Behavioral responses to a change in stimulus: transport vs. attack behavior in a carnivorous land snail, Haplotrema concavum

Abstract. Haplotrema concavum , a carnivorous land snail, responds differently to the same prey when hunting versus when transporting food. The ability of these animals to distinguish between fresh, unmanipulated or uneaten prey and manipulated or partially eaten prey was tested by switching prey items while the predator was in the process of transporting the prey. Predators responded to a switch from a manipulated egg of Anguispira alternata to an unmanipulated egg by suspending transport behavior while the new egg was manipulated. Seven of ten predators responded to a switch from an egg of A. alternata to a conspecific ( H. concavum ) egg by stopping transport and abandoning the less-preferred food. Predators responded to a switch from partially eaten hatchlings of A. alternata to fresh but injured hatchlings by stopping transport after the retrieval stage to eat the new hatchling, after which transport was resumed. Predators which were not engaged in transport behavior attacked rather than retrieved fresh, injured hatchling prey. These results suggest that by manipulating its prey, the predator alters the sensory stimulus of the prey by marking the prey with predator mucus which, in turn, elicits the transport behavior by the predatory snail.     

Inducible defenses,competition and shared predation in planktonic food chains

Ecologists have long debated the role of predation in mediating the coexistence of prey species. Theory has mainly taken a bitrophic perspective that excludes the effects of inducible defenses at different trophic levels. However, inducible defenses could either limit or enhance the effects of predation on coexistence, by means of effects on bottom-up control and population stability. Our aim was to investigate how inducible defenses at different trophic levels affect the possibilities for predator-mediated coexistence, as opposed to competitive exclusion, in replicated experimental plankton communities. In particular, we analyzed how the presence or absence of inducible defenses in algal basal prey affected the outcome of competition between an inducible defended and an undefended herbivore, in the presence or absence of a carnivore. We found the undefended herbivore to be a superior competitor in the absence of predation. This outcome was reversed in the presence of a shared carnivore: populations of the undefended herbivore then strongly declined. The extent of this population decline differed between food webs based on undefended as opposed to inducible defended algal prey. In the former the undefended herbivore became undetectable for most of the duration of the experiment. In the latter the undefended herbivore also crashed to low densities, but it could still be detected during most of the experiment. In food webs based on inducible defended algae, the carnivore failed to reach high densities and exerted weaker top-down control on the two competing herbivores. We conclude that the inducible defense in one of our two competing herbivores allowed the outcome of competition to be reversed when a shared carnivore was added. Inducible defenses in algae did not change this outcome, but they significantly delayed extinction of the undefended herbivore. Predation itself did not promote coexistence in these experimental plankton communities.     

Carnivore mollusks as natural enemies of invasive land flatworms

There are several records of the carnivorous behaviour of land flatworms, considered to be top‐predators in their micro‐habitats, by preying upon various species of invertebrates. However, there is little knowledge of predators on land‐flatworms. The possible impact of invasive land flatworms on prey populations has caused widespread concern, when considering their predatory behaviour, combined with recent human influence on the distribution of certain species. This work is the first record of predation on land flatworms by a carnivorous snail. Various‐sized land flatworms of 10 native species of the subfamily Geoplaninae, as well as the exotic species Bipalium kewense (subfamily Bipaliinae), were offered to Rectartemon depressus (Gastropoda, Streptaxidae), which accepted all. The predator also fed on the snail Bradybaena similaris. The snails were maintained in laboratory for an average period of 12 months based on a mixed diet of flatworms and B. similaris, suggesting that the snail is a polyphagous predator. Because certain land‐flatworm species have been described as invasive species which may have a potential impact on prey populations in native and man‐made ecosystems, it is proposed that carnivorous snails of other native species, as potential predators of flatworms, should be tested for possible use in biological control programmes of these invasive planarians.     

Temperature-mediated dynamics of planktonic food chains: the effect of an invertebrate carnivore

1. This study involves an examination of two- and three-trophic-level food chains at two temperatures (18 and 25 °C) in order to determine how the addition of a carnivore to a predator–prey system can alter the dynamics of populations and how this effect may be temperature mediated. The system consisted of phytoplankton, Daphnia pulex and the flatworm Mesostoma ehrenbergii .
2. Although the plant–herbivore system is inherently unstable at 25 °C, the addition of the carnivore led to a further destabilization of the Daphnia –algal dynamics at the higher temperature. No destabilization effect of the carnivore was noted at 18 °C. At the lower temperature, all populations persisted and the carnivore induced changes only in the age structure of the Daphnia populations rather than in overall biomass.
3. The differential effects of the carnivore at two temperatures can be attributed to shifts in the life history, physiological rates and the reproductive strategy employed by Mesostoma .
4. Previous theoretical work has predicted that the addition of a third trophic level to an unstable predator–prey system should stabilize dynamics. Our results indicate that the effect of a carnivore on plant–herbivore dynamics can be significantly affected by ambient temperature.     

Temperature-mediated dynamics of planktonic food chains: the effect of an invertebrate carnivore

1. This study involves an examination of two- and three-trophic-level food chains at two temperatures (18 and 25 °C) in order to determine how the addition of a carnivore to a predator–prey system can alter the dynamics of populations and how this effect may be temperature mediated. The system consisted of phytoplankton, Daphnia pulex and the flatworm Mesostoma ehrenbergii .
2. Although the plant–herbivore system is inherently unstable at 25 °C, the addition of the carnivore led to a further destabilization of the Daphnia –algal dynamics at the higher temperature. No destabilization effect of the carnivore was noted at 18 °C. At the lower temperature, all populations persisted and the carnivore induced changes only in the age structure of the Daphnia populations rather than in overall biomass.
3. The differential effects of the carnivore at two temperatures can be attributed to shifts in the life history, physiological rates and the reproductive strategy employed by Mesostoma .
4. Previous theoretical work has predicted that the addition of a third trophic level to an unstable predator–prey system should stabilize dynamics. Our results indicate that the effect of a carnivore on plant–herbivore dynamics can be significantly affected by ambient temperature.     

Predator–prey interactions amongst Permo‐Triassic terrestrial vertebrates as a deterministic factor influencing faunal collapse and turnover

Unlike modern mammalian communities, terrestrial Paleozoic and Mesozoic vertebrate systems were characterized by carnivore faunas that were as diverse as their herbivore faunas. The comparatively narrow food base available to carnivores in these paleosystems raises the possibility that predator–prey interactions contributed to unstable ecosystems by driving populations to extinction. Here, we develop a model of predator–prey interactions based on diversity, abundance and body size patterns observed in the Permo‐Triassic vertebrate fossil record of the Karoo Basin, South Africa. Our simulations reflect empirical evidence that despite relatively high carnivore: herbivore species ratios, herbivore abundances were sufficient for carnivores to maintain required intake levels through most of the Karoo sequence. However, high mortality rates amongst herbivore populations, even accounting for birth rates of different‐sized species, are predicted for assemblages immediately preceding the end‐Guadalupian and end‐Permian mass extinctions, as well as in the Middle Triassic when archosaurs replaced therapsids as the dominant terrestrial fauna. These results suggest that high rates of herbivore mortality could have played an important role in biodiversity declines leading up to each of these turnover events. Such declines would have made the systems especially vulnerable to subsequent stochastic events and environmental perturbations, culminating in large‐scale extinctions.     

Can plants betray the presence of multiple herbivore species to predators and parasitoids? The role of learning in phytochemical information networks

In response to feeding by phytophagous arthropods, plants emit volatile chemicals. This is shown to be an active physiological response of the plant and the released chemicals are therefore called herbivore-induced plant volatiles (HIPV). One of the supposed functions of HIPV for the plant is to attract carnivorous natural enemies of herbivores. Depending on which plant and herbivore species interact, blends of HIPV show qualitative and quantitative variation. Hence, one may ask whether this allows the natural enemies to discriminate between volatiles from plants infested by herbivore species that are either suitable or unsuitable as a food source for the natural enemy. Another question is whether natural enemies can also recognise HIPV when two or more herbivore species that differ in suitability as a food source simultaneously attack the same plant species. By reviewing the literature we show that arthropod predators and parasitoids can tell different HIPV blends apart in several cases of single plant–single herbivore systems and even in single plant–multiple herbivore systems. Yet, there are also cases where predators and parasitoids do not discriminate or discriminate only after having learned the association between HIPV and herbivores that are either suitable or non-suitable as a source of food. In this case, suitable herbivores may profit from colonising plants that are already infested by another non-suitable herbivore. The resulting temporal or partial refuge may have important population dynamical consequences, as such refuges have been shown to stabilise otherwise unstable predator–prey models of the Lotka-Volterra or Nicholson-Bailey type.     

Influence of snail mucus trails and first snail meal on the behavior of malacophagous sciomyzid larvae

There has been considerable focus on the natural enemies of snails, particularly those of medical and veterinary significance. Much attention has focussed on members of the Family Sciomyzidae (Diptera), the majority of which feed on a range of mollusc species. However, little is known about the influence of first snail meal on subsequent prey choices, an important consideration in biocontrol. We examined neonate larval responses of Ilione albiseta to fresh and aged snail mucus trails of three snail species. Median neonate response rates to aged mucus trails for all three snail species tested were significantly (P < 0.001) weaker than for fresh mucus trails indicating a strategy which enhances the likelihood of reaching prey snail species without expending energy following “cold” trails. More than 78% of first instar larvae, fed on one snail species (Radix peregra or Stagnicola palustris) and subsequently offered a choice of these two snail species for the second snail meal, selected the snail species of the first snail meal suggesting that the first snail meal influences subsequent prey selection. However, the impact of the first snail meal on larval trail-following behavior is less clear-cut. While there may be some preference for the mucus trail of the snail species on which neonate larvae have fed, this does not exclude the larvae from following the mucus trails of other snail species. The results are discussed in the context of the potential use of I. albiseta as a biocontrol agent of vectors of snail borne diseases.     

Assessing species traits and landscape relationships of the mammalian carnivore community in a neotropical biological corridor

Mammalian carnivores play an important role in regulating food webs and ecosystems. While many carnivore populations are facing various threats such as habitat loss and fragmentation, poaching, and illegal trade, others have adapted to human-dominated landscapes. Information about Neotropical carnivore communities in particular is limited, especially in disturbed landscapes. We conducted a camera trap survey at 38 sites across the San Juan–La Selva Biological Corridor in Costa Rica to assess occupancy and detection probabilities of the carnivore community. We developed hypotheses within a likelihood-based framework in order to determine the landscape features and species traits (diet and size) that influenced their occupancy. We detected nine of the 13 native carnivores predicted to occur in the corridor. When modeled separately, each species responded to land cover changes differently, suggesting no strong community-wide predictors of occupancy. We then modeled three separate guilds within the carnivore community: omnivorous mesopredators, obligate carnivorous mesopredators, and apex predators. These community guild models revealed a negative relationship between omnivorous mesopredators and increasing forest and tree plantation cover, suggesting omnivores utilize forest fragments and edge habitats in agricultural landscapes. Obligate carnivorous mesopredator models did not reveal any strong habitat relationships, but landscape effects tended to contradict our a priori predictions. Apex predators were positively associated with increasing forest and tree plantation cover, protected areas, and increasing distances to villages. Alarmingly, apex predators and obligate carnivorous mesopredators were generally rare within the biological corridor. A lack of top-down control alone might result in heightened occupancy for all mesopredators, but because the community is dominated by omnivorous species, bottom-up release from human-induced land cover changes and resource provision may better explain their high occupancy.     

Tentacles of in vitro-grown round-leaf sundew (<Emphasis Type="Italic">Drosera rotundifolia</Emphasis>L.) show induction of chitinase activity upon mimicking the presence of prey

Induction of plant-derived chitinases in the leaves of a carnivorous plant was demonstrated using aseptically grown round-leaf sundew (Drosera rotundifolia L.). The presence of insect prey was mimicked by placing the chemical inducers gelatine, salicylic acid and crustacean chitin on leaves. In addition, mechanical stirring of tentacles was performed. Chitinase activity was markedly increased in leaf exudates upon application of notably chitin. Application of gelatine increased the proteolytic activity of leaf exudates, indicating that the reaction of sundew leaves depends on the molecular nature of the inducer applied. In situ hybridization of sundew leaves with a Drosera chitinase probe showed chitinase gene expression in different cell types of non-treated leaves, but not in the secretory cells of the glandular heads. Upon induction, chitinase mRNA was also present in the secretory cells of the sundew leaf. The combined results indicate that chitinase is likely to be involved in the decomposition of insect prey by carnivorous plants. This adds a novel role to the already broad function of chitinases in the plant kingdom and may contribute to our understanding of the molecular mechanisms behind the ecological success of carnivorous plants in nutritionally poor environments.     

Subtidal food thieves: interactions of four invertebrate kleptoparasites with the sea star Leptasterias polaris

We used time-lapse underwater video systems in the subtidal zone of the Mingan Islands, eastern Canada, to examine behavioural interactions of four kleptoparasites with the sea star Leptasterias polaris when it was feeding on infaunal (buried) clams. Departures of L. polaris from its prey coincided with interactions with kleptoparasites in 10 out of 10 filmed feeding bouts on the large clam Spisula polynyma, compared with only four out of 10 of filmed bouts on the smaller clam Mya truncata. The sea star's abandoning of the prey was most often caused by interactions with another sea star, Asterias vulgaris, a potential predator. The whelk (a carnivorous snail), Buccinum undatum, and the crabs Hyas araneus and Cancer irroratus, also kleptoparasitized L. polaris, especially when A. vulgaris was present. Comparisons of feeding bouts in the presence and absence of kleptoparasites showed that at least 10.4% of the prey mass captured by L. polaris was lost to kleptoparasites. Simultaneous current meter data showed that the movement of A. vulgaris and B. undatum to sites where L. polaris was feeding on S. polynyma was upstream, suggesting chemodetection of food odours. Crabs in the vicinity may also use visual cues. Leptasterias polaris is the only carnivore that can efficiently extract large clams from the sediments and therefore its foraging may supply a substantial part of the diet of kleptoparasites. Copyright 2000 The Association for the Study of Animal Behaviour.     

Harnessing disorder: onychophorans use highly unstructured proteins,not silks,for prey capture

Onychophora are ancient, carnivorous soft-bodied invertebrates which capture their prey in slime that originates from dedicated glands located on either side of the head. While the biochemical composition of the slime is known, its unusual nature and the mechanism of ensnaring thread formation have remained elusive. We have examined gene expression in the slime gland from an Australian onychophoran, Euperipatoides rowelli, and matched expressed sequence tags to separated proteins from the slime. The analysis revealed three categories of protein present: unique high-molecular-weight proline-rich proteins, and smaller concentrations of lectins and small peptides, the latter two likely to act as protease inhibitors and antimicrobial agents. The predominant proline-rich proteins (200 kDa+) are composed of tandem repeated motifs and distinguished by an unusually high proline and charged residue content. Unlike the highly structured proteins such as silks used for prey capture by spiders and insects, these proteins lack ordered secondary structure over their entire length. We propose that on expulsion of slime from the gland onto prey, evaporative water loss triggers a glass transition change in the protein solution, resulting in adhesive and enmeshing thread formation, assisted by cross-linking of complementary charged and hydrophobic regions of the protein. Euperipatoides rowelli has developed an entirely new method of capturing prey by harnessing disordered proteins rather than structured, silk-like proteins.     

Stability of ecosystems with complex food webs.

In a complex food web, various interaction pathways may connect pairs of species. A high carnivore might prey upon both a lesser carnivore and the prey of the lesser carnivore, forming a “loop” in the food web. This report contains one technical definition for stability of an ecosystem. Sufficient conditions for stability are presented. The conditions require that loops in food webs be in a sense “balanced.”     

Food partitioning among carnivores within feeding guild structure of fishes inhabiting a mudflat ecosystem of Indian Sundarbans

The present study was aimed to determine dietary composition and feeding guild structure of the fishes inhabiting mudflat habitat of Indian Sundarbans. In addition, partitioning of food resources by the fish species belonging to the carnivorous feeding guild was also performed to understand the survival strategies of fish in a mudflat estuarine habitat. Seventeen prey categories were isolated from the stomachs of 31 studied fish species. Overall, five feeding guilds (viz. plankti-benthivore: 12 species, herbivore: one species, detritivore: three species, omnivore: one species and carnivore: 14 species) were identified on the basis of the prey abundance within their stomachs, considering 64 % Bray–Curtis similarity. Among the carnivorous, maximum trophic richness was obtained for Uroconger lepturus followed by Ophichthus apicalis. Teleost and decapods were the main animal prey items preferred by majority of the carnivorous fishes. However, O. apicalis and Terapon jarbua showed their preference toward maximum number of prey categories among carnivores, which was also ratified by the high values for standardized niche breadth presented by them. The maximum degree of interspecific dietary overlap was found between Uropterygius marmoratus and Pseudapocryptes elongatus as both of them were recognized as cranci-piscivore. The lowest was observed between Hyporhamphus limbatus and Coilia neglecta. As food resources are not limiting in the mudflats of Indian Sundarbans, the general patterns of resource partitioning and niche differentiation in resident ecological communities will improve the understanding of the mechanisms underlying species coexistence and community structure.     

Changes in the Cell Volume of Didinium nasutum During Population Increase*

SYNOPSIS. In 3 species of carnivorous protozoa, the rate of individual food intake per generation declines with an increase in the density of the population. In all, the rate of division remains constant. Three hypotheses may be proposed to explain these phenomena: the individual size decreases, thus bringing about a decline in food need; the individuals in the earlier stages of population growth consume excess food which is passed on to later generations to supplement their food intake; an increase in density within the carnivore population decreases the activity rate, thus reducing the food requirement. Experiments using Didinium nasutum as the carnivore and Paramecium aurelia, syngen 4, as the food source were conducted to test these 3 hypotheses. The results contradicted the 1st hypothesis and part of the 2nd. A new working hypothesis based on the remaining hypotheses is proposed.     

The Coupling Between Grazing and Detritus Food Chains and the Strength of Trophic Cascades Across a Gradient of Nutrient Enrichment

A minimal food web model was constructed comprising one grazing and one detritus food chain coupled by nutrient cycling and generalist carnivores to investigate how prey preference by carnivores may affect the strength of trophic cascades across a gradient of nutrient enrichment. The equilibrium or mean abundance of each food web component and the magnitude of the carnivore effect on lower trophic levels were calculated for different values of the prey preference and nutrient input parameters. Our model predicts that nutrient enrichment increases the mean abundances of carnivores, autotrophs and detritus, but the magnitude of this effect is dependent on the prey preference term. On the other hand, herbivores and detritivores are relatively unaffected by enrichment but are strongly affected by carnivore preference. Carnivores have a negative effect on herbivores and a positive effect on autotrophs and detritus, whereas the effect on detritivores can be both positive and negative. At high preference for herbivores, carnivores have a positive effect on detritivores, because the positive effect of increased detritus availability due to reduced herbivore grazing outweighs the negative effect of predation. At high preference for detritivores, the balance is changed in the other direction. We argue that in systems where authochtonous primary production is the major source of detritus, herbivores can control the rates of detritus production and have indirect effects on detritivores, which may feed back into effects on herbivores through their shared enemies. This positive feedback is probably one mechanism affecting the resilience of alternative stable states in shallow lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions  J.L.A. conceived the model and the study, J.R. wrote the Matlab programs and ran the simulations and J.L.A. wrote most of the paper.     

A food web perspective on large herbivore community limitation

The exceptional diversity of large mammals in African savannas provides an ideal opportunity to explore the relative importance of top‐down and bottom‐up controls of large terrestrial herbivore communities. Recent work has emphasized the role of herbivore and carnivore body size in shaping these trophic relationships. However, the lack of across‐ecosystem comparisons using a common methodology prohibits general conclusions. Here we used published data on primary production, herbivore and carnivore densities and diets to estimate the consumption fluxes between three trophic levels in four African savanna ecosystems. Our food web approach suggests that the body size distribution within and across trophic levels has a strong influence on the strength of top‐down control of herbivores by carnivores and on consumption fluxes within ecosystems, as predicted by theoretical food web models. We generalize findings from the Serengeti ecosystem that suggest herbivore species below 150 kg are more likely to be limited by predation. We also emphasize the key functional role played by the largest species at each trophic level. The abundance of the largest herbivore species largely governs the consumption of primary production in resident communities. Similarly, predator guilds in which the largest carnivore species represent a larger share of carnivore biomass are likely to exert a stronger top‐down impact on herbivores. Our study shows how a food web approach allows integrating current knowledge and offers a powerful framework to better understand the functioning of ecosystems.     

The role of exotic mammals in the diet of native carnivores from South America

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Human activity helps prey win the predator-prey space race

Predator-prey interactions, including between large mammalian wildlife species, can be represented as a "space race", where prey try to minimize and predators maximize spatial overlap. Human activity can also influence the distribution of wildlife species. In particular, high-human disturbance can displace large carnivore predators, a trait-mediated direct effect. Predator displacement by humans could then indirectly benefit prey species by reducing predation risk, a trait-mediated indirect effect of humans that spatially decouples predators from prey. The purpose of this research was to test the hypothesis that high-human activity was displacing predators and thus indirectly creating spatial refuge for prey species, helping prey win the "space race". We measured the occurrence of eleven large mammal species (including humans and cattle) at 43 camera traps deployed on roads and trails in southwest Alberta, Canada. We tested species co-occurrence at camera sites using hierarchical cluster and nonmetric multidimensional scaling (NMS) analyses; and tested whether human activity, food and/or habitat influenced predator and prey species counts at camera sites using regression tree analysis. Cluster and NMS analysis indicated that at camera sites humans co-occurred with prey species more than predator species and predator species had relatively low co-occurrence with prey species. Regression tree analysis indicated that prey species were three times more abundant on roads and trails with >32 humans/day. However, predators were less abundant on roads and trails that exceeded 18 humans/day. Our results support the hypothesis that high-human activity displaced predators but not prey species, creating spatial refuge from predation. High-human activity on roads and trails (i.e., >18 humans/day) has the potential to interfere with predator-prey interactions via trait-mediated direct and indirect effects. We urge scientist and managers to carefully consider and quantify the trait-mediated indirect effects of humans, in addition to direct effects, when assessing human impacts on wildlife and ecosystems.     

Trail following and stowaway behaviour of the myrmecophilous staphylinid beetle,Homoeusa acuminata,during foraging trips of its hostLasius fuliginosus (Hymenoptera: Formicidae)

Summary The behaviour of adultHomoeusa acuminata on trails of its hostLasius fuliginosus was investigated both in the field and in the laboratory. The beetles were active from May to September, accurately following the foraging trails of their hosts up to 20 metres from the nest. Most of the time, they were ignored by the ants, but if attacked they raised their abdomen as a possible appeasement or defensive behaviour. On trails the beetles most probably act as food robbers, feeding on prey collected by ants. The following method, called stowaway behaviour, was used by the beetles: when a beetle encountered an ant carrying a prey back to the nest it jumped on the prey, probably feeding on it while being transported.Laboratory experiments on circular artificial trails demonstrated thatH. acuminata follows a water extract of hindguts of the ants, the source of the trail pheromone. Both beetles and ants responded to an artificial trail of 0.03 hindgut equivalent per cm, but the mean distance followed by the beetles was about twelve times higher than that covered by the ants themselves. In contrast, experiments with solutions of the six fatty acids reported as the active components of the trail pheromone showed that the beetles did not respond at all, and that the ants only respond to the fatty acids at a very high concentration.