Jellyfish as Prey: Frequency of Predation and Selective Foraging of Boops boops (Vertebrata,Actinopterygii) on the Mauve Stinger Pelagia noctiluca (Cnidaria,Scyphozoa)

In recent years, jellyfish blooms have attracted considerable scientific interest for their potential impacts on human activities and ecosystem functioning, with much attention paid to jellyfish as predators and to gelatinous biomass as a carbon sink. Other than qualitative data and observations, few studies have quantified direct predation of fish on jellyfish to clarify whether they may represent a seasonally abundant food source. Here we estimate predation frequency by the commercially valuable Mediterranean bogue, Boops boops on the mauve stinger jellyfish, Pelagia noctiluca, in the Strait of Messina (NE Sicily). A total of 1054 jellyfish were sampled throughout one year to quantify predation by B. boops from bite marks on partially eaten jellyfish and energy density of the jellyfish. Predation by B. boops in summer was almost twice that in winter, and they selectively fed according to medusa gender and body part. Calorimetric analysis and biochemical composition showed that female jellyfish gonads had significantly higher energy content than male gonads due to more lipids and that gonads had six-fold higher energy content than the somatic tissues due to higher lipid and protein concentrations. Energetically, jellyfish gonads represent a highly rewarding food source, largely available to B. boops throughout spring and summer. During the remainder of the year, when gonads were not very evident, fish predation switched towards less-selective foraging on the somatic gelatinous biomass. P. noctiluca, the most abundant jellyfish species in the Mediterranean Sea and a key planktonic predator, may represent not only a nuisance for human leisure activities and a source of mortality for fish eggs and larvae, but also an important resource for fish species of commercial value, such as B. boops.     

A demographic approach to evaluating the impact of stressors on bumble bee colonies

1. Natural and anthropogenic stressors threaten the sustainability of bumble bees and evaluating their impact is essential to the stewardship of these valuable pollinators. Demographic modelling provides a framework for testing hypotheses about the impacts of stressors, but it has not previously been applied to bumble bees. 2. I therefore formulated a demographic model for a bumble bee colony and then quantified the impact of two stressors, pesticide exposure and spider predation, by perturbing it with their known effects. 3. By simulating a laboratory exposure of Bombus terrestris L. to dietary imidacloprid (a neonicotinoid insecticide), I tested whether the observed colony decline was explained solely by a toxic effect on the fecundity of the foundress queen. By simulating field observations of B. terricola Kirby, I tested whether predation by crab spiders reduced colony fitness sufficiently to provide an adaptive explanation for avoidance behaviours seen when bumble bees encounter spiders. 4. In B. terrestris, a dose‐appropriate decrease in fecundity predicted the observed colony decline, which implicates this as a principal mechanism of toxicity. In B. terricola, doubling the rate of spider predation reduced a colony's production of new queens by 11%, which implies that spider avoidance is highly adaptive. 5. These analyses illustrate the utility of demographic modelling for quantifying the impacts of stressors on bumble bees. In the future, models of this type could be used to investigate a wider range of stressors and to produce thereby knowledge and tools useful for safeguarding bumble bees and the pollination services that they provide.     

Temporal and spatial variability of pelagic wild fish assemblages around Atlantic bluefin tuna Thunnus thynnus farms in the eastern Adriatic Sea

The abundance and size structure of wild fishes aggregated around the sea‐cages of two commercial Thunnus thynnus farms, including control locations, were assessed and compared over a 1 year period. The T. thynnus farms were located in the eastern Adriatic Sea, offshore of the islands of Ugljan and Bra?. Fish assemblages were evaluated through visual census using scuba at 2 month intervals at two sites within each farm. The data suggest that wild fish assemblages at the study sites differed greatly; 20 species occurred at the Ugljan farm and 17 at the Bra? farm, while only seven species were observed at the control locations. The abundance and diversity of wild fish assemblages were greater at the farms in comparison to control locations. The most abundant families were Sparidae and Belonidae (>80% of aggregated fishes). At both farms, the abundance and diversity of wild fishes were highest during summer, while diversity was lowest in winter and was mainly characterized by schools of bogue Boops boops and garfish Belone belone. Variability was also detected in spatial assemblages between farms; B. boops and B. belone were the most abundant species for the overall study at the Bra? farm, while B. belone and saddled bream Oblada melanura were the most abundant at the Ugljan farm. The settlement also played a significant role in farm‐associated fish assemblages, as both juveniles and advanced juveniles were common residents at farms. The majority of species which settled at the farms belonged to the sparids. Results indicate that aggregations of wild fishes at T. thynnus farms are persistent year‐round, though the assemblage compositions and size structures of dominant species vary in respect to location and season.     

Overdispersion of ant colonies: a test of hypotheses

Summary The nest locations of two ant species in the Colorado Desert are intraspecifically overdispersed. Intraspecific overdispersion has been thought to represent strong intraspecific competition. Here we consider this hypothesis along with three competing hypotheses: microhabitat selection by foundress queens, predation on foundress queens, and predation on established colonies. To test these hypotheses five types of data were collected: (1) the forager population sizes of Veromessor pergandei and Pogonomyrmex californicus, (2) the response of the territory use of V. pergandei to varying levels of food, (3) the encounter rates of conspecifics and other ant species to foundress queens artificially placed near and far from conspecific colonies, (4) predation on colonies as a function of colony spacing, and (5) the relationship between the plant microhabitat at the nest and colony spacing. The results show that established colonies have no apparent selectivity for a particular type of plant microhabitat nor do foundress queens show avoidance or attraction toward conspecifics. V. pergandei workers show only a slight ability to find V. pergandei queens that are artificially placed near their entrances. Certain spiders are the most common ant predators on our study area. Direct observations on spiders indicate that colonies with closer neighbors are not prone to a higher risk of predation. In addition, the estimates of the death rate of workers from a mark-recapture technique indicate that colonies with closer neighbors lose similar numbers of workers as compared to colonies with further neighbors. In favor of the competition hypothesis, the summed size of intraspecific nearest neighbor pairs are larger for colonies that are spaced further apart than those colonies that are spaced closer together. We also develop an index of foraging directionality for the column foraging species V. pergandei. Using this measure, we find that nearest neighbors tend to avoid foraging toward each other. The response of territory use to food levels was tested with experiments involving patches of cracked wheat. These experiments showed that patches away from nearest neighbors were found significantly sooner than patches toward nearest neighbors. The above five sets of data together suggest that resource competition and perhaps queen predation by established colonies account for the intraspecific spatial patterns of these species.     

Predation Risk Avoidance by Terrestrial Amphibians: The Role of Prey Experience and Vulnerability to Native and Exotic Predators

We studied avoidance, by four amphibian prey species (Rana luteiventris, Ambystoma macrodactylum, Pseudacris regilla, Tarichia granulosa), of chemical cues associated with native garter snake (Thamnophis elegans) or exotic bullfrog (R. catesbeiana) predators. We predicted that avoidance of native predators would be most pronounced, and that prey species would differ in the intensity of their avoidance based on relative levels of vulnerability to predators in the wild. Adult R. luteiventris (presumably high vulnerability to predation) showed significant avoidance of chemical cues from both predators, A. macrodactylum (intermediate vulnerability to predation) avoided T. elegans only, while P. regilla (intermediate vulnerability to predation) and T. granulosa (low vulnerability to predation) showed no avoidance of either predator. We assessed if predator avoidance was innate and/or learned by testing responses of prey having disparate levels of prior exposure to predators. Wild‐caught (presumably predator‐exposed) post‐metamorphic juvenile R. luteiventris and P. regilla avoided T. elegans cues, while laboratory‐reared (predator‐naive) conspecifics did not; prior exposure to R. catesbeiana was not related to behavioural avoidance among adult or post‐metamorphic juvenile wild‐reared A. macrodactylum and P. regilla. These results imply that (i) some but not all species of amphibian prey avoid perceived risk from garter snake and bullfrog predators, (ii) the magnitude of this response probably differs according to prey vulnerability to predation in the wild, and (iii) avoidance tends to be largely learned rather than innate. Yet, the limited prevalence and intensity of amphibian responses to predation risk observed herein may be indicative of either a relatively weak predator–prey relationship and/or the limited importance of predator chemical cues in this particular system.     

Riffles as barriers to interpool movement by three cyprinids (Notropis boops,Campostoma anomalum and Cyprinella venusta)

1. The effects of riffles as barriers to movement of stream fish was investigated in a set of eight large outdoor artificial streams. Pools were 183 cm in diameter and 45 cm deep; riffles were 183 cm long and 43 cm wide. Rates of movement of three species of minnows (Cyprinidae) (Campostoma anomalum, Cyprinella venusta and Notropis boops) among pools were measured at four riffle current velocities (0, 15, 30 and 45 cm s^–1), three thalweg depths (10, 50 and 100 mm), two riffle lengths (183 and 549 cm), and with and without the threat of predation. Visual observation and video cameras were used to quantify movement rate. 2. Mean movement rate (percentage of fish crossing a riffle each 30 min) was 18.1% at 0 cm s^–1 and only 1.8 at 45 cm s^–1. Movement rate was 7.2% with no predators present and 20.2% with caged predators in pools. Notropis boops had a lower rate of movement than C. venusta or C. anomalum across all trials. The mean group size (number of individuals crossing a riffle together) was 1.2 fish overall, indicating most movement was by individuals and not groups. Group size was significantly greater only with shallow riffles or under the threat of predation. 3. Overall biotic and abiotic factors in these artificial streams do influence movement rates and may affect movement among pools in natural streams.     

Effects of large Saduria entomon (Isopoda) on spatial distribution of their small S. entomon and Monoporeia affinis (Amphipoda) prey

We performed laboratory experiments to investigate the effects of predator avoidance and numerical effects of predation on spatial distribution of small Saduria entomon (Isopoda) and Monoporeia affinis (Amphipoda), with large S. entomon as predators. The horizontal distribution and mortality of the prey species, separately and together, were studied in aquaria with a spatial horizontal refuge. We also estimated effects of refuge on mortality of small S. entomon and M. affinis by experiments without the refuge net. In addition, we investigated whether predation risk from large S. entomon influenced the swimming activity of M. affinis, to clarify the mechanisms behind the spatial distribution. Both small S. entomon and M. affinis avoided large S. entomon. The avoidance behaviour of M. fffinis contributed about 10 times more to the high proportion in the refuge than numerical effects of predation. Due to the low mortality of small S. entomon the avoidance behaviour of this species was even more important for the spatial distribution. The combined effect of avoidance behaviour and predation in both species was aggregation, producting a positive correlation between the species in density. M. affinis showed two types of avoidance behaviour. In the activity experiments they reduced activity by 36% and buried themselves in the sediment. In the refuge experiments we also observed avoidance behaviour with the emigration rate from the predator compartment being twice the immigration rate. The refuge did not lower predation mortality in M. affinis, probably due to the small scale of the experimental units in relation to the mobility of the species. Predation mortality in small S. entomon was higher in absence of a refuge and especially high in absence of M. affinis.     

Predator effects on prey population dynamics in open systems

Animal population dynamics in open systems are affected not only by agents of mortality and the influence of species interactions on behavior and life histories, but also by dispersal and recruitment. We used an extensive data set to compare natural loss rates of two mayfly species that co-occur in high-elevation streams varying in predation risk, and experience different abiotic conditions during larval development. Our goals were to generate hypotheses relating predation to variation in prey population dynamics and to evaluate alternative mechanisms to explain such variation. While neither loss rates nor abundance of the species that develops during snowmelt (Baetis bicaudatus) varied systematically with fish, loss rates of the species that develops during baseflow (Baetis B) were higher in streams containing brook trout than streams without fish; and surprisingly, larvae of this species were most abundant in trout streams. This counter-intuitive pattern could not be explained by a trophic cascade, because densities of intermediate predators (stoneflies) did not differ between fish and fishless streams and predation by trout on stoneflies was negligible. A statistical model estimated that higher recruitment and accelerated development enables Baetis B to maintain larger populations in trout streams despite higher mortality from predation. Experimental estimates suggested that predation by trout potentially accounts for natural losses of Baetis B, but not Baetis bicaudatus. Predation by stoneflies on Baetis is negligible in fish streams, but could make an important contribution to observed losses of both species in fishless streams. Non-predatory sources of loss were higher for B. bicaudatus in trout streams, and for Baetis B in fishless streams. We conclude that predation alone cannot explain variation in population dynamics of either species; and the relative importance of predation is species- and environment-specific compared to non-predatory losses, such as other agents of mortality and non-consumptive effects of predators. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Lunar Philia in a Nocturnal Primate

The influence of moonlight on behavior has been well documented for many nocturnal mammals, including rodents, lagomorphs, badgers and bats. These studies have consistently shown that nocturnal mammals respond to bright moonlight by reducing their foraging activity, restricting their movement, and reducing their vocalizations. Lunar phobia among nocturnal mammals is generally believed to be a form of predator avoidance: numerous studies indicate that predation increases during moonlit nights. A study I conducted at Tangkoko Nature Reserve in Sulawesi, Indonesia, demonstrates that spectral tarsiers, (Tarsius spectrum), are not lunar phobic, but are lunar philic; they become more active during full moons. During full moons, spectral tarsiers increased foraging, decreased resting, increased travel (distance traveled per unit time, nightly path length, and home range size), increased the frequency of group travel and decreased the frequency of olfactory communication. I explore several potential hypotheses to account for the lack of lunar phobia and potential increased risk of predation resulting from this unusual behavior. Two hypotheses that may account for the behavior are that: 1) foraging efficiency increases during full moons and outweighs the increased risk of predation, and 2) predation risk is not greater during full moons. Instead, predation risk increases during new moons.     

Unveiling trade‐offs in resource selection of migratory caribou using a mechanistic movement model of availability

Habitat selection is a multi‐level, hierarchical process that should be a key component in the balance between food acquisition and predation risk avoidance (food–predation trade‐off). However, to date, studies have not fully elucidated how fine‐ and broad‐scale habitat decisions by individual prey can help balance food versus risk. We studied broad‐scale habitat selection by Newfoundland caribou Rangifer tarandus, focusing on trade‐offs between predation risk versus access to forage during the calving and post‐calving period. We improved traditional measures of habitat availability by incorporating fine‐scale movement patterns of caribou into the availability kernel, thus enabling separation of broad and fine scales of selection. Remote sensing and field surveys served to create a spatio‐temporal model of forage availability, whereas GPS telemetry locations from 66 black bears Ursus americanus and 59 coyotes Canis latrans provided models of predation risk. We then used GPS telemetry locations from 114 female caribou to assess food–predation trade‐offs through the prism of our refined model of caribou habitat availability. We noted that migratory movements of caribou were oriented mainly towards habitats with abundant forage and lower risk of bear and (to a lesser extent) coyote encounter. These findings were generally consistent across caribou herds and would not have been evident had we used traditional methods instead of our refined model when estimating habitat availability. We interpret these findings in the context of stereotypical migratory behaviour observed in Newfoundland caribou, which occurs despite the extirpation of wolves Canis lupus nearly a century ago. We submit that caribou are able to balance food acquisition against predation risk using a complex set of factors involving both finer and broader scale selection. Accordingly, our study provides a strong argument for using refined habitat availability estimates when assessing food–predation trade‐offs.     

Distribution and species composition of juvenile and adult scombropids (Teleostei,Scombropidae) in Japanese coastal waters

Two scombropid fishes, Scombrops boops and Scombrops gilberti, are closely related and commercially important species in Japan. These species are often confused in commercial markets because of their morphological similarity. In this study, scombropid specimens collected from various Japanese coastal waters were subjected to polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analysis and phylogenetic analysis of the 16S rRNA gene in mitochondrial DNA. These analyses showed that all the scombropid specimens collected from localities in the Sea of Japan were identified as S. boops, whereas those from the Pacific Ocean included two species, S. boops and S. gilberti. Almost all juvenile (<200 mm standard body length, S_L) S. gilberti originated from the Pacific coastal waters of the northern Japan, whereas adults (>400 mm S_L) were found only in deep water off the Izu Peninsula to the Izu Islands. This suggests that S. gilberti might migrate extensively during its life cycle. In addition, differences in the number of specimens and the distribution between the two species suggest that S. gilberti is less abundant than S. boops in Japanese waters.     

Effects of structural heterogeneity provided by the floating macrophyte <Emphasis Type="Italic">Eichhornia azurea</Emphasis> on the predation efficiency and habitat use of the small Neotropical fish <Emphasis Type="Italic">Moenkhausia sanctaefilomenae</Emphasis>

Macrophytes have a fundamental structuring role in aquatic environments. Several authors have suggested that trophic interactions are particularly mediated by aquatic plants. In the current article, we evaluated the effects of the structural heterogeneity provided by Eichhornia azurea (Kunth) roots on predation and habitat use by the small fish Moenkhausia sanctaefilomenae (Steindachner). We tested the hypotheses that (i) high structural heterogeneity protects macroinvertebrates against predation by M. sanctaefilomenae; (ii) distinct prey types are differently protected by the refuge provided by roots; and (iii) the behavior of M. sanctaefilomenae is affected by the structural heterogeneity provided by macrophyte roots. To test these hypotheses, we performed an experiment in 20 l aquaria in which macroinvertebrates (Cypricercus sp. and Chironomus sp.) were exposed to M. sanctaefilomenae predation for 4 h under three structural heterogeneities, represented by different root densities. High structural heterogeneity protected macroinvertebrates against predation. Additionally, E. azurea roots similarly protected different prey species. The macrophyte spatial structure substantially changed the habitat use of M. sanctaefilomenae. In general, our results corroborated the hypothesis that the structural heterogeneity provided by E. azurea roots significantly affects predation and habitat use by M. sanctaefilomenae. Handling editor: S. Declerck     

Short-term predator avoidance behavior by invasive and native amphipods in the Great Lakes

Understanding predator avoidance behavior by prey remains an important topic in community and invasion ecology. Recently, the Ponto-Caspian amphipod Echinogammarus ischnus (Stebbing 1898) was accidentally introduced into the Great Lakes. Since its introduction, it has displaced the native amphipod, Gammarus fasciatus (Say 1818), from several locations in the lower lakes. To assess whether behavioral differences in predator avoidance might be a causal mechanism increasing the success of the invasive amphipods, two experiments were conducted examining (1) native and invasive amphipod behavioral responses to five fish species with different foraging behaviors, and (2) amphipod responses to different densities of round gobies, a hyper-abundant benthic invertivore. Echinogammarus reduced its distance moved in the presence of all fish species tested, whereas Gammarus reduced its distance moved only after exposure to round gobies, black crappies, and rainbow darters. Both amphipod species increased the time spent motionless following exposure to round gobies, but not after encountering the scent of most of the remaining fish predators. The exception was that Echinogammarus also responded to black crappie scent whereas Gammarus did not. Although both amphipod species exhibited behavioral responses to many of the fish predators, the magnitude of their responses differed only after exposure to the brown bullhead. In the bullhead trials, Echinogammarus reduced its distance traveled significantly more than Gammarus. Both amphipod species increased their avoidance response to increasing goby density, however, the pattern of avoidance behavior was different. Invasive E. ischnus exhibited a consistently strong avoidance response to round gobies over the test duration. Native G. fasciatus initially avoided goby scent, but then either ceased their avoidance response or showed a hyper-avoidance response, depending on goby density. These results suggested (1) both species of amphipods were able to differentiate and react to a variety of fish predators, (2) invasive Echinogammarus amphipods avoided a larger range of fish predators than the native Gammarus, (3) increased avoidance behavior was associated with an increased density of fish, and (4) the avoidance response patterns of invasive Echinogammarus when faced with round goby predators might lead to increased predation on native Gammarus in habitats where they co-occur.     

Multiple Ecological Factors Influence the Location of Proboscis Monkey (<Emphasis Type="Italic">Nasalis larvatus</Emphasis>) Sleeping Sites in West Kalimantan,Indonesia

Multiple ecological factors have been hypothesized to influence primate sleeping site selection. Testing multiple hypotheses about sleeping site selection permits examination of the relative strength of distinct ecological factors and expands our ability to understand how selection pressures influence primate sleeping behavior. Here we examine how avoidance of biting insects, thermoregulation, foraging efficiency, tree stability, and interspecific competition influence selection of sleeping sites by proboscis monkeys (Nasalis larvatus) in Indonesian Borneo. We collected data on relative insect abundance, temperature, rainfall, food availability, group size, sleeping site location, and presence of other primates for 12 mo. Using formal model comparison and information criteria, we analyzed the relative importance of these ecological factors in determining one aspect of sleeping site location: distance from the river. Our models supported the avoidance of biting insects and the foraging efficiency hypotheses. Proboscis monkeys slept further inland on nights when the abundance of sandflies was high along the river, and when less food was available along the river. Many studies suggest that primates select sleeping trees and locations to reduce predation risk; our study indicates that additional factors may also be important in determining sleeping site selection.     

Spatio-temporal dynamics and management implications of the nightly appearance of Boops boops (Acanthopterygii,Perciformes) juvenile shoals in the anthropogenically modified Mediterranean littoral zone

A remarkable phenomenon of dense Boops boops shoals appearing almost adjacent to the shoreline during nighttime is known to the locals of island communities of the Aegean Sea (eastern Mediterranean). In this work, we investigated this appearance testing the hypotheses that (a) it may occur only in anthropogenically modified locations (as suggested by previous observations), (b) the migration pattern to the littoral is not arbitrary but synchronized to the sunset/sunrise, (c) fish abundance is affected by location, season and/or natural (moon) light fluctuations. Quantitative sampling included visual observations from the coast at five stations in Syros (Cyclades, Greece) from July 2009 to September 2010. Both hypotheses concerning occurrence only in anthropogenically modified locations and timing with sunset/sunrise were confirmed. Fish abundance was modelled using generalized additive models, demonstrating a seasonal pattern and revealing significant differences among sampling stations, but no moon-phase effects. The phenomenon investigated here has implications for fisheries management, as the shoal proximity to the shore renders them prone to illegal harvesting (seasonally at high abundances), aggravating the problem of illegal, unreported and unregulated fishing. Further considerations on the integrated management of the coastal zone arise, especially concerning the effects of habitat structural modification and light pollution.     

Variation in horizontal zooplankton abundance in mountain lakes: shore avoidance or fish predation?

Hypothesizing that fish predation, active shore avoidance and outlet stream avoidance may be separately affecting horizontal zooplankton distribution, the effects of fish presence, sampling location (midlake, outlet and non-outlet shore) and time (day or night) on zooplankton abundance and body size were tested. Statistically significant horizontal zooplankton abundance gradients occurred in both fish-present and fish-absent lakes. Fish may strengthen zooplankton spatial patterns common to both fish-present and fish-absent lakes, as abundance differences among locations were often greater in fish-present systems compared to fish-absent systems. Horizontal zooplankton abundance gradients differed through a diel cycle, but were species specific with some species exhibiting gradients only during the day, while others exhibit gradients only during the night. Avoidance of the outlet over and above active shore avoidance appeared to take place in Daphnia sp. Other taxa provided equivocal support of active outlet avoidance with most showing no significant difference between shore and outlet abundance (seven of nine), one taxa showing a significant decrease and one a significant increase in outlet compared to shore abundance. No gradients in zooplankton body size were found.      

Predator-mediated genotypic shifts in a prey population: experimental evidence

We demonstrate the effect of fish predation on genotype frequencies in a laboratory population composed of two Daphnia magna clones, with historically contrasting exposures to fish predation. The two clones differed in their responsiveness to predation via differential avoidance/escape behavior. The clone which coexists with fish in nature is more responsive to the presence of a fish predator, while the clone not exposed to fish predation does not exhibit the defensive reaction. Fish caused a rapid (within 18 h) and significant shift in Daphnia clonal composition, from 1:1 to 8:1, in favor of the responsive clone. Genotype-specific defensive abilities (modus defendi) can contribute greatly to the phenomenon of genotype replacement under selective predation.     

Predator avoidance,microhabitat shift,and risk-sensitive foraging in larval dragonflies

Summary Dragonfly larvae (Odonata: Anisoptera) are often abundant in shallow freshwater habitats and frequently co-occur with predatory fish, but there is evidence that they are underutilized as prey. This suggests that species which successfully coexist with fish may exhibit behaviors that minimize their risk of predation. I conducted field and laboratory experiments to determine whether: 1) dragonfly larvae actively avoid fish, 2) microhabitat use and foraging success of larvae are sensitive to predation risk, and 3) vulnerability of larvae is correlated with microhabitat use. I experimentally manipulated the presence of adult bluegills (Lepomis macrochirus) in defaunated patches of littoral substrate in a small pond to test whether colonizing dragonfly larvae would avoid patches containing fish. The two dominant anisopteran species, Tetragoneuria cynosura and Ladona deplanata (Odonata: Libellulidae), both strongly avoided colonizing patches where adult bluegills were present. Laboratory experiments examined the effects of diel period and bluegills on microhabitat use and foraging success, using Tetragoneuria, Ladona and confamilial Sympetrum semicictum, found in a nearby fishless pond. Tetragoneuria and Ladona generally occupied microhabitats offering cover, whereas Sympetrum usually occupied exposed locations. Bluegills induced increased use of cover in all three species, and use of cover also tended to be higher during the day than at night. Bluegills depressed foraging in Tetragoneuria and to a lesser extent in Ladona, but foraging in Sympetrum appeared unaffected. Other laboratory experiments indicated that Sympetrum were generally more vulnerable than Tetragoneuria or Ladona to bluegill predation, and that vulnerability was positively correlated with use of exposed microhabitats. Both fixed (generally low use of exposed microhabitats, diel microhabitat shifts) and reactive (predator avoidance, predator-sensitive microhabitat shifts) behavioral responses appear to reduce risk of predation in dragonfly larvae. Evidence indicates that vulnerability probably varies widely among species and even among instars within species, and suggests that spatial distributions of relatively vulnerable species may be limited by their inability to avoid predation.     

Strategies for Nest-Site Selection by King Eiders

ABSTRACT Nest site selection is a critical component of reproduction and has presumably evolved in relation to predation, local resources, and microclimate. We investigated nest-site choice by king eiders (Somateria spectabilis) on the coastal plain of northern Alaska, USA, 2003–2005. We hypothesized that nest-site selection is driven by predator avoidance and that a variety of strategies including concealment, seclusion, and conspecific or inter-specific nest defense might lead to improved nesting success. We systematically searched wetland basins for king eider nests and measured habitat and social variables at nests (n = 212) and random locations (n = 493). King eiders made use of both secluded and concealed breeding strategies; logistic regression models revealed that females selected nests close to water, on islands, and in areas with high willow (Salix spp.) cover but did not select sites near conspecific or glaucous gull (Larus hyperboreus) nests. The most effective nest-placement strategy may vary depending on density and types of nest predators; seclusion is likely a mammalian-predator avoidance tactic whereas concealment may provide protection from avian predators. We recommend that managers in northern Alaska attempt to maintain wetland basins with islands and complex shorelines to provide potential nest sites in the vicinity of water.     

Predation risk and resource availability explain roost locations of Whimbrel Numenius phaeopus in a tropical mangrove delta

Roosts are important sites for shorebirds in non‐breeding areas at night and during high tides. How the spatial configuration of food and risk of predation and disturbance influence roost site use in tropical locations remains poorly known. We analysed the locations of nocturnal roosts of Whimbrel Numenius phaeopus in mangroves of Sanquianga National Park, Colombia, with respect to variation in spatial variables related to food resources and risk of predation and disturbance. We contrasted characteristics of all 13 known nocturnal roost locations with those of all other mangrove islands (n = 209) within the limits of the park. We estimated the distance from roosts and other mangrove islands to foraging sites, and sources of predators and human disturbance. Larger areas of feeding habitat surrounded nocturnal roosts than other mangrove islands, and the average distance to individual feeding patches was shorter. Roosts were also more isolated than other islands, but proximity to sources of human disturbance did not differ. We conclude that Whimbrel roost site use in Sanquianga was best explained by a combination of access to feeding territories and isolation from potential sources of mainland predators, but not by avoidance of human disturbance. Beyond identifying factors influencing roost site selection, the large aggregations of individuals in single locations may suggest that presence of conspecifics itself also plays a role in the formation of Whimbrel roosts. We highlight the interaction of food and risk landscapes with intraspecific attraction on the roost site selection by Whimbrels and the importance of mangroves as roosting sites in tropical regions.