Prey selection by the dobsonfly larva, Protohermes grandis (Megaloptera: Corydalidae)

SUMMARY 1. Prey selection by the dobsonfly larva, Protohermes grandis (Thunberg), was studied in stony riffles of the Yataro River, central Japan. The density, size distribution and taxonomic composition of available prey were assessed for 2 years. In order to know the encounter rate between prey and this ambush predator, prey mobility was also estimated from patterns of colonization of experimentally detiuded stones.
2. Foregut analyses revealed that maximum size of prey eaten increased with larval size, and large larvae did not take the smallest prey in spite of high availability in all seasons.
3. Charnov's (1976) optimal diet model quantitatively predicted such size-selective feeding under seasonally fluctuating conditions of water temperature and prey availability. Larvae maximized the feeding rate by selecting prey.
4. Maximum width of prey eaten coincided approximately with larval mandible length. Mandible size seemed to play an important role in the selection of prey in the optimal size range.     

Prey selection by juvenile cyprinids from running water

SUMMARY. 1. The juveniles of dace ( Leuciscus leuciscus (L.)) and roach ( Rtuihts rtuihis (L.)) and yearling minnows ( Phoxtnus phoxinus (L.)) exhibited generally similar patterns of feeding behaviour and preferences for particular prey types.
2. Factors affecting prey selection included predator and prey size, prey activity and environmental conditions.
3. All three species preferred prey sizes approximately 0.6x their maximum gape.
4. Prey motion was essential to induce attack, and prey were usually selected in proportion to their relative activities.
5. Specialization on one prey type was observed in still water, and was always directed at the most active prey type. In flowing water no specialization was observed, and size selection was suppressed in the juvenile fish. Yearling minnows, however, were size selective in both still and flowing water.
6. The presence of macrophyte cover did not significantly change the pattern of prey selection, but did reduce the predation rale.
7. These results are discussed in the context of previous studies of prey selection and optimal foraging.     

Community-wide character displacement in New Zealand skinks

Aim  To test for community-wide character displacement in New Zealand skinks.
Location  Four small islands in the New Zealand archipelago.
Methods  (1) We conducted a field experiment on a single island to evaluate whether prey size selection is correlated with lizard body size. We pitfall trapped 69 skinks from three species, measured several aspects of their morphology and presented each animal with a variety of different-sized prey in a food choice experiment. (2) We tested whether the morphological characteristics associated with prey size selection were evenly partitioned in four island skink communities using null models.
Results  Prey size selection was associated with skink morphology; larger skinks consumed larger prey. Null model analyses showed support for evenly displaced body sizes on one island, weak support on one island and no support on two islands.
Main conclusions  Results showed mixed support for community-wide character displacement in New Zealand skinks. Differences in body sizes appear to reflect the use of different-sized prey. Even differentiation in body sizes on one island suggests that species coexistence is facilitated by interspecific differences in prey size selection. However, little support was found on other islands, suggesting that other factors, such as interspecific differences in habitat selection and/or diurnal activity patterns, may interact with differences in prey size selection to promote coexistence among New Zealand skinks.     

Prey capture by a benthic coral reef hydrozoan

 The natural diet and prey abundance of the benthic coral reef hydrozoan Nemalecium lighti, a common tropical species, were studied by analysing the gastrovascular contents of polyps. Prey capture was estimated from 10 samples collected at 3-h intervals during a single diel cycle (1–2 September, 1995) in the San Blas Islands (Panamá). Prey size ranged from 5 to 550 μm, with invertebrate larvae being the main contributor to the diet of the species. Prey items were found in 56–88% of the polyps over the entire diel cycle. Gastrovascular contents varied between 0.93 and 2.13 prey items per polyp. These capture incidences are among the highest reported for cnidarian species. Such rates would allow for high production rates for Nemalecium lighti, consistent with reports of the species’ fast growth and high reproduction rates. The observations suggest that some hydrozoans may be active heterotrophic components in coral reef ecosystems. Accepted: 3 January 1999     

Importance of consumptive and non-consumptive prey mortality in a coupled predator–prey system

1. Laboratory experiments were completed to identify the mechanisms by which the predatory flatworm, Dugesia tigrina , imposes mortality on its Aedes aegypti and Daphnia magna prey. Feeding trials were completed in glass microcosms which contained one of three – nine densities of small or large individuals of each prey species.
2. Mortality by Dugesia on small and large Aedes followed a type II functional response, whereas the mortality of Daphnia resembled a type III functional response. Prey mortality imposed by Dugesia consisted of consumptive and non-consumptive elements. Non-consumptive mortality occurred when prey individuals trapped in mucus trails subsequently died but were not ingested.
3. Additional experiments were conducted to quantify consumptive (capture followed by ingestion) and non-consumptive mortality (death not followed by ingestion).
4. Consumptive mortality followed a type II functional response for small and large individuals of both prey species, whereas non-consumptive mortality increased with prey density, although the relationships differed with prey species and size. The non-consumptive mortality of large Daphnia increased at an accelerating rate with prey density and exceeded consumptive mortality at intermediate prey abundances. In contrast, non-consumptive mortality of small Aedes and small Daphnia was lower than consumptive mortality and either increased with prey density at a decelerating (small Aedes ) or accelerating (small Daphnia ) rate.
5. These results suggest that the importance of consumptive and non-consumptive mortality to total prey mortality needs to be considered when modelling predator–prey dynamics.     

Prey vulnerability to a gape-size limited predator: behavioural and morphological impacts on northern pike piscivory

The northern pike ( Esox lucius ) is a selective and important predator in lake ecosystems. Prey size in pike is limited by pike gape size, which is a linear function of pike body length. Here we show that the absolute gape-size limit in pike is greater than previously considered, and that maximum ingestible prey size is limited by prey body depth. Further, we experimentally show that pike prefer shallow-bodied roach before deeper-bodied common bream, and small prey sizes within each prey species. Handling time in pike increases with prey body depth, and since common bream are deeper-bodied than roach, handling time is longer for bream than for roach of the same length, but equal considering body depth. Prey handling time is suggested to be a major cost to the pike, since it increases the risk of losing the prey, as well as exposure to predation, kleptoparasitism and cannibalism. Consequently, prey vulnerability is determined by risk of predation and intraspecific interactions, and behavioural preferences in the pike, and not by pike gape-size limits. The consequences for natural populations is evaluated by analysing size structures of predator and prey fish populations in a eutrophic lake.     

Gastric emptying in Scyliorhinus canicula (L.): a comparison of surface-dependent and non-surface dependent models

The usefulness of both surface-dependent and non-surface-dependent models for describing the gastric emptying process in Scyliorhinus canicula (L.) was examined. In most cases the surface-dependent models produced better fits than did the non-surface-dependent models. The exponential model (non-surface-dependent) only adequately described gastric evacuation in the case of small prey items with thin exoskeletons. On the other hand, the exponential model was clearly better than the linear model in all cases. Prey size had little effect on digestion rate. However, the number of prey items ingested was an important factor influencing digestion.     

Predator gut state and prey detectability using electrophoretic analysis of gut contents

Abstract. I. An investigation of the effects of position of food in the gut (gut state), time since feeding, meal size and environmental temperature on the detection of prey using the electrophoretic separation of proteins with esterase activity was undertaken. The waterbug Notonecta glauca Linn (Hemiptera/Heteroptera) was used as the model predator.
2. Seven easily recognizable gut states can be identified at different times since commencement of a meal.
3. Specific prey esterase 'fingerprint' bands can be detected whilst food remains in the foregut.
4. Meal size and environmental temperature (affecting digestion and assimilation rates) both affect the rate of filling and emptying of the foregut, and hence the time period over which prey proteins can be detected by electrophoresis following a meal. Small meals and high temperatures lead to quite rapid changes in gut state with time and thus reduce the time period over which prey can be identified from gut contents (20 h). Large meals and low temperatures extend this period (48 h).
5. Predator gut state is the most important parameter in prey detectability from gut contents, and from a visual examination of the gut state it is immediately evident whether or not prey detection by electrophoresis will be possible.
6. There is further evidence in support of a twocompartment food processing model for Nofoonecru , and the significance of this type of processing in relation to gut content analysis of sucking predators is discussed.     

A method for in situ estimation of prey selectivity and predation rate in large plankton, exemplified with the jellyfish Aurelia aurita (L.)

A method to estimate predation rates of large predatory zooplankton, such as jellyfish and ctenophores, is outlined. Large plankton size allows direct visual tracking of the predator during the process of foraging. The presented method is novel in the sense that it measures predation rate of a specific individual plankton predator in situ.After prey has been evacuated from the gut of an individual predator, the predator is incubated in situ, and observed by SCUBA-divers who recapture the individual after a defined time. Given that this incubation time is shorter than prey digestion time, predation rate can be calculated as increase in gut content over time. Clearance rates for different prey can be calculated from predation rates and prey concentrations in the water, allowing accurate estimates of prey selectivity. Thus, the problem of unknown feeding history and feeding environment, which can otherwise be a problem in prey selectivity studies of in situ-captured predators, is circumvented. Benefits and limitations of the method are discussed.The method was applied to adult medusae of the common jellyfish Aurelia aurita. A large variation in number of captured prey was detected both among individual jellyfish and among the various oral arms and gastric pouches within individuals. Clearance rates varied strongly with prey type. The medusae selected large crustacean prey (cladocerans and copepods/copepodites) over echinoderm larvae and copepod nauplii. Prey distribution within the medusae indicates that both tentacles and oral arms were used as prey capturing sites. Food passage time from prey capturing organs to gastric pouches was estimated.     

Prey preparation by adult Great Tits Parus major feeding nestlings

Some birds prepare food items before giving them to their nestlings. We studied the relationships between the degree of prey preparation and prey size, nestling age, brood size and time of season. We estimated the degree of preparation of 513 animal prey items, taken by using neck collars, brought to nestling Great Tits Parus major. Prey preparation increased with prey size and decreased as the nestlings grew older, as brood size increased and as the season progressed. Other factors, such as nutrient concentration (through removal of low-quality or deleterious parts) or palatability (considering scaly moth forewings unpalatable), seem also to be important in determining prey preparation. Our results suggest that the degree of prey preparation is a compromise between the benefits gained by the nestlings (ingestion and digestion of prey is facilitated) and the costs to the parents (mainly time allocated to prey preparation).     

A surface-dependent gastric evacuation model for fish

A gastric evacuation curve expresses how fast prey disappear from the stomach, and empirical models are used generally for the relationship between weight of prey remaining ( W_t ) and time ( t ) after a meal. Unfortunately, empirical models are likely to have restricted applicability because their parameters often represent limited biological mechanisms. This paper develops a simple digestion model. The simplest form of the model has four parameters; the digestion velocity (expressing enzymatic breakdown of prey), prey length, initial prey radius and the density of the prey. Two more parameters are included in an extended version; a time-delay before digestion starts and environmental temperature. The approach is based on the assumption that prey digestion is a surface process in that digestive enzymes attack progressively deeper into a prey of known size and shape so that the average digestion rate is proportional to the prey radius r (m). This process is characterized by the digestion velocity d_s (m s ^–1). Unknown parameters are estimated with uncertainty using the maximum likelihood technique. Model evaluation using published data sets demonstrated that the new model is flexible. Prey geometry is incorporated into the model and temperature effects upon gastric evacuation are linked directly to the digestion velocity.     

Foraging habitat determines predator–prey size relationships in marine fishes

Predator–prey size (PPS) relationships are determined by predator behaviour, with the likelihood of prey being eaten dependent on their size relative to that of the consumer. Published PPS relationships for 30 pelagic or benthic marine fish species were analysed using quantile regression to determine how median, lower and upper prey sizes varied with predator size and habitat. Habitat effects on predator foraging activity/mode, morphology, growth and natural mortality are quantified and the effects on PPS relationships explored. Pelagic species are more active, more likely to move by caudal fin propulsion and grow more rapidly but have higher mortality rates than benthic species, where the need for greater manoeuvrability when foraging in more physically complex habitats favours ambush predators using pectoral fin propulsion. Prey size increased with predator size in most species, but pelagic species ate relatively smaller prey than benthic predators. As pelagic predators grew, lower prey size limits changed little, and prey size range increased but median relative prey size declined, whereas the lower limit increased and median relative prey size was constant or increased in benthic species.     

Diet and predation by three leaf-associated stoneflies (Plecoptera) in an Arkansas mountain stream

SUMMARY. 1. Life cycles, food habits, mouthpart morphologies, prey preferences, and predator-prey size relationships were investigated for the three most common stoneflies in leaf packs in the Little Missouri River, Arkansas: Clioperla clio (Newman), Isoperla namata Prison, and Perlesta spp. Each species was univoltine and had a fast seasonal cycle, with extended egg or nymphal diapause, rapid nymphal growth and emergence in spring to early summer.
2. Foregut analysis showed predictable dietary shifts by C. clio and Perlesta spp., from detritus and diatoms in early instars to invertebrate prey in later instars. Isoperla namata fed facultatively on insects, detritus and diatoms, with no conspicuous ontogenetic shifts. The mouthparts of all three species strongly reflected their diet.
3. Electivity analysis indicated opportunistic feeding by the three species, with feeding rarely differing significantly from random.
4. Sizes of stonefly predators and their ingested prey were highly correlated ( P <0.01). However, rather than shifting toward larger prey sizes, growing predators expanded their size thresholds and continued to include numerous small prey items in their diet. Prey choice was governed most directly by prey availability, because stoneflies selected the most abundant groups, irrespective of size or taxon.
5. The likelihood of competition among the three stoneflies for prey was minimized by timing of their life cycles, differential food use and probably the temporary nature of leaf pack habitats.     

Size-dependent prey selection in piscivorous pikeperch Sander lucioperca and Volga pikeperch Sander volgensis shaped by bimodal prey size distribution

Prey size and species selection of pikeperch Sander lucioperca and Volga pikeperch Sander volgensis were investigated in relation to predator size in the shallow Lake Balaton, Hungary. Although their gape sizes were similar, S. lucioperca shifted to piscivory earlier and consumed fewer, but larger, prey than S. volgensis. Prey species preference of the two piscivores also differed. A bimodal prey size distribution resulted in a reclining sigmoid curve for the life span predator size to prey size relationship with inflexion points between 266 and 284 mm predator standard length (L(S) ) in S. lucioperca. In S. volgensis, as well as in S. lucioperca L(S) ≤ 350 mm, prey size increased monotonically with predator L(S) , following a power trend for all prey size variables. Prey depth to predator L(S) relationship varied significantly with prey species and prey number in both piscivores, and prey depth tended to be smaller in predators consuming more than one prey. Both predator species characteristically selected less active, benthic prey fishes in spite of their spiny fin rays, and small- and mid-sized predators selected for small prey. Relatively large prey were also eaten, however, especially by the smallest and largest S. lucioperca.     

Seasonal shifts in predator body size diversity and trophic interactions in size-structured predator-prey systems

1.?Theory suggests that the relationship between predator diversity and prey suppression should depend on variation in predator traits such as body size, which strongly influences the type and strength of species interactions. Prey species often face a range of different sized predators, and the composition of body sizes of predators can vary between communities and within communities across seasons. 2.?Here, I test how variation in size structure of predator communities influences prey survival using seasonal changes in the size structure of a cannibalistic population as a model system. Laboratory and field experiments showed that although the per-capita consumption rates increased at higher predator-prey size ratios, mortality rates did not consistently increase with average size of cannibalistic predators. Instead, prey mortality peaked at the highest level of predator body size diversity. 3.?Furthermore, observed prey mortality was significantly higher than predictions from the null model that assumed no indirect interactions between predator size classes, indicating that different sized predators were not substitutable but had more than additive effects. Higher predator body size diversity therefore increased prey mortality, despite the increased potential for behavioural interference and predation among predators demonstrated in additional laboratory experiments. 4.?Thus, seasonal changes in the distribution of predator body sizes altered the strength of prey suppression not only through changes in mean predator size but also through changes in the size distribution of predators. In general, this indicates that variation (i.e. diversity) within a single trait, body size, can influence the strength of trophic interactions and emphasizes the importance of seasonal shifts in size structure of natural food webs for community dynamics.     

Tadpoles of three common anuran species from Thailand do not prey on mosquito larvae

Tadpoles are often considered to be predators of mosquito larvae and are therefore beneficial for the control of certain disease vectors. Nevertheless, only a few species have actually been recorded to prey on mosquito larvae. The mosquito larvae predation rates of tadpoles of three common Thai anuran species (Bufo melanostictus, Kaloula pulchra and Hylarana raniceps) were experimentally tested. Tadpoles in varying developmental stages were used to assess a size/age effect on the predation rate. In addition, different instars of Culex quinquefasciatus were used in order to assess a prey size effect on the predation rates. All three species failed to show any evidence of mosquito larvae predation. Neither small nor large tadpoles fed on mosquito larvae. Prey size also did not affect predation. Although tadpoles do not feed on mosquito larvae, there may be other direct or indirect inter‐specific interactions that adversely impact the development of larvae in shared habitats with tadpoles.     

The influence of prey size,sediment thickness and fish size on consumption in common sole (Solea solea L.)

This study determined prey consumption in common sole as a function of prey size (0–0.5, 1–1.5, 2–2.5 and 4–5 g), sediment thickness (20 cm and 2 cm) and fish size (50 g, 125 g or 300 g). Prey consumption (in numbers of prey eaten per fish per day) was reduced with increasing prey size and sediment thickness, and was increased with increasing fish size (p < .001 for all factors). All 3 factors showed significant two way interactions (p < .001) when expressed in numbers of prey eaten. Prey consumption decreased with prey size when prey could not escape by burying (2 cm of sediment thickness) irrespective of fish size. We suggest that increasing effort to ingest and handle larger prey played a role. Prey consumption increased with fish size when prey could not bury (2 cm of sediment thickness). However, when prey was able to bury (at 20 cm sediment thickness) prey consumption was similar irrespective of fish size (p < .001 for interaction fish size × sediment). This interaction suggests that with increasing fish size there is an increasing mismatch between foraging adaptation and prey burial depth. This may explain the dominance of crustaceans in the diet of adult common sole in nature, despite the high abundance of polychaetes.     

The effects of hydrological disturbance on the densities of macroinvertebrate predators and their prey in a coastal stream

1. Environmental Stress Models (ESMs) predict that abiotic disturbance or harshness will differentially affect predators and prey. Consumer Stress Models (CSMs) predict that consumers will be relatively more inhibited by disturbance than prey, and therefore predator impacts will be reduced. Conversely, Prey Stress Models (PSMs) predict that prey will be more adversely affected and consequently predator impacts will increase in disturbed habitats. This study compared the relative tolerances of lotic invertebrate predators and their prey to hydrological disturbance in an Australian coastal stream to test the initial predictions of ESMs.
2. Macroinvertebrates were sampled with a suction sampler at monthly intervals and immediately following four high flow events at five sites on the Cumberland River, in south-west Victoria, Australia. Various statistical procedures were used to compare the relative resistance and resilience of predatory and prey taxa to each high flow event.
3. The relative resistances of seven predator and nine prey taxa to four floods over a 12-month period were highly variable between floods and between runs within the same flood. Prey taxa appeared to be more resilient than predators to the largest flood event, but there were no differences in the resilience of predators and prey following smaller floods. If disturbance tolerance is determined by resistance and resilience, then there was no consistent pattern of differential tolerance to floods among invertebrate predators and prey in this system.
4. The variability in the relative tolerances of taxa to different disturbance events makes general predictions about the effects of disturbance on the community-wide impact of predation extremely difficult.     

Molecular scatology as a tool to study diet: analysis of prey DNA in scats from captive Steller sea lions

The DNA of prey present in animal scats may provide a valuable source of information for dietary studies. We conducted a captive feeding trial to test whether prey DNA could be reliably detected in scat samples from Steller sea lions (Eumetopias jubatus). Two sea lions were fed a diet of fish (five species) and squid (one species), and DNA was extracted from the soft component of collected scats. Most of the DNA obtained came from the predator, but prey DNA could be amplified using prey-specific primers. The four prey species fed in consistent daily proportions throughout the trial were detected in more than 90% of the scat DNA extractions. Squid and sockeye salmon, which were fed as a relatively small percentage of the daily diet, were detected as reliably as the more abundant diet items. Prey detection was erratic in scats collected when the daily diet was fed in two meals that differed in prey composition, suggesting that prey DNA is passed in meal specific pulses. Prey items that were removed from the diet following one day of feeding were only detected in scats collected within 48 h of ingestion. Proportions of fish DNA present in eight scat samples (evaluated through the screening of clone libraries) were roughly proportional to the mass of prey items consumed, raising the possibility that DNA quantification methods could provide semi-quantitative diet composition data. This study should be of broad interest to researchers studying diet since it highlights an approach that can accurately identify prey species and is not dependent on prey hard parts surviving digestion.     

The effect of bladderwort (Utricularia) predation on microcrustacean prey

SUMMARY 1. The effects of the carnivorous plant Utricularia ( bladderwort) on its microcrustacean and macroinvertebrate prey were studied under seminatural and natural conditions. The results suggest that Utricularia is a strong interactor in littoral communities that influences its prey populations by direct predation and indirect facilitation.
2. In an 8-week enclosure experiment, effects on prey density were compared in three treatments with (1) U. vulgaris with intact trapbladders, (2) U. vulgaris without bladders and (3) no Utricularia present.
3. Utricularia predation caused a decrease in prey density over time, whereas presence of Utricularia without bladders increased prey density. In the controls without Utricularia , prey density was relatively constant over time.
4. Field samples were collected to quantify predation rates of three Utricularia species on two natural prey populations. Daily consumption rates on prey peaked from mid-July to mid-August for all Utricularia species, but were low in June and September. This pattern was explained mainly by a high number of trapbladders at this time, but also by a slight increase in the number of prey caught per bladder. Per capita prey mortality rates caused by Utricularia were substantial and ranged between 0.14 and 0.43 day⁻¹ for copepods, 0.1–0.27 day⁻¹ for ostracods and 0.04–0.2 day⁻¹ for chydorid cladocerans.
5. Predation and facilitation effects were observed for total prey and separately for epiphytic and benthic prey. Planktonic microcrustaceans showed no response to Utricularia presence.