Evolutionary trends within bivalve prey of chesapeake group naticid gastropods

Five genera of Miocene bivalves evolved antipredatory adaptations in response to predation by drilling naticid gastropods. I examined the evolution of two traits affecting predator‐prey interaction, prey shell thickness (TH) and internal volume (IV). Thickness controls predation costs by determining drilling time, and internal volume influences the benefit derived by the predator.

Internal volume showed no consistent pattern of temporal change among the taxa studied. IV fluctuated nondirectionally during the history of most genera, though both increasing and decreasing trends occurred within species ofAstarte. In contrast, all five genera exhibited significant thickness increases (from 8–157%) during the three‐million‐year interval. Both gradual intraspecific and interspecific directional changes occurred. Taxa with the greatest predation intensities displayed the most change, suggesting that predation selected for the thickness increases. Increased thickness apparently reduced predation; a significant negative correlation between TH and predation intensity occurred within four of the bivalve genera. Improvement of predator capabilities apparently did not keep pace with increased antipredatory morphologic adaptations over the interval studied.     

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

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

Mortality by moonlight: predation risk and the snowshoe hare

Optimal behavior theory suggests that prey animals will reduceactivity during intermittent periods when elevated predationrisk outweighs the fitness benefits of activity. Specifically,the predation risk allocation hypothesis predicts that preyactivity should decrease dramatically at times of high predationrisk if there is high temporal variation in predation risk butshould remain relatively uniform when temporal variation inpredation risk is low. To test these predictions we examinedthe seasonably variable response of snowshoe hares to moonlightand predation risk. Unlike studies finding uniform avoidanceof moonlight in small mammals, we find that moonlight avoidanceis seasonal and corresponds to seasonal variation in moonlightintensity. We radio-collared 177 wild snowshoe hares to estimatepredation rates as a measure of risk and used movement distancesfrom a sample of those animals as a measure of activity. Inthe snowy season, 5-day periods around full moons had 2.5 timesmore predation than around new moons, but that ratio of theincreased predation rate was only 1.8 in the snow-free season.There was no significant increase in use of habitats with morehiding cover during full moons. Snowshoe hares' nightly movementdistances decreased during high-risk full-moon periods in thesnowy season but did not change according to moon phase in thesnow-free season. These results are consistent with the predationrisk allocation hypothesis.     

Use of a thermal and light refugium by Daphnia and its effects on foraging pumpkinseeds

The effects of thermal stratification and light gradients on the feeding behavior of pumpkinseeds, Lepomis gibbosus, were tested in vertical aquarium columns. Successful captures, unsuccessful captures and unsuccessful searches by foraging sunfish on Daphnia pulex were recorded. Clearance and feeding rates of the sunfish were lowest when prey densities remained high, indicating that the Daphnia were occupying an area that could not be searched by the sunfish. Thermal stratification limited the searching volume and prey availability of the sunfish, while creating a refuge for the Daphnia. Light intensities ≤ 4.2 × 10^-3 W m^-2 decreased the searching and capture abilities of the sunfish under isothermal conditions. Thermal stratification had more of an effect than the light gradients, creating a refuge for the Daphnia causing them to be unavailable and less vulnerable to predation by the sunfish. This revised version was published online in August 2006 with corrections to the Cover Date.     

Status of the Nemertea as predators in marine ecosystems

The ecology of nemertean predators in marine ecosystems is reviewed. Nemerteans occur in most marine environments although usually in low abundances. Some species, particularly in intertidal habitats, may reach locally high densities. During specific time periods appropriate for hunting, nemerteans roam about in search of prey. Upon receiving a stimulus (usually chemical cues), many nemertean species actively pursue their prey and follow them into their dwellings or in their tracks. Other species (many hoplonemerteans) adopt a sit-and-wait strategy, awaiting prey items in strategic locations. Nemerteans possess potent neurotoxins, killing even highly mobile prey species within a few seconds and within the activity range of its attacker. Most nemertean species prey on live marine invertebrates, but some also gather on recently dead organisms to feed on them. Heteronemerteans preferentially feed on polychaetes, while most hoplonemerteans prey on small crustaceans. The species examined to date show strong preferences for selected prey species, but will attack a variety of alternative prey organisms when deprived of their favourite species. Ontogenetic changes in prey selection appear to occur, but no further information about, e.g. size selection, is available. Feeding rates as revealed from short-term laboratory experiments range on the order of 1–5 prey items d^–1. These values apparently are overestimates, since long-term experiments report substantially lower values (0.05–0.3 prey items d^–1). Nemerteans have been reported to exert a strong impact on the population size of their prey organisms through their predation activity. Considering low predation rates, these effects may primarily be a result of indirect and additive interactions. We propose future investigations on these interactive effects in combination with other predators. Another main avenue of nemertean ecological research appears to be the examination of their role in highly structured habitats such as intertidal rocky shore and coral reef environments.     

The feeding behaviour of Leptodora kindti and its impact on the zooplankton community of Neusiedler See (Austria)

Leptodora kindti is a very efficient invertebrate predator. Its searching mode of preying is tactile. The setae of the first thoracic limb act as mechanoreceptors, the other thoracic limbs, thorax and head together form the shape of an open basket in which after encounter the prey is pushed in by the aid of the first thoracic limbs and the furca. In Neusiedler See, small individuals of Diaphanosoma brachyurum (0.6–0.9 mm) are the preferred prey, rarely copepods are taken. The predation rate is influenced by temperature, prey density and predator size and varies between less than one and 12 prey items per predator per day. At high predator densities, Leptodora will have a substantial effect on the Diaphanosoma population of Neusiedler See.     

Prey classification by an araneophagic ant-like jumping spider (Araneae: Salticidae)

What to attack is one of the most basic decisions predators must make, and these decisions are reliant upon the predator's sensory and cognitive capacity. Active choice of spiders as preferred prey, or araneophagy, has evolved in several distantly related spider families, including jumping spiders (Salticidae), but has never been demonstrated in ant-like jumping spiders. We used prey-choice tests with motionless lures to investigate prey-choice behaviour in Myrmarachne melanotarsa , an East African ant-like salticid that normally lives in aggregations and often associates with other spider species. We show that M . melanotarsa chooses spiders as prey in preference to insects and, furthermore, discriminates between different types of spiders. Myrmarachne melanotarsa 's preferred prey were juvenile hersiliids and its second most preferred were other salticids. To date, all documented examples of araneophagic salticids have been from the basal subfamily Spartaeinae. Myrmarachne melanotarsa is the first non-spartaeine and also the first ant-like salticid for which araneophagy has been demonstrated.     

Molluscan predation by Ovalipes punctatus (De Haan) (Crustacea: Brachyura: Portunidae)

Molluscan predation by the three-spot swimming crab was investigated. The dentition of the heteromorphic chelae allowed crushing, shearing, cutting and holding of prey. Laboratory investigations indicated that small mussels and gastropods were crushed, the larger mussels were prized open, and the foot of the larger gastropods shredded and bits removed. Stomach contents of freshly captured crabs indicated that the crabs are selective carnivores and preferred prey species which are not most abundant in situ (crabs from Kings Beach, Donax serra Röding; crabs from Maitlands River Beach, Bullia rhodostoma Reeve). Ovalipes punctatus (De Haan) foraged on a variety of prey and had no upper prey size limit, but the crabs did show preferences for certain prey sizes. Data indicate that the swimming crabs can effectively utilize the entire mollusc populations on the beaches as prey items.     

Eine überschuss produzierende Population des Hausrotschwanzes(Phoenicurus ochruros) in Ortschaften mit hoher Hauskatzendichte(Felis catus)

Zusammenfassung Zwischen März 1994 und April 1997 bestimmten wir in zwei Schweizer Bergdörfern mit hoher Hauskatzendichte den Source-Sink-Status einer Hausrotschwanzpopulation (Phoenicurus ochruros) und quantifizierten die Verluste durch Hauskatzen. Die ermittelte Wachstumsrate der Population lag bei 1,06±0,09 ( ± s. e.), d. h. der lokale Bestand erzeugte während den drei Untersuchungsjahren mehr Brutvögel als er durch Mortalität verlor. Eine alternative Berechnungsmethode — Verhältnis zwischen der Anzahl produzierter Brutvögel (103) und Neuansiedlern (88) — ergab eine Wachstumsrate von 1,17. Die Population befand sich somit in einem Source-Status. Während der Untersuchungszeit sind insgesamt 307 Junge ausgeflogen. Die Überlebensrate der Jungen zwischen Ausfliegen und der nächsten Brutsaison wurde auf 0,336 geschätzt. Somit gingen 103 neue Brutvögel aus den 307 ausgeflogenen Jungen hervor. Dies sind mehr als Brutvögel starben (94). Bei Wegfallen der beobachteten Katzenprädation hätte die Wachstumsrate der Beutepopulation mindestens 1,20 betragen, falls dabei die anderen Populationsparameter unverändert geblieben wären. Raubverluste durch Katzen verminderten also die Produktivität der untersuchten Beutepopulation um mindestens 12 % (von 1,20 auf 1,06) ohne einen Sink-Status herbeizuführen. Ein Vergleich der Sterblichkeit von Eiern, Nestlingen und Jungvögeln nach Mayfield (1961, 1975) in Kombination mit Direktbeobachtungen wies darauf hin, dass Jungvögel vor dem Selbständigwerden besonders gefährdet sind. Prädation durch Katzen verursachte nachweislich 33 % aller Eiverluste, 20 % der Nestlingsverluste, 10 % der Verluste an Jungen zwischen Ausfliegen und Selbständigwerden und 3 % der Adulttier-Verluste.

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A source population of Black Redstarts(Phoenicurus ochruros) in villages with a high density of feral cats(Felis catus)

Summary Between March 1994 and April 1997 we determined the source-sink status of a population of Black Redstarts (Phoenicurus ochruros) in two adjacent mountain villages in Switzerland, and estimated mortality attributable to feral cats (Felis catus). The finite rate of increase of the population was 1.06±0.09 ( ± s. e.). Hence, during the three study years the local population had a source status (>1). The ratio between the number of produced (103) and absorbed breeders (88) — another way to evaluate source-sink status — revealed a rate of increase of 1.17. In total, 307 young fledged during the study period. Survival rate between fledging and the next breeding season was estimated to be 0.336, so 103 new breeders were produced out of the 307 fledglings. This number is slightly higher than the 94 breeders the population lost through mortality. The finite rate of increase of the population without the observed losses from cat predation would be =1.20, assuming that this would not affect fecundity and mortality from other sources. Therefore, predation by cats reduced the productivity of this population by at least 12% (from 1.20 to 1.06) but did not convert it into a sink population. A comparison of the mortality of eggs, nestlings and juveniles, in combination with direct observations, suggested that juveniles were most vulnerable to cat predation between fledging and their independence after eight days. Predation by cats caused 33% of egg fatalities, 20 % of nestling fatalities, 10 % of fledgling fatalities and 3% of adult fatalities.

     

Prey Selection by the Lady Beetle Harmonia axyridis: The Influence of Prey Mobility and Prey Species

The influence of prey mobility and species on prey selection by the coccinellid Harmonia axyridis Pallas was determined under laboratory conditions for two prey species, Hyaliodes vitripennis (Say) and Tetranychus urticae Koch. Prey selection was influenced by prey mobility. In the presence of active prey, the coccinellid selected T. urticae while in presence of immobilized prey, H. vitripennis was preferred. Harmonia axyridis searching time was longer in the presence of active H. vitripennis than in the presence of active T. urticae. Moreover, the coccinellid capture rate was lower for active H. vitripennis caused by effective defensive mechanisms. Prey suitability was affected by prey mobility and species. Immobilized H. vitripennis were the most profitable prey, i.e. induced a shorter developmental time and no mortality. However, active H. vitripennis were not a suitable food source for H. axyridis. Our results suggested that three factors are involved in prey selection by H. axyridis: (i) prey mobility; (ii) prey defensive mechanisms; and (iii) prey species.     

Intraguild predation among the predatory mites Amblyseius eharai,Amblyseius cucumeris and Amblyseius barkeri

Predator species with the same prey interact not only by competition for food and space but also by intraguild predation (IGP). The impact of IGP on introduced phytoseiid mites and native species in the context of biological control is a matter of considerable debate. Amblyseius eharai is the dominant native citrus species in central China, while Amblyseius cucumeris and Amblyseius barkeri are candidates for importation. All three species can feed on the spider mite Panonychus citri, which is the main pest in citrus. This study investigated, in the laboratory, possible IGP among these species in the absence and presence of P. citri, respectively. IGP in different densities of shared prey and intraguild (IG) prey was also studied. All three species consumed heterospecific larvae and eggs but not adults, and the IGP rate of larvae was significantly higher than that of eggs in the absence of shared prey. Additionally, the IGP rate of each group was reduced dramatically in the presence of both shared and IG prey when compared to the absence of shared prey. This occurs most likely because the three species prefer to feed on their natural prey P. citri, rather than on IG prey. Our results showed that A. eharai seems to be a more voracious IG predator than A. cucumeris. A. eharai was much more prone to IGP than A. barkeri.     

Diurnal Tiger Beetles (Coleoptera: Cicindelidae) Capture Prey Without Sight

Although much research has examined the process of prey capture by tiger beetles, an underlying assumption in this work is that tiger beetles are principally, or even exclusively, visual predators. Because this assumption is untested, we performed a series of experiments on four diurnally active tiger beetles in the Genus Cicindela. Individual beetles were placed in chambers in complete darkness and allowed to forage on apterous Drosophila for 6 h. Contrary to expectations, adults of all tested species captured more than 90% of prey items. These results show that other modalities can be used by tiger beetles during prey capture. Beyond potentially providing an explanation for observed night activity in tiger beetle species, the significance of these findings lies in the need to test underlying assumptions of even well-studied organisms.     

Prey selection by the Indian tiger (Panthera tigris tigris) in Nagarjunasagar Srisailam Tiger Reserve, India

Very little is known about prey selection by the Indian tiger Panthera tigris tigris in tropical dry deciduous forests or in wild herbivore-depleted habitats with high livestock pressures. We undertook a short-term study in a large south Indian tiger reserve and examined two intensive study areas (SA's) from October 1998 – June 1999. In each area, herbivore sightings were recorded, scats were collected, and wild prey and livestock kills documented. Chital, wild boar and sambar were the most abundant among the wild herbivores in the study area. Scat analysis revealed wild boar (Sus scrofa) being the most common prey followed by chital (Axis axis) and sambar (Rusa unicolor). Livestock comprised less than 7% of diet intake. Here, the tigers consumed a lower mean prey mass (56.3 kg) than in other reserves. Our study suggests that in tropical dry deciduous forests with low natural prey density, smaller prey species, and high livestock biomass, tigers preferentially kill smaller prey and generally avoid livestock predation.     

Size-dependent predator–prey relationships between perch and their fish prey

Size-dependent interactions between piscivorous perch Perca fluviatilis (age ≥1 year) and their fish prey age 0 year perch, pikeperch Sander lucioperca and roach Rutilus rutilus in the biomanipulated Bautzen Reservoir indicated that the highest ratio of prey total length ( L _T) to predator L _T was 59%. Perch L _T and prey fish L _T were positively and linearly related. Perch L _T was strongly related with both gape width and gape height. Within the range 80–110 mm L _T, the gape height of perch exceeded gape width, while beginning at 120 mm L _T the gape width exceeded gape height. The minimum, maximum and mean prey L _T and prey body depths of all three prey species increased with increasing predator size, but the increases in mean sizes of perch and pikeperch as prey were less than that of roach. The low limit of the 'predation window' observed in this study coupled with results of previous studies on perch in the Bautzen Reservoir indicated that perch had a major impact on the population dynamics of both perch and pikeperch.