Recent records of the Nile Soft-shelled Turtle,Tríonyx triunguis,and of the Euphrates Soft-shelled Turtle,Tríonyx euphratìcus,in the Middle East

Abstract

Thriving populations of the Spiny Oyster Spondylus spinosus Schreibers, 1793, an Indo-Pacific bivalve, are reported from off the Mediterranean coast of Israel. The taxonomy and nomenclature of this new Lessepsian migrant are briefly discussed. The polymorphism of the Mediterranean specimens is discussed. Their habitat is described, and the epibionts and predators in the Mediterranean are enumerated.     

How does flow recruit epibionts onto brachiopod shells? Insights into reciprocal interactions within the symbiotic framework

Computational fluid dynamics simulations were performed to examine the passive recruitment of epibionts onto Devonian spiriferide brachiopod host shells. Because many planktonic larvae and spores are propulsion-inefficient swimmers, we determined the areas most prone to settlement in terms of inertial impaction and direct interception, which are characteristic of higher and lower pressure, respectively. Simulations on a unique specimen of Paraspirifer with a geopetal structure of broken brachidia suggest that the larva of Aulopora on the shell was transported and had settled onto the shell through inertial impaction after the host was dead and overturned on the sea floor. In the case of an ideal life posture, the spiriferide models received higher pressure on the shell surfaces at the forward and rearward stagnation areas and lower pressure along the shell margins and the anterior part of the sulcus, regardless of whether the ventral or the dorsal valve was facing upstream. Both sites seem to be available for epibionts by way of direct interception or inertial impaction. Our results indicate that the initial recruitment of most epibionts is accidental and passive, whereas the directions and patterns of epibiont growth suggest a biological response to ambient conditions.     

Innate threat-sensitive foraging: black-tailed deer remain more fearful of wolf than of the less dangerous black bear even after 100 years of wolf absence

Anti-predator behaviors often entail foraging costs, and thus prey response to predator cues should be adjusted to the level of risk (threat-sensitive foraging). Simultaneously dangerous predators (with high hunting success) should engender the evolution of innate predator recognition and appropriate anti-predator behaviors that are effective even upon the first encounter with the predator. The above leads to the prediction that prey might respond more strongly to cues of dangerous predators that are absent, than to cues of less dangerous predators that are actually present. In an applied context this would predict an immediate and stronger response of ungulates to the return of top predators such as wolves (Canis lupus) in many parts of Europe and North America than to current, less threatening, mesopredators. We investigated the existence of innate threat-sensitive foraging in black-tailed deer. We took advantage of a quasi-experimental situation where deer had not experienced wolf predation for ca. 100 years, and were only potentially exposed to black bears (Ursus americanus). We tested the response of deer to the urine of wolf (dangerous) and black bear (less dangerous). Our results support the hypothesis of innate threat-sensitive foraging with clear increased passive avoidance and olfactory investigation of cues from wolf, and surprisingly none to black bear. Prey which have previously evolved under high risk of predation by wolves may react strongly to the return of wolf cues in their environments thanks to innate responses retained during the period of predator absence, and this could be the source of far stronger non-consumptive effects of the predator guild than currently observed.     

Nature and role of newly described symbiotic associations between a sea anemone and gastropods at bathyal depths in the NW Atlantic

The hormathiid sea anemone Allantactis parasitica was found living as an epibiont on numerous species of gastropods at depths of 725-1100 m along the continental slope of eastern Canada. The proportion of bathyal gastropods hosting 1-6 sea anemones reached 72.5% in a single trawl. Although A. parasitica was occasionally found on other substrata (i.e. empty shells, pebbles), laboratory trials confirmed that they preferably associate with living gastropods. Settlement of planula larvae occurred significantly more often on the shells of live bathyal gastropods than on all other substrata present in the tanks. Juvenile sea anemones (∼ 1 mm diameter) readily moved from the mud or other inert substrata onto shells of burrowed bathyal gastropods. Conversely, larvae, juveniles and adults of A. parasitica never associated with any shallow-water gastropods when given the opportunity. Trials exposing predatory sea stars (Leptasterias polaris) from shallow and bathyal depths to bathyal gastropods (Buccinum undatum) with epibiotic A. parasitica, and to asymbiotic bathyal and shallow-water B. undatum, revealed adaptive behaviours in both prey and predator. Shallow-water gastropods (devoid of epibionts) reacted defensively to L. polaris, whereas bathyal gastropods relied mostly on their epibionts to protect them, thus falling prey to L. polaris when the epibionts were removed. L. polaris from bathyal depths typically ignored symbiotic gastropods, but they consistently preyed on asymbiotic ones, while L. polaris from shallow areas initially attempted to prey on all gastropods, but learned to avoid those harbouring sea anemones. Furthermore, living as epibionts afforded sea anemones a means to escape one of their own predators, the sea star Crossaster papposus. The mutualistic relationship between hormathiid sea anemones and bathyal gastropods from the NW Atlantic may have evolved in response to predation pressure.     

Optimal foraging versus shared doom effects: interactive influence of mussel size and epibiosis on predator preference

In the western Baltic Sea, the highly competitive blue mussel Mytilus edulis tends to monopolize shallow water hard substrata. In many habitats, mussel dominance is mainly controlled by the generalist predator Carcinus maenas. These predator-prey interactions seem to be affected by mussel size (relative to crab size) and mussel epibionts.There is a clear relationship between prey size and predator size as suggested by the optimal foraging theory: Each crab size class preferentially preys on a certain mussel size class. Preferred prey size increases with crab size.Epibionts on Mytilus, however, influence this simple pattern of feeding preferences by crabs. When offered similarly sized mussels, crabs prefer Balanus-fouled mussels over clean mussels. There is, however, a hierarchy of factors: the influence of attractive epibiotic barnacles is weaker than the factor ‘mussel size’. Testing small mussels against large mussels, presence or absence of epibiotic barnacles does not significantly alter preferences caused by mussel size. Balanus enhanced crab predation on mussels in two ways: Additional food gain and, probably more important, improvement in handling of the prey. The latter effect is illustrated by the fact that artificial barnacle mimics increased crab predation on mussels to the same extent as do live barnacles.We conclude that crab predation preferences follows the optimal foraging model when prey belong to different size classes, whereas within size classes crab preferences is controlled by epibionts.     

Trophic niche partitioning between two prey and their incidental predators revealed various threats for an endangered species

Documenting trophic niche partitioning and resource use within a community is critical to evaluate underlying mechanisms of coexistence, competition, or predation. Detailed knowledge about foraging is essential as it may influence the vital rates, which, in turn, can affect trophic relationships between species, and population dynamics. The aims of this study were to evaluate resource and trophic niche partitioning in summer/autumn between the endangered Atlantic‐Gaspésie caribou (Rangifer tarandus caribou) population, moose (Alces americanus) and their incidental predators, the black bear (Ursus americanus) and coyote (Canis latrans), and to quantify the extent to which these predators consumed caribou. Bayesian isotopic analysis showed a small overlap in trophic niche for the two sympatric ungulates suggesting a low potential for resource competition. Our results also revealed that caribou occupied a larger isotopic niche area than moose, suggesting a greater diversity of resources used by caribou. Not surprisingly, coyotes consumed mainly deer (Odocoileus virginianus), moose, snowshoe hare (Lepus americanus), and occasionally caribou, while bears consumed mainly vegetation and, to a lesser extent, moose and caribou. As coyotes and bears also feed on plant species, we documented trophic niche overlap between caribou and their predators, as searching for similar resources can force them to use the same habitats and thus increase the encounter rate and, ultimately, mortality risk for caribou. Although the decline in the Gaspésie caribou population is mostly driven by habitat‐mediated predation, we found evidence that the low level of resource competition with moose, added to the shared resources with incidental predators, mainly bears, may contribute to jeopardize the recovery of this endangered caribou population. Highlighting the trophic interaction between species is needed to establish efficient conservation and management strategies to insure the persistence of endangered populations. The comparison of trophic niches of species sharing the same habitat or resources is fundamental to evaluate the mechanisms of coexistence or competition and eventually predict the consequences of ecosystem changes in the community.     

Seasonal Effects of Habitat on Sources and Rates of Snowshoe Hare Predation in Alaskan Boreal Forests

Survival and predation of snowshoe hares (Lepus americanus) has been widely studied, yet there has been little quantification of the changes in vulnerability of hares to specific predators that may result from seasonal changes in vegetation and cover. We investigated survival and causes of mortalities of snowshoe hares during the late increase, peak, and decline of a population in interior Alaska. From June 2008 to May 2012, we radio-tagged 288 adult and older juvenile hares in early successional and black spruce (Picea mariana) forests and, using known-fate methods in program MARK, evaluated 85 survival models that included variables for sex, age, and body condition of hares, as well as trapping site, month, season, year, snowfall, snow depth, and air temperature. We compared the models using Akaike’s information criterion with correction for small sample size. Model results indicated that month, capture site, and body condition were the most important variables in explaining survival rates. Survival was highest in July, and more generally during summer, when alternative prey was available to predators of hares. Low survival rates coincided with molting periods, breeding activity in the spring, and the introduction of juveniles to the sample population in the fall. We identified predation as the cause of mortality in 86% of hare deaths. When the source of predation could be determined, hares were killed more often by goshawks (Accipiter gentilis) than other predators in early successional forest (30%), and more often by lynx (Lynx canadensis) than other predators in black spruce forest (31%). Great horned owls (Bubo virginianus) and coyotes (Canis latrans) represented smaller proportions of hare predation, and non-predatory causes were a minor source (3%) of mortality. Because hares rely on vegetative cover for concealment from predators, we measured cover in predation sites and habitats that the hares occupied and concluded that habitat type had a greater influence on the sources of predation than the amount of cover in any given location within a habitat. Our observations illustrate the vulnerability of hares to predators in even the densest coniferous habitat available in the boreal forest, and indicate strong seasonal changes in the rates and sources of predation.     

Cue reliability, risk sensitivity and inducible morphological defense in a marine snail

Reliable cues that communicate current or future environmental conditions are a requirement for the evolution of adaptive phenotypic plasticity, yet we often do not know which cues are responsible for the induction of particular plastic phenotypes. I examined the single and combined effects of cues from damaged prey and predator cues on the induction of plastic shell defenses and somatic growth in the marine snail Nucella lamellosa. Snails were exposed to chemical risk cues from a factorial combination of damaged prey presented in isolation or consumed by predatory crabs (Cancer productus). Water-borne cues from damaged conspecific and heterospecific snails did not affect plastic shell defenses (shell mass, shell thickness and apertural teeth) or somatic growth in N. lamellosa. Cues released by feeding crabs, independent of prey cue, had significant effects on shell mass and somatic growth, but only crabs consuming conspecific snails induced the full suite of plastic shell defenses in N. lamellosa and induced the greatest response in all shell traits and somatic growth. Thus the relationship between risk cue and inducible morphological defense is dependent on which cues and which morphological traits are examined. Results indicate that cues from damaged conspecifics alone do not trigger a response, but, in combination with predator cues, act to signal predation risk and trigger inducible defenses in this species. This ability to “label” predators as dangerous may decrease predator avoidance costs and highlights the importance of the feeding habits of predators on the expression of inducible defenses.     

Predators modify the evolutionary response of prey to temperature change

As climate regimes shift in many ecosystems worldwide, evolution may be a critical process allowing persistence in rapidly changing environments. Organisms regularly interact with other species, yet whether climate-mediated evolution can occur in the context of species interactions is not well understood. We tested whether a species interaction could modify evolutionary responses to temperature. We demonstrate that predation pressure by Dipteran larvae (Chaoborus americanus) modified the evolutionary response of a freshwater crustacean (Daphnia pulex) to its thermal environment over approximately seven generations in laboratory conditions. Daphnia kept at 21°C evolved higher population growth rates than those kept at 18°C, but only in those populations that were also reared with predators. Furthermore, predator-mediated selection resulted in the evolution of elevated Daphnia thermal plasticity. This laboratory natural selection experiment demonstrates that biotic interactions can modify evolutionary adaptation to temperature. Understanding the interplay between multiple selective forces can improve predictions of ecological and evolutionary responses of organisms to rapid environmental change.     

Fish predation can induce mesohabitat-specific differences in food web structures in small stream ecosystems

Diverse benthic communities in streams include a wide variety of predators with different habitat preferences, e.g. for pools or riffles. We hypothesised that these preferences result in mesohabitat-specific predator community structures with quantitative differences concerning predation intensity by vertebrate and invertebrate predators, importance of intraguild predation, or top–down pressure. This hypothesis was evaluated for a small submontane stream by means of mesohabitat-specific quantification of prey consumption by two benthivorous fish species (Gobio gobio and Barbatula barbatula) and several invertebrate predators. The estimation was based on daily food rations and diet composition of predators and mesohabitat-specific predator biomass. We found clear differences between the two mesohabitat types. Predator food webs were less complex in pools than in riffles. Fish predation was more important than invertebrate predation in pools, and intraguild predation had a higher relative importance in these mesohabitats. These differences were probably caused by the mesohabitat use of G. gobio, the largest top predator, which preferred pools. Consequently, the predator food webs were more similar between the mesohabitats when fish were absent. Top–down pressure on primary consumers by all predators together was lowest in pools without fish, but the effect was not significant. Omnivory (including cannibalism) was intense, but its potentially destabilising effects were probably counterbalanced by mesohabitat connectivity. From the results of our experimental study, we conclude that even in small stream ecosystems, food web structures and predation pathways can differ between mesohabitats and that a mesohabitat-specific consideration will help to explain the variety of top–down effects on benthic communities.     

Predator-induced plasticity in web-building behaviour

Many orb-web weaving spiders add conspicuous silken structures, called stabilimenta, to the hub of their webs, which are hypothesized to attract more prey. However, they may also attract predators. Orb spiders should therefore alter their web-building behaviour to minimize predation risk. We tested this hypothesis by experimentally examining web-building responses of the St Andrew cross spider, Argiope versicolor, to predation risk from one of its natural predators, the jumping spider Portia labiata. We randomly assigned A. versicolor juveniles to one of three treatments: (1) blank control (clean blotting paper: no odour from the predator or nonpredator); (2) predator odour cues from P. labiata; and (3) nonpredator control (odour cues from Leucauge decorata). Each individual of A. versicolor was monitored until it had built five consecutive webs (two webs before and three webs after the introduction of predator cues). When exposed to predator cues, the juveniles not only decreased the frequency of stabilimentum building but also refrained from increasing stabilimentum area, capture area and capture silk thread with subsequent webs compared with the blank control and the nonpredator control. Web-building traits, however, were not significantly different between the blank control and the nonpredator control. One plausible explanation is that A. versicolor juveniles can detect and discriminate between predators and nonpredators through olfactory cues and alter stabilimentum building and other web traits in response to the risk of predation. This is the first demonstration of an adaptive, plastic web-building behavioural response induced by chemical cues from a predator.     

Wave-related abrasion induces formation of extended spines in a marine bryozoan

Inducible morphology, the conditional expression of morphological characters under certain environmental regimes, is a trait usually found in organisms subject to discrete environmental variability. In marine invertebrates, inducible changes in morphology are usually linked to unpredictable attack by predators or overgrowth competition. We present here evidence that extended spine formation in the marine bryozoan Electra pilosa is inducible by an abiotic cue, wave-related abrasion. In a laboratory experiment, we induced the formation of extended spines by subjecting colonies of E. pilosa to abrasion by seaweeds. We also investigated the potential role of Adalaria proxima, a specialist suctorial nudibranch predator of E. pilosa, in the formation of extended spines. While the presence of the predator does not itself induce extended spine formation, the spines do have a fortuitous anti-predator effect, discouraging predation both by A. proxima and another nudibranch, Polycera quadrilineata. We suggest that extended spines in E. pilosa constitute an adaptation for the protection of feeding polypides in high-energy environments, and that plasticity for the trait is of adaptive value this passively dispersed organism, which exploits in a diverse range of substrata and epifaunal habitats.     

Strike Fast,Strike Hard: The Red-Throated Caracara Exploits Absconding Behavior of Social Wasps during Nest Predation

Red-throated Caracaras Ibycter americanus (Falconidae) are specialist predators of social wasps in the Neotropics. It had been proposed that these caracaras possess chemical repellents that allow them to take the brood of wasp nests without being attacked by worker wasps. To determine how caracaras exploit nests of social wasps and whether chemical repellents facilitate predation, we: (1) video recorded the birds attacking wasp nests; (2) analyzed surface extracts of the birds'' faces, feet, and feathers for potential chemical repellents; and (3) inflicted mechanical damage on wasp nests to determine the defensive behavior of wasps in response to varying levels of disturbance. During caracara predation events, two species of large-bodied wasps mounted stinging attacks on caracaras, whereas three smaller-bodied wasp species did not. The “hit-and-run” predation tactic of caracaras when they attacked nests of large and aggressive wasps reduced the risk of getting stung. Our data reveal that the predation strategy of caracaras is based on mechanical disturbance of, and damage to, target wasp nests. Caracara attacks and severe experimental disturbance of nests invariably caused wasps to abscond (abandon their nests). Two compounds in caracara foot extracts [sulcatone and iridodial] elicited electrophysiological responses from wasp antennae, and were also present in defensive secretions of sympatric arboreal-nesting Azteca ants. These compounds appear not to be wasp repellents but to be acquired coincidentally by caracaras when they perch on trees inhabited with Azteca ants. We conclude that caracara predation success does not depend on wasp repellents but relies on the absconding response that is typical of swarm-founding polistine wasps. Our study highlights the potential importance of vertebrate predators in the ecology and evolution of social wasps.     

Associational resistance of fouled blue mussels (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) against starfish (<Emphasis Type="Italic">Asterias rubens</Emphasis>) predation: relative importance of structural and chemical properties of the epibionts

Several epibiotic species reduce starfish (Asterias rubens) preference for the blue mussel Mytilus edulis in the Baltic. The aim of this study was to reveal whether this associational resistance was caused by structural or chemical aspects of the different epibionts. To assess structural epibiont effects, an in situ experiment was conducted with unfouled mussels and mussels equipped with artificial epibionts (dummies) exposed to natural predation by A. rubens. The chemically inert dummies closely matched the structural properties of the locally common epibionts Balanus improvisus (barnacle), Ceramium strictum (red alga), Halichondria panicea (sponge), and Laomedea flexuosa (hydrozoan). Starfish fed indiscriminately in all treatments. Chemical effects of epibionts on the attractiveness of mussels for A. rubens were investigated by incorporating freeze-dried epibionts or mussel tissue into Phytagel pellets at natural concentrations. Starfish were allowed to choose among these structurally similar but chemically different prey items in an in vitro experiment. The predators exhibited significant preferences among the food pellets, which closely matched their preferences for corresponding natural mussel–epibiont associations. Thus, chemical aspects of epibionts appear to play a larger role in this associational resistance than do structural aspects. Implications of these indirect interactions for benthic communities are discussed.Communicated by H.-D. Franke     

Predation on Multiple Trophic Levels Shapes the Evolution of Pathogen Virulence

The pathogen virulence is traditionally thought to co-evolve as a result of reciprocal selection with its host organism. In natural communities, pathogens and hosts are typically embedded within a web of interactions with other species, which could affect indirectly the pathogen virulence and host immunity through trade-offs. Here we show that selection by predation can affect both pathogen virulence and host immune defence. Exposing opportunistic bacterial pathogen Serratia marcescens to predation by protozoan Tetrahymena thermophila decreased its virulence when measured as host moth Parasemia plantaginis survival. This was probably because the bacterial anti-predatory traits were traded off with bacterial virulence factors, such as motility or resource use efficiency. However, the host survival depended also on its allocation to warning signal that is used against avian predation. When infected with most virulent ancestral bacterial strain, host larvae with a small warning signal survived better than those with an effective large signal. This suggests that larval immune defence could be traded off with effective defence against bird predators. However, the signal size had no effect on larval survival when less virulent control or evolved strains were used for infection suggesting that anti-predatory defence against avian predators, might be less constrained when the invading pathogen is rather low in virulence. Our results demonstrate that predation can be important indirect driver of the evolution of both pathogen virulence and host immunity in communities with multiple species interactions. Thus, the pathogen virulence should be viewed as a result of both past evolutionary history, and current ecological interactions.     

Living in the danger zone: Exposure to predators and the evolution of spines and body armor in mammals

Mammals have independently evolved a wide variety of morphological adaptations for use in avoiding death by predation, including spines, quills, dermal plates, and noxious sprays. Although these traits appear to protect their bearer from predatory attack, it is less obvious why some species evolved them and others have not. We investigated the ecological correlates favoring the evolution of specialized defenses in mammals, focusing on conspicuousness to predators due to body size and openness of habitat. We scored species for the degree to which they are protected by spines, quills, dermal plating, and sprays and used phylogenetic comparative analyses to study the morphological and ecological factors that may favor their evolution. We show that medium‐sized insectivorous mammals (～800 g to 9 kg) that live in open habitats are more likely to possess one of these defensive traits to reduce predation. Smaller species (<200 g) and those in closed habitats can typically rely on crypsis to avoid predators, and larger species (>10 kg) are less susceptible to predation by most small‐ to medium‐sized predators. We discuss how diet, metabolic rate, and defensive strategy evolve in concert to allow species to exploit this ecomorphological “danger zone” niche.     

Predation by blue crabs, Callinectes sapidus, on rapa whelks, Rapana venosa: possible natural controls for an invasive species?

Blue crabs Callinectes sapidus are voracious predators in Chesapeake Bay and other estuarine habitats. The rapa whelk Rapana venosa is native to Asian waters but was discovered in Chesapeake Bay in 1998. This predatory gastropod grows to large terminal sizes (in excess of 150 mm shell length (SL)) and has a thick shell that may contribute to an ontogenetic predation refuge. However, juvenile rapa whelks in Chesapeake Bay may be vulnerable to predation by the blue crab given probable habitat overlap, relative lack of whelk shell architectural defenses, and the relatively large size of potential crab predators. Feeding experiments using three size classes of blue crab predators in relation to a size range of rapa whelks of two different ages (Age 1 and Age 2) were conducted. Blue crabs of all sizes tested consumed Age 1 rapa whelks; 58% of all Age 1 whelks offered were eaten. Age 2 rapa whelks were consumed by medium (67% of whelks offered were eaten) and large (70% of whelks offered were eaten) blue crabs but not by small crabs. The attack methods of medium and large crabs changed with whelk age and related shell weight. Age 1 whelks were typically crushed by blue crabs while Age 2 whelk shells were chipped or left intact by predators removing prey. Rapa whelks less than approximately 35 mm SL are vulnerable to predation by all sizes of blue crabs tested. Rapa whelk critical size may be greater than 55 mm SL in the presence of large blue crabs indicating that a size refugia from crab predation may not be achieved by rapa whelks in Chesapeake Bay until at least Age 2 or Age 3. Predation by blue crabs on young rapa whelks may offer a natural control strategy for rapa whelks in Chesapeake Bay and other estuarine habitats along the North American Atlantic coast.     

Shell utilization,predation pressure,and thermal stress in panamanian hermit crabs: An interoceanic comparison

Shell preference patterns of two common hermit crabs from hard bottom reef flats on the Caribbean coast of Panama are examined in relation to the predation pressures and physical stresses of their habitat. Clibanarius antillensis Stimpson lives in the high intertidal habitat and minimizes exposure to predators by seeking refuge during high tides. It prefers high-spired shells which maximize protection from thermal stress. Calcinus tibicen Herbst avoids tidal emersion and prefers low-spired shells which enhance resistance to the predators common on Caribbean reef flats.The results are compared with similar results from the tropical eastern Pacific Bay of Panama. Shell-crushing predation on Caribbean hermit crabs is suggested to differ quantitatively and qualitatively from predation on hermit crabs in the Bay of Panama. Predation on hermit crabs in the Bay of Panama is more intense and effects larger individuals than predation on Caribbean reef flat hermit crabs. In addition, shell-crushing predation on hermit crabs in the Bay of Panama is primarily from teleost fish predators (Diodon spp.), while predation on Caribbean hermit crabs is primarily by bottom-dwelling crustaceans.Differences in predation pressures and tidal regimes between the Caribbean and Pacific coasts of Panama are reflected in the shell preferences and behavior of hermit crabs from the two areas.     

Influence of intraguild predation among generalist insect predators on the suppression of an herbivore population

We evaluated the influence of intraguild predation among generalist insect predators on the suppression of an herbivore, the aphid Aphis gossypii, to test the appropriateness of the simple three trophic level model proposed by Hairston, Smith, and Slobodkin (1960). We manipulated components of the predator community, including three hemipteran predators and larvae of the predatory green lacewing Chrysoperla carnea, in field enclosure/exclosure experiments to address four questions: (1) Do generalist hemipteran predators feed on C. carnea? (2) Does intraguild predation (IGP) represent a substantial source of mortality for C. carnea? (3) Do predator species act in an independent, additive manner, or do significant interactions occur? (4) Can the experimental addition of some predators result in increased densities of aphids through a trophic cascade effect? Direct observations of predation in the field demonstrated that several generalist predators consume C. carnea and other carnivorous arthropods. Severely reduced survivorship of lacewing larvae in the presence of other predators showed that IGP was a major source of mortality. Decreased survival of lacewing larvae was primarily a result of predation rather than competition. IGP created significant interactions between the influences of lacewings and either Zelus renardii or Nabis predators on aphid population suppression. Despite the fact that the trophic web was too complex to delineate distinct trophic levels within the predatory arthropod community, some trophic links were sufficiently strong to produce cascades from higher-order carnivores to the level of herbivore population dynamics: experimental addition of either Z. renardii or Nabis predators generated sufficient lacewing larval mortality in one experiment to release aphid populations from regulation by lacewing predators. We conclude that intraguild predation in this system is wide-spread and has potentially important influences on the population dynamics of a key herbivore.     

Inducible defences as key adaptations for the successful invasion of Daphnia lumholtzi in North America?

The mechanisms underlying successful biological invasions often remain unclear. In the case of the tropical water flea Daphnia lumholtzi, which invaded North America, it has been suggested that this species possesses a high thermal tolerance, which in the course of global climate change promotes its establishment and rapid spread. However, D. lumholtzi has an additional remarkable feature: it is the only water flea that forms rigid head spines in response to chemicals released in the presence of fishes. These morphologically (phenotypically) plastic traits serve as an inducible defence against these predators. Here, we show in controlled mesocosm experiments that the native North American species Daphnia pulicaria is competitively superior to D. lumholtzi in the absence of predators. However, in the presence of fish predation the invasive species formed its defences and became dominant. This observation of a predator-mediated switch in dominance suggests that the inducible defence against fish predation may represent a key adaptation for the invasion success of D. lumholtzi.