey availability.     

The influence of initial age structure on predator–prey interaction

Initial age structure influences the growth of a prey population and the outcome of the predator–prey interaction. In order to quantify that influence, we employed a simple numerical model using experimental data from the system Tetranychus urticae–Phytoseiulus persimilis. Four major points were drawn from the results: (1) A population created by young females grows much faster than a population created by the same number of females but distributed among the stable age structure. Final number of individuals after a few generations is then much higher than what a plant could support. Consequently, a stable age structure is probably never achieved under these conditions; (2) In the presence of a predator, such a population can persist for a sufficiently long time to overexploit its host plant and to produce enough individuals to allow dispersal; (3) The impact of the predator on the prey population is drastically different according to its own age structure at the beginning of the interaction; and (4) Predators disturb the prey age structure during the course of interactions and thus maintain the prey growth potential at a high level. These points constitute an important adaptation that determine the persistence of the prey and the predator at a metapopulation level. They bring a new insight on the adaptive characters of young female dispersal.     

Evaluating the impact of caribou habitat restoration on predator and prey movement

Fragmentation of the boreal forest by linear features, including seismic lines, has destabilized predator–prey dynamics, resulting in the decline of woodland caribou (Rangifer tarandus caribou) populations. Restoration of human-altered habitat has therefore been identified as a critical management tool for achieving self-sustaining woodland caribou populations. However, only recently has testing of the response of caribou and other wildlife to restoration activities been conducted. Early work has centered around assessing changes in wildlife use of restored seismic lines. We evaluated whether restoration reduces the movement rates of predators and their associated prey, which is expected to decrease predator hunting efficiency and ultimately reduce caribou mortality. We developed a new method for using cameras to measure fine-scale movement by measuring speed as animals traveled between cameras in an array. We used our method to quantify speed of caribou, moose (Alces alces), bears (Ursus americanus), and wolves (Canis lupus) on treated (restored) and untreated seismic lines. Restoration treatments reduced travel speeds along seismic lines of wolves by 1.38 km/h, bears by 0.55 km/h, and caribou by 1.57 km/h, but did not reduce moose travel speeds. Reduced predator and caribou speeds on treated seismic lines are predicted to decrease encounter rates between predators and caribou and thus lower caribou kill rates. However, further work is needed to determine whether reduced movement rates result in reduced encounter rates with prey, and ultimately reduced caribou mortality.     

Cannibalism in an age-structured predator–prey system

The effect of cannibalism on an age-structured predator–prey system is studied. Three stable equilibrium states are found. Using a Hopf bifurcation analysis, it is found that the non washout steady state looses its stability as the cannibalism attack rate increases past a bifurcation point S_c. The dependence of the bifurcation point on the other parameters in the model is found. It is shown that the trajectory of the solution spirals in for attack rates S<S_c and exhibits limit cycle behavior for S>S_c.     

Prey defense,predator preference,and nonrandom diet: The interactions between Pycnopodia helianthoides and two species of sea urchins

Interactions between the predatory sea star Pycnopodia helianthoides (Brandt, 1835) and two of its natural prey, the sea urchins Strongylocentrotus purpuratus (Stimpson, 1857) and S. franciscanus (Agassiz, 1863), are examined with regard to predator preference, predator diet, and prey defenses. The sea star is able to detect both species of sea urchin upstream in a Y-trough, but does not consistently choose one over the other (i.e., no preference). However, when the sea star is presented with equal numbers of similar-sized specimens of the two species of sea urchin, its diet is markedly nonrandom, since S. purpuratus is eaten almost 98% of the time. The defensive responses of the two species of sea urchin differ in form and effectiveness. S. franciscanus employs its long spines as defensive weapons, pinching the rays of an attacking sea star. This defensive response is more effective than the pedicellarial response used by S. purpuratus. The nonrandom diet of the predator seems to result primarily from prey defensive responses that differ in effectiveness, rather than from an intrinsic, behavioral preference of the predator at an earlier stage in the predator/prey interaction.     

Predicting functional response and size selectivity of juvenile Notonecta maculata foraging on Daphnia magna

A mechanistic model was developed to assess the impact of predation of juvenile Notonecta maculata on size structured Daphnia magna populations and to provide a framework for quantifying the backswimmers uptake of food. Results of experiments and model predictions clearly demonstrate selective predation of backswimmers when fed with a choice of daphnid size classes, with patterns of selectivity differing across N. maculata instars. The model describes the foraging process empirically on the base of a general predation cycle including four conditional events instead of using classic functional response curves. For model parameterisation components of predation, namely probability of encounter, attack and success as well as time spent on handling prey was directly observed by means of video tracking experiments. Since attack rate, capture success and handling time appeared to be a function of prey size differing between Notonecta instars, the model takes into account ontogenic changes in both predator and prey characteristics. Independent data of functional response and size selectivity experiments were used for model validation and proved the model outcome to be consistent with observations.     

The influence of intraguild predation on prey suppression and prey release: a meta-analysis

Intraguild predation (IGP) occurs when one predator species consumes another predator species with whom it also competes for shared prey. One question of interest to ecologists is whether multiple predator species suppress prey populations more than a single predator species, and whether this result varies with the presence of IGP. We conducted a meta-analysis to examine this question, and others, regarding the effects of IGP on prey suppression. When predators can potentially consume one another (mutual IGP), prey suppression is greater in the presence of one predator species than in the presence of multiple predator species; however, this result was not found for assemblages with unidirectional or no IGP. With unidirectional IGP, intermediate predators were generally more effective than the top predator at suppressing the shared prey, in agreement with IGP theory. Adding a top predator to an assemblage generally caused prey to be released from predation, while adding an intermediate predator caused prey populations to be suppressed. However, the effects of adding a top or intermediate predator depended on the effectiveness of these predators when they were alone. Effects of IGP varied across different ecosystems (e.g., lentic, lotic, marine, terrestrial invertebrate, and terrestrial vertebrate), with the strongest patterns being driven by terrestrial invertebrates. Finally, although IGP theory is based on equilibrium conditions, data from short-term experiments can inform us about systems that are dominated by transient dynamics. Moreover, short-term experiments may be connected in some way to equilibrium models if the predator and prey densities used in experiments approximate the equilibrium densities in nature.     

Predators of juvenile sea urchins and the effect of habitat refuges

We evaluated the effects of potential predators from intertidal habitats on Strongylocentrotus purpuratus survival using laboratory experiments and assessed abundances of main predatory species along the Pacific coast of North America. The interactive effects of urchins’ and predators’ sizes in mediating predation were quantified. Habitat complexity (substrate pits, adult spine canopy) was manipulated to examine its effects on predation of most susceptible individuals (<14 mm). Pachygrapsus crassipes was identified as a major predator of urchins up to ≈30 mm. A positive effect of predator size on consumption of progressively larger urchins was detected, probably due to a mechanical limitation on crabs’ ability to consume large prey. Larger claws of males with respect to females of comparable sizes facilitated the handling of larger prey. Substrate refuges significantly reduced mortality on juvenile urchins. These results show that crab predation may be important in organizing intertidal communities, despite multiple ecological mechanisms promoting sea urchin survival.