Prey selection by thaidid gastropods: some observational and experimental field tests of foraging models

Summary Field observations and experiments revealed that predatory intertidal gastropods of the genus Thais (or Nucella) were able both to recognize the expected food value of encountered prey (expected energy or growth potential gained per unit handling time) and to monitor their average yield over time (average energy or growth potential gained per unit foraging time). They appeared to discriminate not only among prey species, but also among different sized individuals of the same prey species. The evidence supporting these interpretations included: 1) field observations of snails feeding preferentially on prey types of higher expected food value even though lower value prey types were available and abundant, 2) a very limited number of direct underwater observations of foraging snails rejecting encountered items that were either of lower expected value than the item finally eaten or not measurably different from it, and 3) field (=arena) experiments in which both average yield, and the distribution and abundance of potential prey were controlled: snails conditioned at a high average yield fed preferentially on high value items, while those animals conditioned at a low yield consumed prey in the proportions that they were encountered. These behaviors are all consistent with a prey-selection decision motivated by energetic considerations. Further, the field experiments indicated that these predatory gastropods could select items from a diverse array of prey so as to maximize growth in their natural environment. The behaviors were not consistent with three alternative foraging hypotheses: non-selective foraging, frequency-dependent foraging on prey types (here, sizes of particular prey species), and frequency-dependent foraging on prey species. Deviations from some of the quantitative predictions of optimal foraging theory appeared related to learning and risk.     

Growth rate as a measure of food value in thaidid gastropods: Assumptions and implications for prey morphology and distribution

Rates of body growth were measured for three species of rocky intertidal gastropods: Thais (or Nucella) lamellosa (Gmelin), T. (or N.) canaliculata (Duclos), and T. (or N.) emarginata (Deshayes). Three size classes of each predator species were grown experimentally in cages at two tidal heights on four size classes of each prey species: Semibalanus cariosus (Pallas), Balanus glandula Darwin, and Mytilus edulis L. Growth was also assessed for Thais canaliculata feeding on Mytilus californianus Conrad, and Thais emarginata feeding on Chthamalus dalli Pilsbry.Rates of body growth varied as a function of predator size, prey size and prey species. With one exception (large Thais canaliculata) intermediate sized Balanus glandula promoted the most rapid growth for all sizes of all three species of Thais; thus these potentially competing predator species have the same highest ranked prey. Among prey promoting slower growth, those encountered commonly in the normal habitats of the predator promoted more rapid growth than those encountered rarely, suggesting that past evolutionary experience has influenced present food value (= growth potential) of prey. For some predators, the ranking of prey species changed with predator size; Chthamalus dalli and similar sized Balanus glandula promoted comparable growth in small Thais emarginata but Chthamalus dalli promoted much slower growth than similar sized Balanus glandula in larger Thais emarginata. Rank differences based on prey size in Balanusglandula became more pronounced and in some instances changed with increasing predator size; larger B. glandula promoted relatively faster growth for large snails than for small snails in all Thais species. Finally, growth rates were correlated with two important attributes of fitness in T. canaliculata and T. emarginata; in general, prey promoting more rapid growth also resulted in an earlier age of first reproduction for initially immature snails and higher rate of egg capsule production for mature individuals. Thus, these growth rates provide a basis from which to examine quantitatively patterns of prey selection from an energy- or growth-maximization perspective.These patterns of predator growth also permit inferences about the possible influence of predation on the evolution of prey morphology, life-history, and microhabitat distribution. Size refuges from Thais predation were confirmed for both Semibalanus cariosus and Mytilus californianus. Both Balanus glandula and Mytilus edulis, on the other hand, were vulnerable to predation by all sizes of Thais examined (> 10 mm) regardless of prey size; thus, neither of these species achieves a size refuge. In addition, on rocky shores, vertical distributions of the prey species reflect general preference patterns of their predators: higher value prey species, i.e. those with less well developed defensive morphologies (Balanus glandula, Mytilus edulis), generally occur higher on the shore. This pattern would be expected if preferred prey are consumed first lower on the shore where they are more available.     

PHYSIOLOGICAL STRESS AND COLOR POLYMORPHISM IN THE INTERTIDAL SNAIL NUCELLA LAPILLUS

The intertidal snail Nucella lapillus exhibits considerable variation in shell color both within and between populations differentially exposed to wave action. Populations from high-wave-energy shores tended to be highly polymorphic and were dominated by pigmented morphs (especially brown), while those at more sheltered locations exhibited less polymorphism and were predominantly white. Field and laboratory experiments were conducted to determine the role of physiological stress and selective predation in maintaining the observed distribution of color morphs. The results demonstrated that 1) physiological stress from high temperature and desiccation during periods of tidal emersion was greater on protected shores, 2) under similar natural conditions, brown morphs heated up faster, attained higher temperatures, desiccated more rapidly, and suffered greater mortality than did white morphs, and 3) when pairs of brown and white morphs were tethered intertidally there was virtually no mortality of either morph on the exposed shore or in shaded microhabitats on the protected shore, but brown morphs suffered much greater mortality in sunny microhabitats on the protected shore. These findings demonstrate that the interpopulation variation in shell color of N. lapillus is in part a response to a selective gradient in physiological stress. Selection for crypsis by visually hunting predators did not appear to play a prominent role; however, only adults were considered, and the predation experiments were conducted in the fall before shorebirds that prey on whelks had arrived from their summer feeding grounds. Further experimentation to quantify the effects of visual predators such as birds and fish, particularly on juvenile snails, is necessary to assess adequately the importance of predation.     

Resource levels and prey state influence antipredator behavior and the strength of nonconsumptive predator effects

The risk of predation can drive trophic cascades by causing prey to engage in antipredator behavior (e.g. reduced feeding), but these behaviors can be energetically costly for prey. The effects of predation risk on prey (nonconsumptive effects, NCEs) and emergent indirect effects on basal resources should therefore depend on the ecological context (e.g. resource abundance, prey state) in which prey manage growth/predation risk tradeoffs. Despite an abundance of behavioral research and theory examining state‐dependent responses to risk, there is a lack of empirical data on state‐dependent NCEs and their impact on community‐level processes. We used a rocky intertidal food chain to test model predictions for how resources levels and prey state (age/size) shape the magnitude of NCEs. Risk cues from predatory crabs Carcinus maenas caused juvenile and sub‐adult snails Nucella lapillus to increase their use of refuge habitats and decrease their growth and per capita foraging rates on barnacles Semibalanus balanoides. Increasing resource levels (high barnacle density) and prey state (sub‐adults) enhanced the strength of NCEs. Our results support predictions that NCEs will be stronger in resource‐rich systems that enhance prey state and suggest that the demographic composition of prey populations will influence the role of NCEs in trophic cascades. Contrary to theory, however, we found that resources and prey state had little to no effect on snails in the presence of predation risk. Rather, increases in NCE strength arose because of the strong positive effects of resources and prey state on prey foraging rates in the absence of risk. Hence, a common approach to estimating NCE strength – integrating measurements of prey traits with and without predation risk into a single metric – may mask the underlying mechanisms driving variation in the strength and relative importance of NCEs in ecological communities.     

Predator identity and consumer behavior: differential effects of fish and crayfish on the habitat use of a freshwater snail

Predators can alter the outcome of ecological interactions among other members of the food web through their effects on prey behavior. While it is well known that animals often alter their behavior with the imposition of predation risk, we know less about how other features of predators may affect prey behavior. For example, relatively few studies have addressed the effects of predator identity on prey behavior, but such knowledge is crucial to understanding food web interactions. This study contrasts the behavioral responses of the freshwater snail Physellagyrina to fish and crayfish predators. Snails were placed in experimental mesocosms containing caged fish and crayfish, so the only communication between experimental snails and their predators was via non-visual cues. The caged fish and crayfish were fed an equal number of snails, thereby simulating equal prey mortality rates. In the presence of fish, the experimental snails moved under cover, which confers safety from fish predators. However, in the presence of crayfish, snails avoided benthic cover and moved to the water surface. Thus, two species of predators, exerting the same level of mortality on prey, induced very different behavioral responses. We predict that these contrasting behavioral responses to predation risk have important consequences for the interactions between snails and their periphyton resources. Received: 1 June 1998 / Accepted: 12 October 1998     

Predation rates of nemertean predators: the case of a rocky shore hoplonemertean feeding on amphipods

Estimates of the predation rates of benthic nemerteans are often based on observations of single individuals, and consequently they may not be representative for all members of a population of these predators. Herein we conducted controlled and repeatable laboratory experiments on the predation rate of the hoplonemertean Amphiporus nelsoni Sánchez 1973, which is common at exposed rocky shores along the central Chilean coast. During the austral fall (April, May 2000), nemerteans were observed in relatively high numbers crawling in the intertidal zone during early morning or late-afternoon low tides. When these nemerteans were offered living amphipods held by a forceps, they immediately attacked the amphipods and fed on them. In the laboratory experiments, nemerteans preferred the amphipod Hyale maroubrae Stebbing, 1899, which is also very common in the natural habitat of A. nelsoni. The nemerteans preyed to a higher extent on small males and non-ovigerous females than would have been expected from their abundance. We suggest that these (non-reproductive) stages of H. maroubrae are very mobile and therefore have a high likelihood of encounters with nemerteans. Predation rates reached maxima when nemerteans were provided prey densities of four or more of their preferred prey species, H. maroubrae, furthermore indicating that encounter rates with prey may affect predation rates. In long-term laboratory experiments, A. nelsoni consumed more amphipods during low tide conditions than during high tide conditions. Many nemerteans in the field prefer particular environmental conditions (e.g. nocturnal low tides), which restricts the time available for successful feeding. In the long-term experiment, predation rates of A. nelsoni never exceeded 0.5 amphipods nemertean^–1 d^–1. Maximum feeding events were 3 or 4 amphipods nemertean^–1 d^–1, but this only occurred during 10 out of a possible 2634 occasions. Nemerteans that had consumed 3 or 4 amphipods during 1 day, consumed substantially less prey during the following days. Towards the end of the long-term experiment, average predation rates decreased to 0.2 amphipods nemertean^–1 d^–1, corresponding to predation rates reported for other nemertean species (0.1–0.3 prey items nemertean^–1 d^–1). We suggest that predation rates from laboratory experiments represent maximum estimates that may not be directly transferable to field populations. Additionally, low predator–prey encounter rates with preferred prey in the field may further limit the predation impact of nemertean predators in natural habitats.     

Leech predation on juvenile freshwater snails: effects of size,species and substrate

Summary The leechGlossiphonia complanata does not appear to have substantial impact on snail populations, but this may be due to most studies focusing on adult snails rather than juvenile snails. In this study I investigated how predation rates ofG. complanata feeding on newly-hatched and juvenile snails was affected by snail species, snail size, snail density and substrate, in a laboratory experiment. Number of snails eaten increased with increasing density resulting in a type II functional response curve. Predation rates were higher when leeches were feeding onLymnaea emarginata than onPhysa gyrina, whereas there was no significant difference in predation rates when they were feeding onL. emarginata andHelisoma anceps. Sandy substrates and greater snail size resulted in decreased predation rates. Sand reduced movement speed ofG. complanata, which probably reduced encounter rates. Thus, there was a comparatively large effect of leech predation on newly-hatched snails, due to a high probability of encounter and high predation rates, but spatial and temporal refuges probably reduce the importance of leech predation as a structuring force in freshwater snail assemblages.     

Plant-specific expression of antipredator behaviour by larval damselflies

Summary In this study of interactions between larval damselflies (Zygoptera: Coenagrionidae) and pumpkinseed sunfish (Lepomis gibbosus) we focus on the behaviour of the damselfly prey. First we document a prey behaviour in which damselflies use plant stems or leaves to hide from pumpkinseeds. We then test two hypotheses: (1) that damselfly hiding is a specific antipredator behaviour and (2) that hiding occurs more frequently in plant habitats where damselflies experience greater risk of predation. Since plant species growth forms can influence predation risk, our second hypothesis implies that hiding behaviour is conditional upon the type of vegetation providing habitat structure. Conditional expression of antipredator behaviour according to vegetation type may be important in littoral environments, since predator-prey interactions can occur in habitats with a wide range of macrophyte growth forms. The first hypothesis was supported by our findings that damselfly hiding increased in frequency in the presence of pumpkinseeds, that it was related to the frequency of predator approaches, and that its use reduced damselfly predation risk in high risk habitats. The second hypothesis was supported by our results that damselfly hiding rates were greater in the high risk Scirpus habitats than in the lower risk Potamogeton habitats. These results indicate that prey behaviour can influence predator-prey interactions, and that variation in plant growth form can influence prey behaviour, thus contributing to the impact of habitat structure on predator-prey dynamics.     

Effects of predation pressure on the feeding behaviour of the serpa tetra Hyphessobrycon eques (Ostariophysi, Characidae)

The indirect, non-lethal results of predation, such as reduction in feeding time or restraint in seeking sexual partners and/or natural resources, have a drastic effect on prey populations. In this study, we investigated the behaviour of two serpa tetras Hyphessobrycon eques groups, one wild and the other born and raised in captivity, to evaluate how their feeding behaviour is affected by the avian predator, the rufescent tiger-heron Trigrisoma lineatum (using a taxidermy specimen). For a total of 133 observation hours, the feeding behaviour of each fish group was observed according to the absence (control) or presence (treatments) of a predator near the aquarium surface. The results showed that the presence of a predator on the surface inhibits the feeding behaviour of H. eques. The differences observed between the groups are probably due to the fishes experiences with predators. Our results suggest that fish-eating birds may affect prey populations in streams and ponds perhaps more through non-lethal effects, on feeding behaviour for example, than directly through death rates.     

Scaling the effects of predation and disturbance in a patchy environment

The effects of hydraulic disturbances on the impact of two predatory benthic invertebrates on their prey were examined in a stream at two distinct spatial scales. At the scale of small habitat patches (0.0625 m²), hydraulic patch type was an important determinant of the microdistribution of prey and predators. Prey abundances were similar across all patch types at baseflow, but local densities were higher in patches identified as low-flow refugia after periods of high and fluctuating flow. The microdistribution pattern of predatory larvae of a caddisfly, Plectrocnemia conspersa, was similar to that of its prey, whereas predatory larvae of an alderfly, Sialis fuliginosa, did not shift their microdistribution significantly with discharge and were always most abundant in lowflow refugia. There was little evidence of an aggregative response of predators with prey, even though both predators and prey are mobile. Both predator species showed similar patch-specific patterns of per capita consumption rates: uniform consumption rates across hydraulic patch types at low and moderate flows, but highest in flow refugia during high flows. Species-specific patterns, however, were apparent in the magnitude and direction of differences between consumption rates during disturbance events, and in comparable patches at base flow: At high flow, consumption rates for P. conspersa were exaggerated (3.9 times higher) in flow refugia but at par in other patches; for S. fuliginosa they were at par in flow refugia but reduced in other patches (up to 3.3. times lower). These differences may be related to species-specific foraging behaviours (search vs ambush predators) and the influence of prey movements on feeding success. Using the patch-scale results only, it is difficult to predict the effects of physical disturbance on predation intensity at the larger scales of whole habitats, populations or communities. At the large scale (>200 m²), net predator impacts were estimated over the stream reach, using a spatially explicit model that accounts, in an additive way, for habitat heterogeneity and patch-specific responses of predators and prey. The relationship between predator impact over the whole reach and hydraulic disturbance differed for the two predators. The predator impact of S. fuliginosa decreased with increasing hydraulic disturbance, as predicted by the harsh-benign hypothesis. There was no directional trend for P. conspersa, however, and maximum predator impact may occur at intermediate disturbance levels. For the prey community in this stream, predation pressure from S. fuliginosa appears to fluctuate directly with the discharge hydrograph, whereas predation from P. conspersa may be more persistent. Flow refugia may play a dual role in the sructure of stream communities by preventing catastrophic mortality of animals (predators and prey) from physical forces during disturbances, and by maintaining (or perhaps increasing) predation pressure. Summing the effects of species interactions in small habitat patches to the larger scale of a whole stream reach indicates that the scale of approach influences the observed patterns and their implied underlying process.     

Predator identity and time of day interact to shape the risk–reward trade-off for herbivorous coral reef fishes

Non-consumptive effects (NCEs) of predators occur as prey alters their habitat use and foraging decisions to avoid predation. Although NCEs are recognized as being important across disparate ecosystems, the factors influencing their strength and importance remain poorly understood. Ecological context, such as time of day, predator identity, and prey condition, may modify how prey species perceive and respond to risk, thereby altering NCEs. To investigate how predator identity affects foraging of herbivorous coral reef fishes, we simulated predation risk using fiberglass models of two predator species (grouper Mycteroperca bonaci and barracuda Sphyraena barracuda) with different hunting modes. We quantified how predation risk alters herbivory rates across space (distance from predator) and time (dawn, mid-day, and dusk) to examine how prey reconciles the conflicting demands of avoiding predation vs. foraging. When we averaged the effect of both predators across space and time, they suppressed herbivory similarly. Yet, they altered feeding differently depending on time of day and distance from the model. Although feeding increased strongly with increasing distance from the predators particularly during dawn, we found that the barracuda model suppressed herbivory more strongly than the grouper model during mid-day. We suggest that prey hunger level and differences in predator hunting modes could influence these patterns. Understanding how context mediates NCEs provides insight into the emergent effects of predator–prey interactions on food webs. These insights have broad implications for understanding how anthropogenic alterations to predator abundances can affect the spatial and temporal dynamics of important ecosystem processes.     

Lack of preference for low-predation-risk habitats in larval damselflies explained by costs of intraspecific interactions

Many studies indicate prey organisms select microhabitats with high structural complexity as a way of reducing risk of predation. We used laboratory experiments to show that damselfly larvae, Ischnura verticalis, suffer higher predation rates from pumpkinseed sunfish in low-density vegetation. However, larvae do not preferentially occupy microhabitats with high vegetation density in either the presence or absence of sunfish; when given a choice, the number of larvae per stem of vegetation was equal across all densities of vegetation. That larvae do not congregate in dense vegetation may reflect costs of aggressive interactions. Results from laboratory experiments indicated larval interactions increase conspicuous behaviours (most notably swimming) and consequently increase fish predation. A subsequent experiment indicated that frequency of larval interactions increases with increased vegetation density when number of larvae/stem is constant. Thus, larval microhabitat selection may reflect a trade-off between reduced risk of predation in areas of high vegetation density, caused by reduced fish foraging ability, and increased aggressive larval interactions, due to decreased proximity of larvae. Copyright 2000 The Association for the Study of Animal Behaviour.     

Effects of predation risk and plant resistance on Manduca sexta caterpillar feeding behaviour and physiology

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Foraging behaviour of Lesser Sheathbills Chionis minor exploiting invertebrates on a sub-Antarctic island

Summary During winter (May through October) many Lesser Sheathbills Chionis minor at Marion Island in the sub-Antarctic were obliged to leave their preferred foraging habitat in penguin colonies to forage for invertebrates on the island's coastal plain. The study describes factors affecting feeding success, time budgets and predation risk of the sheathbills which exploited these small, patchily dispersed prey. The birds appeared to select prey 1 mm in diameter, and ignore smaller, common invertebrates.Sheathbills were highly selective of foraging habitat. During 17 censuses made through the winter, 97% of the 1,504 birdsightings were at only eight of the 19 available vegetation types. Multiple regression analysis revealed that prey density was the most important criterion in habitat preference, followed by plant canopy height and distance of the habitat from the sea. These variables accounted for 78% of the variance of habitat use. Focal-animal observations in a sample of habitats showed that feeding success was correlated with prey density and distance from the sea. Tall vegetation impeded the locomotion and foraging of sheathbills. The sheathbills reduced predation risk from skuas Catharacta lonnbergi and travelling time by foraging near the shore. The spatial distribution of prey within vegetation types was apparently unimportant in habitat selection.During winter 83% of the sheathbills in the study foraged communally and 98% roosted communally. Flocks occurred only on good quality habitat and flocking probably facilitated habitat selection. Feeding success increased initially with increasing flock size but decreased in flocks greater than 15 birds, which was attributed to localized prey deletions. The sheathbills spent 88% of the daytime foraging; and feeding, looking around and walking comprised 99% of foraging time. Feeding time increased with increasing flock size, looking around decreased but walking was unaffected. Aggression was rare, was unaffected by flock size and did not significantly affect feeding. A probability model showed that sheathbills could greatly reduce predation risk by flocking but the benefits would not improve much in flocks greater than eight birds.The habitat selection, time budgets and feeding success of adults, subadults and juveniles were very similar.The exploitation of terrestrial invertebrates by sheathbills was interpreted as an expansion of the population's trophic niche to tap an underexploited resource on a species-poor island.     

Balancing predation risk,social interference,and foraging opportunities in backswimmers,<Emphasis Type="Italic"> Notonecta maculata</Emphasis>

Loss of foraging opportunities and intraspecific competition for prey may be important costs of using refuges, because a hiding animal is unable to use or defend its foraging area from conspecific intrusions. Thus, animals should balance antipredator demands with other requirements in deciding when to come out from a refuge after a predators unsuccessful attack. Observations on foraging and social interactions of backswimmers Notonecta maculata suggest that foraging may be costly in terms of intraspecific agonistic interactions. When prey density is low, increasing the probability of finding a prey may require active exploration of a larger area, but this also increases the probability of encountering a competitor. After simulated exposure to predators, unfed bugs resumed feeding positions after a significantly shorter hiding period than recently fed bugs. We hypothesized that hiding time may also be reduced by recent interactions with conspecific competitors, due to an increased perceived need to defend feeding opportunities. Thus, when a predator attack occurred immediately after an agonistic conspecific interaction, backswimmers resumed feeding positions more quickly, and closer to the original position from which they were disturbed, suggesting short-term defense of particular positions. We conclude that when foraging, backswimmers balance the benefits of finding prey with the costs of predation risk and social interference in deciding their foraging strategy.Communicated by P.K. McGregor     

A kinematic investigation into the feeding behavior of the Goliath grouper <Emphasis Type="Italic">Epinephelus itajara</Emphasis>

Goliath grouper Epinephelus itajara suffered significant overfishing in the United States until they were protected from harvest in 1990. As the population recovers, interactions between Goliath grouper and anglers have increased, and are often reported to management agencies as complaints after grouper predation upon hooked fish. Goliath grouper are generally characterized as opportunistic predators capable of consuming a wide variety of prey types; however, minimal data are available regarding the prey capture behavior of this species. Kinematic analyses of adult Goliath grouper feeding events demonstrated the capacity of individuals to modulate feeding behavior based upon the mobility and position of ‘prey’ items. Mobile epibenthic food (tethered swimming fish) elicited larger maximum gapes, faster times to food capture, shorter times to mouth closing, and more rapid total bite durations than food items that were not moving (cut dead bait). Feeding sequences involving mobile food items were characterized by a significantly higher degree of ram feeding behavior, while immobile food elicited primarily suction feeding and were preceded by a slower and closer approach to the food item prior to the onset of mouth opening. The findings are discussed in light of predation upon angled species and demonstrate the ability of Goliath grouper to adjust their feeding strategy based upon prey type and condition. This behavior likely allows for the exploitation of a wide variety of prey and provides an expansive dietary breadth for these opportunistic predators.     

Relative vulnerability of six freshwater gastropods to the leech Nephelopsis obscura (Verrill)

SUMMARY. 1. The vulnerability of six species of freshwater snails to the leech Nephelopsis obscura was determined in laboratory predation experiments.
2. Nephelopsis was unable to prey on prosobranch snails with an operculum, but did consume certain pulmonate species though predation rates were low, ranging from one to two snails per leech per night at 20°C. Apparently, Nephelopsis does not actively select prey, but merely consumes those species most easily handled.
3. Among pulmonates, leeches captured species in the following order: Physa gyrina (Say), Helisoma anceps (Menke). Lymnaea emarginata (Say) and Helisoma trivolvis (Say). Susceptibihty of size classes varied among snail species.
4. Neither environmental structure, such as cobble or macrophytes, nor gastropod escape behaviors, such as clamping to substrates or vertical migration away from bottom-feeding leeches, lowered capture rates by Nephelopsis.
5. In 2 years sampling, only one of seven lakes, however, had appreciable abundances of Nephelopsis. It is therefore suggested that the rarity, low feeding rates, and lack of strong gastropod preference limit the effect of Nephelopsis on field gastropod distributions.     

Effects of multiple vertebrate predators on grasshopper habitat selection: trade-offs due to predation risk, foraging, and thermoregulation

Predation risk can influence habitat use and activity of potential prey. I explored how the risk of predation by vertebrates influenced the behavior of grasshoppers. I monitored the height in vegetation and the frequency of resting, moving, and feeding behaviors of both tethered and free-ranging grasshoppers under exposure to various predators. Grasshoppers protected from birds remained high in the vegetation, while those protected only from small mammals and lizards remained low in the vegetation. Grasshoppers exposed to all predators occupied an intermediate height. Lower positions in the vegetation were associated with cooler thermal conditions, lower feeding rates, and lower food availability. My results are consistent with the hypothesis that grasshoppers utilize different microhabitats to balance the trade-off between reducing mortality from predators and experiencing greater food availability, and warmer conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Foraging strategies of an insectivorous marsupial,Sminthopsis youngsoni (Marsupialia: Dasyuridae), in Australian sandridge desert

This study investigated the effects of predation risk, dune position and microhabitat on foraging of the Lesser Hairy‐footed Dunnart Sminthopsis youngsoni, a small insectivorous marsupial, in the Simpson Desert of western Queensland. The intensity of foraging was assessed by establishing feeding stations (dishes containing mealworms) in open and bush microhabitats at three levels on sand dunes, and recording the numbers of prey taken by dunnarts from the stations after nightly bouts of foraging. Risk of predation was manipulated by provision of artificial illumination at alternate feeding stations on each of five occasions when trials were run. The numbers of mealworms left after feeding bouts varied inconsistently across trials, providing little evidence that dunnarts respond to habitat or predation risk while foraging. These results contrast sharply with studies of arid zone rodents, where foraging is usually sensitive to both predation risk and resource distribution. We suggest that S. youngsoni forages equally in all habitats of its sandridge environment, and experiences relatively low risk of predation whilst doing so.     

Difference in response by two cyprinid species to predatory threat from the nocturnal catfish Silurus asotus

Response to predators may not be identical between different prey species with different life histories and body sizes, particularly when the threat of predation is not great. To clarify this hypothesis, we introduced two prey species (10 Japanese dace, Tribolodon hakonensis, and 10 pale chub, Zacco platypus) into each experimental pond (in total, 8 ponds×4 trials) in which benthic algae had been allowed to grow. The presence or absence of Far Eastern catfish, Silurus asotus, and a refuge for prey fish was used to produce four treatments. The presence of catfish and/or a refuge did not affect either the feeding behavior or growth rate of Japanese dace. In contrast, when catfish were present and no refuge was available, the incidence of bottom feeding for pale chub greatly decreased. Pale chub growth rate was low when catfish were present and a refuge was available, indicating that pale chub spent more of their time in the refuge and lost opportunities of acquiring food. Japanese dace can reach a threshold size at which the prey are safe from predation, but pale chub cannot, and this may explain the differences in response to predators of the two species.