Prey choice by marten during a decline in prey abundance

Summary We examined variation in diet choice by marten (Martes americana) among seasons and between sexes and ages from 1980–1985. During this period prey populations crashed simultaneously, except for ruffed grouse (Bonasa umbellus) which was common at the beginning and end of the study, and masked shrews (Sorex cinereus) which were abundant in 1983. Marten were catholic in selection of prey and made use of most available mammalian prey, ruffed grouse, passerine birds, berries, and insects. Diet niche was widest during the latter three years when prey was scare, particularly in late winter. Diet niche breadth was negatively correlated with abundance of all common prey species. Proportion of small prey species in the diet was correlated with absolute abundance of those species, but proportion of some large prey was related to their relative abundance. Diet choice varied among years and among seasons. Berries and insects were common in summer diets while large prey, particularly varying hare (Lepus americanus), were more frequent in winter diet than in summer diet. We found little evidence that any small mammal species was a preferred prey. Sexual size dimorphism between the sexes did not affect prey choice, nor did age. Reduced foraging effort in winter resulted in a wider diet niche only when prey was scarce. The only prediction of optimal foraging models fully supported by our data was a wider diet niche with reduced prey abundance. However, among the three most profitable prey species choice was dependent on the absolute abundance of the most profitable type (varying hare). We suggest that marten primarily forage for large prey but employ a strategy which results in encounters with small prey as well. These small prey are eaten as they provide energy at minimal cost, between captures of large prey.     

The foraging decisions of a central place foraging seabird in response to fluctuations in local prey conditions

During reproduction, seabirds need to balance the demands of self- and offspring-provisioning within the constraints imposed by central place foraging. To assess behavioral adjustments and tolerances to these constraints, we studied the feeding tactics and reproductive success of common murres (also known as common guillemots) Uria aalge , at their largest and most offshore colony (Funk Island) where parents travel long distances to deliver a single capelin Mallotus villosus to their chicks. We assessed changes in the distance murres traveled from the colony, their proximate foraging locations and prey size choice during two successive years in which capelin exhibited an order of magnitude decrease in density and a shift from aggregated (2004) to dispersed (2005) distributions. When capelin availability was low (2005), parental murres increased their maximum foraging distances by 35% (60 to 81 km) and delivered significantly larger capelin to chicks, as predicted by central place foraging theory. Murres preferred large (>140 mm) relative to small capelin (100–140 mm) in both years, but unexpectedly this preference increased as the relative density of large capelin decreased. We conclude that single prey-loading murres target larger capelin during long foraging trips as parents are 'forced' to select the best prey for their offspring. Low fledgling masses suggest also that increased foraging time when capelin is scarce may come at a cost to the chicks (i.e. fewer meals per day). Murres at this colony may be functioning near physiological limits above which further or sustained adjustments in foraging effort could compromise the life-time reproductive success of this long-lived seabird.     

Prey size, prey nutrition, and food handling by shrews of different body sizes

We tested some predictions relating metabolic constraints offoraging behavior and prey selection by comparing food handlingand utilization in four sympatric shrew species: Sorex minutus(mean body mass = 3.0 g), S. araneus (8.0 g), Neomys anomalus(10.0 g), and N. fodiens (14.4 g). Live fly larvae, mealwormlarvae, and aquatic arthropods were offered to shrews as smallprey (body mass <0.1 g). Live earthworms, snails, and smallfish were offered as large prey (>0.3 g). The larvae werethe high-nutrition food (>8 kJ/g), and the other prey werethe low-nutrition food (<4 kJ/g). The smallest shrew, S.minutus, utilized (ate + hoarded) <30% of offered food,and the other species utilized >48% of food. The largerthe shrew, the more prey it ate per capita. However, highlyenergetic insect larvae composed 75% of food utilized by S.minutus and only >40% of the food utilized by the other species. Thus, inverse relationships appeared between shrewbody mass and mass-specific food mass utilization and betweenshrew body mass and mass-specific food energy utilization:the largest shrew, N. fodiens, utilized the least food massand the least energy quantity per 1 g of its body mass. Also,the proportion of food hoarded by shrews decreased with increase in size of shrew. With the exception of S. araneus, the sizeof prey hoarded by the shrews was significantly larger thanthe size of prey eaten. Tiny S. minutus hoarded and ate smallerprey items than the other shrews, and large N. fodiens hoardedlarger prey than the other shrews.     

Prey size,prey perishability and group foraging in a social spider

Summary Selection might favor group foraging and social feeding when prey are distributed in patches that do not last long enough for a solitary individual to consume more than a small fraction of them (Pulliam and Millikan 1982; Pulliam and Caraco 1984). Here we considered the foraging behavior of a social spider, Anelosimus eximius, in light of this ephemeral resource hypothesis. This species builds large webs in which members cooperate to capture a wide variety of different sizes and types of prey, many of which are very large. The capture success of this species was very high across all prey sizes, presumably due to the fact that they foraged in groups. Group consumption times in natural colonies for all prey larger than five mm were less than the time that dead insects remained on the plastic sheets that we used as artificial webs. Solitary consumption estimates, calculated from the rate at which laboratory individuals extracted insect biomass while feeding, were the same as the residence times of insects on artificial webs in the field for insects between 6 and 15 mm in length and were significantly longer than the persistence of insects on plastic sheets for all larger insects. Large prey, that contribute substantially to colony energy supplies, appeared to be ephemeral resources for these spiders that could not be consumed by a single spider in the time they were available. These factors made the food intake of one spider in a group less sensitive to scavenging by others and could act to reinforce the social system of this species.     

Prey size,prey abundance,and temperature as correlates of growth in stream populations of cutthroat trout

Growth and maximum size of stream fishes can be highly variable across populations. For salmonid fishes in streams, individuals from populations confined to headwater streams often exhibit small size at maturity in comparison to populations with access to main-stem rivers. Differences in prey size, prey availability, and metabolic constraints based on temperature may explain patterns of maximum size and growth. In this study, cutthroat trout from headwater stream populations that were isolated above a waterfall were compared to individuals from populations in similar sized streams without a movement barrier and from large main-stem rivers. Cutthroat trout from smaller streams with or without a movement barrier were significantly smaller at a given age than fish from main-stem rivers, where individuals were able to achieve a much larger maximum size. Comparisons of invertebrate drift abundance and size in the three types of streams revealed that drift size did not differ between stream categories, but was highest per volume of water in large main-stem rivers. Across all stream types, prey abundance declined from summer to fall. Temperature declined over the course of the season in a similar manner across all stream types, but remained relatively high later in the season in main-stem river habitats. Prey availability and temperature conditions in main-stem rivers may provide more optimal growing conditions for fish as individuals increase in size and become constrained by prey availability and temperature conditions in small streams. Maintaining connectivity between small spawning and rearing tributary streams and main-stem river habitats may be critical in maintaining large-bodied populations of stream salmonids.     

The role of prey size and abundance in the geographical distribution of spider sociality

1. Social species in the spider genus Anelosimus predominate in lowland tropical rainforests, while congeneric subsocial species occur at higher elevations or higher latitudes. 2. We conducted a comparative study to determine whether differences in total biomass, insect size or both have been responsible for this pattern. 3. We found that larger average insect size, rather than greater overall biomass per se, is a key characteristic of lowland tropical habitats correlating with greater sociality. 4. Social species occupied environments with insects several times larger than the spiders, while subsocial species nearing dispersal occupied environments with smaller insects in either high or low overall biomass. 5. Similarly, in subsocial spider colonies, individuals lived communally at a time when they were younger and therefore smaller than the average insect landing on their webs. 6. We thus suggest that the availability of large insects may be a critical factor restricting social species to their lowland tropical habitats.     

Prey capture and processing behaviors vary with prey size and shape in Australian and subantarctic fur seals

When hunting at sea, pinnipeds should adapt their foraging behaviors to suit the prey they are targeting. We performed captive feeding trials with two species of otariid seal, Australian fur seals (Arctocephalus pusillus doriferus) and subantarctic fur seals (Arctocephalus tropicalis). This allowed us to record detailed observations of how their foraging behaviors vary when presented with prey items that cover the full range of body shapes and sizes encountered in the wild. Small prey were captured using suction alone, while larger prey items were caught in the teeth using raptorial biting. Small fish and long skinny prey items could then be swallowed whole or processed by shaking, while all prey items with body depths greater than 7.5 cm were processed by shaking at the water's surface. This matched opportunistic observations of feeding in wild Australian fur seals. Use of “shake feeding” as the main prey processing tactic also matches predictions that this method would be one of the only tactics available to aquatic tetrapods that are unable to secure prey using their forelimbs.     

The dynamics of prey choice in fish: the importance of prey size and satiation

Over a number of decades the process of prey choice has been investigated using fishes as model predators. Using fishes for the model has allowed the proximate factors that determine how a mobile predator finds and chooses to eat the prey encountered within a variable 3‐D environment to be estimated. During prey choice a number of constraints exist, in particular most fish predators will eat their prey whole thus their jaws and gut create functional limitations once a prey has been attacked. By considering the relationship between the size of the prey and the predator's feeding apparatus and feeding motivation this study explores the link between mechanistic studies and theoretical, optimal foraging based predictions. How the prediction of prey choices made by the fish following prey encounter can be reconciled with what is likely to be found in the fish's stomach is discussed. This study uses a progression of empirical examples to illustrate how the limits of functional constraints and prey choice at different stages of motivation to feed can be taken into account to improve predictions of predator prey choice.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.     

Effects of prey density,prey size and predator size on rates of feeding by an intertidal predatory gastropod Morula marginalba Blainville (Muricidae), on several species of prey

As a prerequisite for models of foraging behaviour of the whelk, Morula marginalba Blainville (Muricidae), the effects of variation in density of prey on the rate of feeding of the predator were examined in field conditions for three coexisting species of prey. Densities of prey used were those at which the prey, two limpets and a barnacle, occurred naturally in the rocky intertidal habitat.Large limpets, Cellana tramoserica (Sowerby) can resist attacks by predatory gastropods by raising the mantle over the outside of the shell. These experiments showed that no C. tramoserica were killed by Morula marginalba even at very great densities and with no alternative prey present. For the small limpet Patelloida latistrigata (Angas), one of the whelk's most highly preferred prey, juveniles were eaten 1.4 times as fast as adults. Fitting the random predator equation gave greater attack coefficients and shorter handling times for juvenile than adult limpets.Sizes of both predator and prey affected rates of eating barnacles, Tesseropora rosea (Krauss), but not in a simple way. Whelks of 15-mm aperture length ate adult barnacles 4.2 times faster than did 12-mm whelks, but there was no significant difference in the rates at which the two sizes of snail ate juvenile barnacles.Rates of feeding on T. rosea and Patelloida latistrigata increased significantly with prey density. These results form a basis for including the density of prey in models of spatial dispersion of the predatory gastropod Morula marginalba.     

Monitoring the dead as an ecosystem indicator

Dead animal biomass (carrion) is present in all terrestrial ecosystems, and its consumption, decomposition, and dispersal can have measurable effects on vertebrates, invertebrates, microbes, parasites, plants, and soil. But despite the number of studies examining the influence of carrion on food webs, there has been no attempt to identify how general ecological processes around carrion might be used as an ecosystem indicator. We suggest that knowledge of scavenging and decomposition rates, scavenger diversity, abundance, and behavior around carrion, along with assessments of vegetation, soil, microbe, and parasite presence, can be used individually or in combination to understand food web dynamics. Monitoring carrion could also assist comparisons of ecosystem processes among terrestrial landscapes and biomes. Although there is outstanding research needed to fully integrate carrion ecology and monitoring into ecosystem management, we see great potential in using carrion as an ecosystem indicator of an intact and functional food web.     

Prey abundance and the strength of interference in a foraging shorebird

Interference is an important component of food competition but is often difficult to detect and measure in natural animal populations. Although interference has been shown to occur between oystercatchers Haematopus ostralegus L. feeding on mussels Mytilus edulis L., four previous studies have not detected interference between oystercatchers feeding on cockles Cerastoderma edule L. In contrast, this study detected interference between cockle-feeding oystercatchers in the Baie de Somme, France. Prey stealing (kleptoparasitism), one of the main causes of interference between mussel-feeders, also occurred between oystercatchers in the Baie de Somme. The kleptoparasitism rate was related to the natural variation in the food supply, tending to be higher when cockles were rare. Feeding rate was negatively related to competitor density, so providing evidence for interference, but, as in mussel-feeders, only above a threshold density of about 50–100 birds ha⁻¹. The strength of interference at a fixed competitor density was related to the cockle food supply, usually being greater when cockles were rare. Previous studies probably failed to detect interference between cockle-feeders because competitor densities were too low, or cockles were too abundant, or because they were not conducted during late winter when interference is most intense. The study shows that natural variation in the food supply can influence the strength of interference within an animal population and provides support for those behaviour-based interference models which predict that the strength of interference will be greatest when competitor densities are high and prey scarce.     

Prey size spectra and prey availability of larval and small juvenile cod

The aim of the present study is to describe the prey preference characteristics of cod larvae and assess preference variability in relation to species and size composition of copepod prey. A further aim is to examine the hypothesis that dietary prey size spectra remain the same during the larval stage when viewed on a relative predator/prey size scale. The study is based on stomach analysis of larval/juvenile cod in the size range 10–35 mm from nursery grounds in the North Sea. Stomach contents (species, size) were compared to environmental composition and preference indices were calculated. Prey size spectra had the expected relationship to larval cod size, and preference for given copepod species could be ascribed to their relative size. Additional species-specific preferences were evident, for example the larger Pseudocalanus and the larger Calanus spp. were highly preferred. Available prey biomass was highest in the areas of a hydrographic front, where larvae have been shown to concentrate. Changes in prey availability with larval growth depend strongly on specific prey biomass spectra at a given location. Both increasing and decreasing prey availability at increasing larval size were indicated, dependent on location. The findings illustrate the usefulness of coupling dietary prey size spectra and biomass spectra of available prey sizes during studies of ichthyoplankton feeding ecology.     

Prey size selection and cannibalistic behaviour of juvenile barramundi Lates calcarifer

This study assessed the cannibalistic behaviour of juvenile barramundi Lates calcarifer and examined the relationship between prey size selection and energy gain of cannibals. Prey handling time and capture success by cannibals were used to estimate the ratio of energy gain to energy cost in prey selection. Cannibals selected smaller prey despite its capability of ingesting larger prey individuals. In behavioural analysis, prey handling time significantly increased with prey size, but it was not significantly affected by cannibal size. Conversely, capture success significantly decreased with the increase of both prey and cannibal sizes. The profitability indices showed that the smaller prey provides the most energy return for cannibals of all size classes. These results indicate that L. calcarifer cannibals select smaller prey for more profitable return. The behavioural analysis, however, indicates that L. calcarifer cannibals attack prey of all size at a similar rate but ingest smaller prey more often, suggesting that prey size selection is passively orientated rather than at the predator's choice. The increase of prey escape ability and morphological constraint contribute to the reduction of intracohort cannibalism as fish grow larger. This study contributes to the understanding of intracohort cannibalism and development of strategies to reduce fish cannibalistic mortalities.     

The effects of prey relative abundance and chemical cues on prey selection in rainbow trout

An optimal foraging model was used to predict prey selection based on both energy maximization and number maximization strategies. The influence of chemical cues and relative abundance on rainbow trout diet selection was examined under laboratory conditions.
In most fish, diet composition was strongly influenced by chemical cues. No fish followed an energy maximization strategy, and selection of prey based on taste persisted despite large caloric penalties associated with these choices. In the absence of chemical cues, diet composition was based on relative abundance of prey (a number maximization strategy). Within the feasible constraints of the optimal foraging model lie a large number of possible diet combinations which would provide sufficient energy for growth and reproduction. This provides a wide scope for feeding flexibility. Response to prey chemical cues may be the basis for observations of individual diet variability in trout.     

Foraging behaviour and prey size of the painted stork

The foraging behaviour of painted stork Mycteria leucocephala was studied during 2004–2006 at 14 different sites in the Delhi region, India. Observations were recorded on 131 individuals, including 29 juvenile birds using a video camera. Recordings were also made at the nesting colony in Delhi zoo to study the prey sizes regurgitated to nestlings. The results confirm that the painted stork is a tactile forager and exclusively piscivorous. Foraging group size ranged from 1 to 18 individuals. Per 5 min foot stirring rates in the vegetated habitats were significantly higher than in non-vegetated habitats. The attempt rate and feeding rate in the breeding season were significantly higher than that in the non-breeding season. Prey sizes taken in the breeding season were significantly smaller than those taken in the non-breeding season. About 80% fish fed to the chicks were smaller than 10 cm. Young chicks were offered smaller prey compared with older chicks. The variations in foraging parameters are discussed in relation to habitats and their conservation in the Delhi region.     

Prey selection by a stonefly: the influence of hunger and prey size

Summary The influences of hunger and prey size on prey selection by the stonefly Hesperoperla pacifica (Perlidae) were explored in the laboratory by observing behavioral responses toward ten prey taxa and three nonprey taxa. Patterns of behavior were consistent with most assumptions and predictions of optimal foraging theory predicting sizebased prey selection by pursuing predators. Handling time appeared to increase as an exponential function of prey mass, and prey profitability (mg/s) was highest for small and intermediate-sized prey. Fasted stoneflies consumed a wide range of prey sizes, whereas well-fed stoneflies concentrated their attacks on intermediate-sized prey. Responses of H. pacifica to nonprey taxa, however, suggest that prey recognition and selection are not based on size alone.     

Animals as an indicator of carbon sequestration and valuable landscapes

Possibilities of the assessment of a landscape with the use of succession development stages, monitored with the value of the Mean Individual Biomass (MIB) of carabid beetles and the occurrence of bird species are discussed on the basis of an example from Poland. Higher variability of the MIB value in space signifies a greater biodiversity. Apart from the variability of MIB, it is suggested to adopt the occurrence of the following animals as indicators, (in the order of importance), representing underlying valuable landscapes: black stork, lesser spotted eagle, white-tailed eagle, wolf, crane and white stork. The higher number of these species and their greater density indicate a higher value of the landscape for biodiversity and ecosystem services, especially carbon sequestration. All these indicators may be useful to assess measures for sustainable land use.