Trait-mediated interactions: influence of prey size, density and experience

1. The role of non-consumptive predator effects in structuring ecological communities has become an important area of study for ecologists. Numerous studies have shown that adaptive changes in prey in response to a predator can improve survival in subsequent encounters with that predator. 2. Prey-mediated changes in the shapes of predators' functional response surfaces determine the qualitative predictions of theoretical models. However, few studies have quantified the effects of adaptive prey responses on the shape of predator functional responses. 3. This study explores how prey density, size and previous predator experience interact to change the functional response curves of different-sized predators. 4. We use a response surface design to determine how previous exposure to small or large odonate predators affected the short-term survival of squirrel tree frog (Hyla squirella) tadpoles across a range of sizes and densities (i.e. the shape of odonate functional response curves). 5. Predator-induced tadpoles in a given size class did not differ in shape, although induction changed tadpole behaviour significantly. Induced tadpoles survived better in lethal encounters with either predator than did similar-sized predator-naive tadpoles. 6. Induction by either predator resulted in increased survival with both predators at a given size. However, different mechanisms led to increased survival for induced tadpoles. Attack rate for the small predators, whereas handling time increased for the large predators.     

Epibenthic gastropods of the middle Florida keys: the role of habitat and environmental stress on assemblage composition

A survey of epibenthic prosobranch gastropods was undertaken in both seagrass and hard substratum (coral or old reef rock) habitats on opposite sides of the Florida Keys (Florida Bay and Hawk Channel) to compare faunal differences attributable to differences in the above two habitats and environments. Additionally, two data sets (26 continuous months) of daytime dissolved oxygen, surface salinity and water temperature from Florida Bay (Long Key) and Hawk Channel (Key Largo) environments were compared to determine differences that might constitute environmental stresses likely to affect the fauna. The above data were collected to determine if several hypotheses concerning effects of stress on organisms, assemblage, community and faunal composition were consistent with data on assemblage structure. These hypotheses were that: (1) stress should reduce the average size of organisms; (2) shorten food chains; (3) reduce predation intensity; (4) reduce species richness and diversity; and (5) increase the relative abundance of predator-susceptible ancestral species (i.e. Archaegastropoda). Water quality data suggest that the two most likely forms of stress in deeper (>1 m) areas of Florida Bay adjacent to the Keys are cold water temperatures associated with winter cold fronts and low predawn oxygen associated with warm summer temperatures, high salinity, and periodic algal and seagrass drift buildups. Seagrass sites had high population densities and low diversity due to the dominance of Astraea americana Gmelin (American star shell) in Florida Bay and Modulus modulus L. in Hawk Channel seagrass habitats. Florida Bay sites had high species richness on a small spatial scale, but Hawk Channel sites had more species and greater encounter rates of new species on a larger scale. Predawn oxygen measurements taken during July in four habitats were positively correlated with prosobranch species richness and diversity. Faunal data, analysed on a population density basis, fit the above hypotheses of body size, trophic level, and evolutionary age of the species. Attempts to measure predation on an experimental prosobranch (A. americana) were unsuccessful but a tethering experiment with a sea urchin (Echinometra lucunter L.) indicated higher predation in the less stressful Hawk Channel than Florida Bay hard substratum sites. Stress appears to reduce the abundance of higher trophic levels (both prosobranch and finfish predators) resulting in the dominance of ancestral forms not adapted to predation but tolerant of environmental stress. Eutrophication or increased oxygen demands in Florida Bay could result in further species richness and diversity declines.     

Response of predators to prey abundance: separating the effects of prey density and patch size

We tested the relative and combined effects of prey density and patch size on the functional response (number of attacks per unit time and duration of attacks) of a predatory reef fish (Cheilodactylus nigripes (Richardson)) to their invertebrate prey. Fish attacked prey at a greater rate and for longer time in large than small patches of prey, but large patches had naturally greater densities of prey. We isolated the effects of patch size and prey density by reducing the density of prey in larger patches to equal that of small patches; thereby controlling for prey density. We found that the intensity at which fish attacked prey (combination of attack rate and duration) was primarily a response to prey density rather than the size of patch they occupied. However, there was evidence that fish spent more time foraging in larger than smaller patches independent of prey density; presumably because of the greater total number of prey available. These experimental observations suggest that fish can distinguish between different notions of prey abundance in ways that enhance their rate of consumption. Although fish may feed in a density dependent manner, a critical issue is whether their rate of consumption outstrips the rate of increase in prey abundance to cause density dependent mortality of prey.     

Variability in the numbers of post-settlement King George whiting (Sillaginidae: Sillaginodes punctata, Cuvier) in relation to predation, habitat complexity and artificial cage structure

The importance of predation by fish in altering abundances of juvenile King George whiting (Sillaginodes punctata) was examined at multiple locations in Port Phillip Bay, Australia, by manipulating the numbers of piscivorous fish in unvegetated sand and seagrass habitats using cages. Additional information regarding the local abundances of, and habitat use by, the most common piscivorous fish, Western Australian salmon (Arripidae: Arripis truttacea, Cuvier), was gathered using netting surveys and underwater video. Regardless of habitat, abundances of S. punctata were similar in partial cages and uncaged areas. In unvegetated sand, S. punctata were more abundant inside cages than partial cages or uncaged areas. In seagrass, there was no difference in the numbers of S. punctata between caging treatments. Patterns in abundances of S. punctata between cage treatments in each habitat were consistent between sites, but the relative difference in the abundances of S. punctata between habitats was site specific. Abundances of A. truttacea varied significantly between sites, and they consumed a variety of epibenthic fishes including atherinids, clupeids, gobiids, syngnathids and pleuronectids. At one site in Port Phillip Bay (Blairgowrie), A. truttacea occurred more commonly in patches of unvegetated sand than seagrass. Over unvegetated sand, abundances of A. truttacea varied little between partial cages and uncaged areas. The numbers of S. punctata varied between caging treatments and habitats in a manner that was consistent with a model whereby seagrass interferes with foraging by predatory fish and provides juvenile fish with a refuge from predation. The almost total absence of A. truttacea in seagrass habitats and the lack of S. punctata in their diets implies, however, that patterns in S. punctata in seagrass/unvegetated sand mosaics are driven by processes other than direct predation.     

Predation by Neomysis mercedis: effects of temperature, Daphnia magna size and prey density on ingestion rate and size selectivity

1. Neomysis mercedis predation rates on Daphnia magna were determined under laboratory conditions. There were generally no consistent differences between the number of Daphnia ingested at 10 and 14°C. 2. At each temperature, the number of prey consumed increased with mysid size and decreased with Daphnia size. 3. For small prey the relationship between ingestion rate and prey density represented a Type II functional response. However, for larger prey there was no significant relationship between density of prey and consumption by mysids. 4. The pattern of size-selective predation by Neomysis was studied to test the optimal foraging hypothesis. For prey populations with mixed size classes, the smallest size of prey was consumed most frequently but intermediate size prey provided the greatest biomass. These observations are contrary to our predictions based on calculations of profitability of different sizes of prey.     

Claw morphology, prey size selection and foraging efficiency in generalist and specialist shell-breaking crabs

Claw morphology, and claw-closing forces of four species of intertidal crabs from San Juan Island, Washington were compared and related these findings were related to prey size selection, shell breaking times and total handling times on their snail prey, Littorina sitkana Philippi. Two functional groups of crabs emerged: generalists and specialists on hard-shelled prey. The generalist, Hemigrapsus nudus (Dana), has an omnivorous diet and possesses weak claws with small, fine denticles and mechanical advantage (MA) of the claw's lever system <0.3, while the specialists, Lophopanopeus bellus (Stimpson), Cancer oregonensis (Dana) and C. productus (Randall), consume hard-shelled prey and possess large, powerful claws with broad, blunt molars and MA>0.3. The claws of the generalist, H. nudus, exhibited weaker claw closing forces (5 N) than those of similar sized specialists (>12 N). When crabs of similar weight were offered four size categories of Littorina sitkana, the generalist, Hemigrapsus nudus, exhibited a consistent preference for the smallest size categories, while the three specialists attacked all size classes offered. Hemigrapsus nudus took significantly longer (134 s) than the specialists (30–52 s) to break open a 4 mm L. sitkana. This difference in shell-breaking time between the generalist and the specialists increased with increasing prey size. The rate of successful attacks on increasingly larger L. sitkana decreased with prey size in the generalist (70% on 4 mm, 37% on 6 mm, and 0% on 8 mm snails), but remained high in the specialists (70–100%). Strength limitation of the claws is the best hypothesis to explain the avoidance of large snails by the generalist, H. nudus.     

Post-settlement effects of habitat type and predator size on cannibalism of glaucothoe and juveniles of red king crab Paralithodes camtschaticus

Postlarval (glaucothoe) and juvenile (first crab stage, C1) red king crab Paralithodes camtschaticus actively select structurally complex substrata for settlement. Such habitats may provide them with shelter from predation during critical early stages. We tested this hypothesis by placing glaucothoe and juvenile crab in aquaria with or without natural or artificial habitats, and with or without predators (1-3-year-old red king crab) of two different sizes. Predators caused increased mortality of glaucothoe, but predator size, habitat presence and habitat type had no effect on survival. Predators caused significant mortality of C1 crabs in the absence of habitat, and mortality was inversely related to predator size. Density of glaucothoe on habitats was similar with or without predators, but density of C1 crab on habitats was higher than that of glaucothoe, and increased in the presence of large predators. Active selection for complex substrata by settling glaucothoe does not reduce cannibalism, but may pre-position them for improved survival after metamorphosis. In contrast, juvenile crabs modify their behavior to achieve higher densities in refuge habitats, which tends to dampen the effect of predation. These survival strategies may have evolved to compensate for the greater risk of predation in open habitats.     

Microhabitat use by a post-settlement stage estuarine fish: evidence from relative abundance and predation among habitats

Seagrass beds provide food and shelter for many fish species. However, the manner in which fishes use seagrass bed habitats often varies with life stage. Juvenile fishes can be especially dependent on seagrass beds because seagrass and associated habitats (drift macroalgae) may provide an effective tradeoff between shelter from predation and availability of prey. This study addressed aspects of habitat use by post-settlement pinfish, Lagodon rhomboides (Linneaus), an abundant and trophically important species in seagrass beds in the western North Atlantic and Gulf of Mexico. Abundance of post-settlement fish in seagrass beds was positively related to volume of drift macroalgae, but not to percent cover of seagrass, indicating a possible shelter advantage of the spatially complex algae. Tethering experiments indicated higher rates of predation in seagrass without drift macroalgae than in seagrass with drift macroalgae. Aquarium experiments showed lower predation with higher habitat complexity, but differences were only significant for the most extreme cases (unvegetated bottom, highest macrophyte cover). Levels of dissolved oxygen did not differ between vegetated and unvegetated habitats, indicating no physiological advantage for any habitat. Seagrass beds with drift macroalgae provide the most advantageous tradeoff between foraging and protection from predation for post-settlement L. rhomboides. The complex three-dimensional shelter of drift macroalgae provides an effective shelter that is embedded in the foraging habitat provided by seagrass. Drift macroalgae in seagrass beds is a beneficial habitat for post-settlement L. rhomboides by reducing the risk of predation, and by providing post-settlement habitat within the mosaic (seagrass beds) of adult habitat, thus reducing risks associated with ontogenetic habitat shifts.     

Predation risk and habitat selection in the persistence of a remnant caribou population

Summary A small caribou herd of 24–77 animals resided on Pic Island (1138 ha) in Lake Superior from 1976 to 1984. Most of the caribou populations on the adjacent mainland had gone extinct earlier in this century. We tested three hypotheses for the persistence of this remanant population: (1) there was more forage available on the island than the mainland, (2) the animals on the island were isolated from white-tailed deer and did not develop the meningeal worm infection, and (3) there was less predation by wolves on the island. Forage was more abundant on the mainland than on Pic Island. The eggs of meningeal worms were not found in the feces of deer on the mainland or Pic island. Wolves seldom visited the island and when they did there was escape habitat available for the caribou. We concluded that the herd persisted because of this reduced predation risk and that the animals were prepared to select a reduced variety and phytomass of forage to remain for long periods in the relatively safe island habitat. When the animals did visit the mainland to feed they sclected forbs that provided a large bite size. On the island food supplies were too meager to select plants that gave a large bite size and the caribou spent long intervals feeding. Caribou by using habitats with a large phytomass and selecting for large bite size should minimize their time feeding which would allow them more time to watch for predators.     

Diet choice in an omnivorous salt-marsh crab: different food types, body size, and habitat complexity

Studies of diet choice by omnivores have the potential to form conceptual links between studies of diet choice by herbivores, frugivores, detritivores, and predators. We examined diet choice in the omnivorous salt marsh crab Armases cinereum (=Sesarma cinereum (Grapsidae)) in a series of laboratory experiments. Armases is sexually dimorphic, with larger males having relatively larger claws than females. In a growth experiment, an invertebrate diet supported better growth than any other single diet; however, growth also occurred on single diets of mud, leaf litter or fresh leaves. Mixed diets provided the best growth. If alternative foods were available, consumption of leaf litter and fresh leaves decreased, but these items were not dropped from the diet completely. In contrast, consumption of invertebrate prey was not affected by the availability of alternative foods. In a predation experiment, crustacean prey (an amphipod and an isopod) were more vulnerable to predation by Armases than were two small gastropod species. Only large male Armases were able to consume large numbers of gastropods. Environmental structure (plant litter or litter mimics) reduced predation rates, especially on crustaceans, which actively utilized the structure to hide from predators. Armases consumes a mixed diet because several factors (prey physical defenses, avoidance behavior of prey, growth benefits of a mixed diet) favor omnivory over a specialized diet. Similar factors may promote minor amounts of “omnivory” by species generally considered to be herbivores, frugivores, detritivores, and predators.     

Predation by the salt marsh killifish Fundulus heteroclitus (L.) in relation to prey size and habitat structure: Consequences for prey distribution and abundance

Laboratory feeding preference experiments show that the maximum size of prey eaten (the snail Melampus bidentatus (Say) and the amphipod Orchestia grillus (Bosc)) increases with increasing size of the predator, Fundulus heteroclitus (L.). Melampus > 7 mm in shell height escape predation by even the largest killifish. In the laboratory, consumption of prey is reduced in high marsh habitat relative to low marsh, particularly in the case of larger fish. Low marsh has few grass stems per unit area, while high marsh is considerably more complex, with dense small stems providing cover for prey and reducing successful fish hunting. The population of Melampus in low marsh within Great Sippewissett salt marsh consists mainly of large snails but this size is rare in high marsh. The size-distribution is inverse for Orchestia, with large amphipods more abundant in high marsh. The construction of fences excluding Fundulus from the marsh surface led to low marsh size-distributions of Melampus and Orchestia resembling those of high marsh, in agreement with the laboratory results. Killifish predation seems to be an important factor regulating the abundance and size-distribution of the two prey species in the two marsh habitats.     

Interactive effects of habitat selection, food supply and predation on recruitment of an estuarine fish

 Seagrass meadows are often important habitats for newly recruited juvenile fishes. Although substantial effort has gone into documenting patterns of association of fishes with attributes of seagrass beds, experimental investigations of why fish use seagrass habitats are rare. We performed two short-term manipulative field experiments to test (1) the effects of food supply on growth and densities of fish, and (2) effects of predation on the density and size distribution of fish recruits, and how this varies among habitat types. Experiments were conducted in Galveston Bay, Texas, and we focused on the common estuarine fish, pinfish Lagodon rhomboides. In the first experiment, replicate artifical seagrass and sand plots were either supplemented with food or left as controls. Recruitment of pinfish was significantly greater to seagrass than sand habitats; however, we detected no effect of food supplementation on the abundance of recruits in either habitat. Pinfish recruits in artifical seagrass grew at a significantly faster rate than those in sand habitats, and fish supplemented with food exhibited a greater growth rate than controls in both sand and artifical grass habitats. In our second experiment, we provided artificial seagrass and sand habitats with and without predator access. Predator access was manipulated with cages, and two-sided cages served as controls. Recruitment was significantly greater to the cage versus cage-control treatment, and this effect did not vary between habitats. In addition, the standard length of pinfish recruits was significantly larger in the predator access than in the predator exclusion treatment, suggesting size-selective predation on smaller settlers or density-dependent growth. Our results indicate that the impact of predation on pinfish recruits is equivalent in both sand and vegetated habitats, and thus differential predation does not explain the higher recruitment of pinfish to vegetated than to nonvegetated habitats. Since predators may disproportionately affect smaller fish, and a limited food resource appears to be more effectively utilized by fish in vegetated than in unvegetated habitats, we hypothesize that pinfish recruits may select vegetated habitats because high growth rates allow them to achieve a size that is relatively safe from predation more quickly. Received: 10 October 1996 / Accepted: 5 April 1997     

Predator foraging success and habitat complexity: quantitative test of the threshold hypothesis

Summary Numerous studies have demonstrated a negative relationship between increasing habitat complexity and predator foraging success. Results from many of these studies suggest a non-linear relationship, and it has been hypothesised that some threshold level of complexity is required before foraging success is reduced significantly. We examined this hypothesis using largemouth bass (Micropterus salmoides) foraging on juvenile bluegill sunfish (Lepomis macrochirus) in various densities of artificial vegetation. Largemouth foraging success differed significantly among the densities of vegetation tested. Regression analysis revealed a non-linear relationship between increasing plant stem density and predator foraging success. Logistic analysis demonstrated a significant fit of our data to a logistic model, from which was calculated the threshold level of plant stem desity necessary to reduce predator foraging success. Studies with various prey species have shown selection by prey for more complex habitats as a refuge from predation. In this stydy, we also examined the effects of increasing habitat complexity (i.e. plant stem density) on choice of habitat by juvenile bluegills while avoiding predation. Plant stem density significantly effected choice of habitat as a refuge. The relationship between increasing habitat complexity and prey choice of habitat was found to be positive and non-linear. As with predator foraging success, logistic analysis demonstrated a significant fit of our data to a logistic model. Using this model we calculated the threshold level of habitat complexity required before prey select a habitat as a refuge. This density of vegetation proved to be considerably higher than that necessary to significantly reduce predator foraging success, indicating that bluegill select habitats safe from predation.Implications of these results and various factors which may affect the relationships described are discussed.     

Variability in early post-settlement mortality in intertidal mussels and the role of size at settlement

Early post-settlement mortality is difficult to measure in mobile benthic taxa such as mussels. Field studies typically employ settlement collectors that often contain empty shells of dead individuals, which are usually disregarded. Here, empty shells of pediveligers, i.e. recent settlers that died before beginning to grow as juveniles, were used to assess temporal and spatial patterns of early mortality in mussels, whether size at settlement determined mortality, and whether mortality was density-dependent. This study took place at two intertidal sites in a large harbour in central New Zealand where monthly cohorts of mussels were collected over a 2-year period. Monthly mortality varied substantially, ranging from 0% to 42.6%, but not across the two sites that were 27?km apart. Although there was no density-dependent mortality evident within cohorts, the proportion of pediveligers which were dead on collection was highly positively correlated across sites. Live settlers were on average larger than dead settlers, and the size distribution of dead settlers was shifted toward smaller sizes compared to live settlers. At the extremes, of all settlers with shell length of 260?µm or less, 72% were dead on collection. By contrast, for settlers 310?µm or greater in length, 6% were dead. Together, these results (1) suggest that in this system size at settlement may be an important determinant of very early, peri-metamorphic mortality in mussels, with smaller settlers at higher risk, and (2) demonstrate the value in evaluating dead pediveligers for assessing natural early post-settlement mortality in mussels.     

Distribution of tetrodotoxin in the xanthid crab (Atergatis floridus) collected in the coastal waters of Kanagawa and Wakayama Prefectures

The objective of this investigation was to study the distribution of tetrodotoxin (TTX) in the xanthid crab (Atergatis floridus) found in the coastal waters of Kanagawa and Wakayama Prefectures of Japan using mouse assay methods. We used 32 crab samples (18 males and 14 females) and toxicity was analyzed on 13 parts of the body of each sample. The muscle of chelipeds was found to be toxic in all the samples with a wide range of toxicity (5–237 MU/g), whereas the toxicity in the muscle of the cephalothorax was found to be non-toxic (below detectable limit) in all the samples [Narita, H., Watanabe, K., Baba, K., Ohgami, H., Ai, T.K., Igarashi, Y., Nara, M., Noguchi, T., Hashimoto, K., 1987. The toxicity of digestive gland of trampet shells inhabiting the coast of Shizuoka Prefecture. J. Food Hyg. Soc. Jpn. 28, 115–118.]. Further investigation of different parts of the chelipeds indicated that the muscle of the palm and carpus are usually toxic and that of merus and ischium are almost non-toxic. Toxicity of the muscles of palm ranged between 7 and 52 MU/g, whereas toxicity of the muscle of ischium was below detectable limit. Results from our study indicate clear contrast in the distribution of tetrodotoxin in muscles of different parts of the xanthid crabs, plausibly due to some inherent physiological mechanism. Further investigation is necessary to understand the mechanism responsible for such contrast.     

Predation on reef-building corals: multiscale variation in the density of three corallivorous gastropods, Drupella spp.

 Feeding aggregations of three corallivorous gastropods, Drupella cornus, D. fragum and D. rugosa, have caused considerable coral damage on reefs across the Indo-West Pacific. Distribution and abundance of these three species were explored at Lizard Island, Great Barrier Reef, to determine within-reef variations in density, and spatial relationships between Drupella and their prey corals. The scales of greatest variation were between reef habitats (combinations of exposure and depth) and individual coral colonies. Density varied 12-fold among four habitats: exposed crests (2.55/m²), exposed slopes (0.22/m²), sheltered crests (0.34/m²) and sheltered slopes (2.07/m²). Species composition also varied markedly between habitats. Individuals were highly aggregated, usually forming small clusters (<10 individuals) on live coral colonies and other substrata, and occasional large aggregations of=200 to>2000. Five basic tenets for sampling Drupella are established, based on patterns of variation in density and species composition, and small-scale habitat use. Accepted: 8 February 1999     

Feeding response of a benthic copepod to ciliate prey type,prey concentration and habitat complexity

1. Protozoans are important consumers within microbial food webs and, in turn, they represent potential prey for small metazoans. However, feeding interactions within these food webs are rarely characterised and this is especially true for freshwater sediments. 2. We aimed to quantify the feeding links between a freshwater meiofaunal copepod and ciliates in two laboratory experiments. The first experiment addressed the response of Eucyclops serrulatus towards ciliate density and type (two ciliate species of the same genus differing in terms of body size). A second experiment assessed the effect of habitat structure on feeding rates by introducing different structural complexity into the feeding arena. In contrast to the first experiment, which was run only for one time period, this experiment also tested three different total feeding times (4, 7 and 9 h). 3. Eucyclops serrulatus exhibited high ingestion rates, with 3–69 ciliates copepod^?1 h^?1 consumed depending on food concentration, food type and habitat complexity. Copepods exhibited a preference for the smaller ciliate when total ciliate concentration was low, but selected both ciliates equally when food concentrations were medium or high. However, at very high food concentration, Eucyclops preferred the larger ciliate (which was 1/3 of its own body size), suggesting that the longer handling times of the larger prey are rewarding when the large prey is present in high numbers. In terms of total numbers consumed, copepods fed on more small ciliates, but in terms of carbon units both ciliates were selected equally when total prey concentration was low or medium. However, copepods derived more carbon from the larger prey at high and very high prey concentrations (up to 0.7 μgC out of a maximum of 1.1 μgC copepod^?1 h^?1). Habitat complexity influenced the feeding of copepods when it was observed over time. 4. The copepod–ciliate link is well known from the pelagic zone of both marine and freshwater habitats. We have shown its potential importance within the benthos, where it can be influenced by food identity, food quantity and possibly by habitat complexity.     

Differential effects of habitat complexity,predators and competitors on abundance of juvenile and adult coral reef fishes

Greater structural complexity is often associated with greater abundance and diversity, perhaps because high complexity habitats reduce predation and competition. Using 16 spatially isolated live-coral reefs in the Bahamas, I examined how abundance of juvenile (recruit) and adult (non-recruit) fishes was affected by two factors: (1) structural habitat complexity and (2) the presence of predators and interference competitors. Manipulating the abundance of low and high complexity corals created two levels of habitat complexity, which was cross-factored with the presence or absence of resident predators (sea basses and moray eels) plus interference competitors (territorial damselfishes). Over 60 days, predators and competitors greatly reduced recruit abundance regardless of habitat complexity, but did not affect adult abundance. In contrast, increased habitat complexity had a strong positive effect on adult abundance and a weak positive effect on recruit abundance. Differential responses of recruits and adults may be related to the differential effects of habitat complexity on their primary predators. Sedentary recruits are likely most preyed upon by small resident predators that ambush prey, while larger adult fishes that forage widely and use reefs primarily for shelter are likely most preyed upon by large transient predators that chase prey. Increased habitat complexity may have inhibited foraging by transient predators but not resident predators. Results demonstrate the importance of habitat complexity to community dynamics, which is of concern given the accelerated degradation of habitats worldwide.     

Effect of prey size on attack components of the functional response by Notonecta undulata

The number of encounters per prey, the proportion of encounters resulting in attacks, and the proportion of attacks that were successful were observed while fourth-instar Notonecta undulata nymphs preyed on smaller N. undulata nymphs. While encounters per prey and proportion of encounters resulting in attacks increased with prey size, the proportion of attacks that were successful decreased. The increase in encounter rate per prey was due in part to an increase in the predator's reactive distance to prey as prey size increased. While none of the attack parameters varied significantly with prey density, logarithmic regression of the number of encounters per unit search time on prey density suggested that prey density tends to have a positive effect on encounters per first-instar prey but a negative effect on encounters per second-instar prey. A functional response model is presented that incorporates components of the predator's attack rate as exponential functions of prey density and allows for effects of the time the predator may spend evaluating prey encountered but not attacked and time spent attacking prey not captured. Estimates of the attack parameters derived from the experimental data are used in the model to generate functional response curves for fourth-instar N. undulata preying on first- or second-instar conspecifics. The predicted curve for second-instar prey is typical type II but the curve for firstinstar prey is slightly positively density dependent at low prey densities, i.e., type III.     

Effects of habitat complexity, prey type, and abundance on intraguild predation between larval odonates

Intraguild predation is an important interaction in which predators feed on a shared prey as well as on each other. It occurs frequently between larval odonates in freshwater lentic communities, and understanding the factors influencing this interaction remains an important objective. An experiment carried out in mesocosms and utilizing a factorial design investigated the strength of intraguild interactions between the dragonfly, Sympetrum vicinum, and the damselfly, Enallagma civile, under two levels each of habitat complexity (high or low), prey abundance (high or low) and prey type (amphipods or blackworms). Effects of treatments on size, mortality and emergence of larval odonates were evaluated. Shared prey abundance had little impact on intraguild interactions, affecting only the mass of the intraguild prey E. civile. Habitat complexity affected the size of E. civile damselflies, as length and wet mass were significantly greater in low complexity mesocosms. Prey type seemed to be the most important factor in the experiment, influencing all response variables measured. When shared prey consisted of larger, more active blackworms, intraguild predation decreased, and E. civile damselflies experienced lower mortality, achieved greater length and mass, and had greater emergence success. Results of this study suggest that prey type and habitat complexity can be more important than prey abundance in mediating intraguild predation.