Predation risk influences food‐web structure by constraining species diet choice

The foraging behaviour of species determines their diet and, therefore, also emergent food‐web structure. Optimal foraging theory (OFT) has previously been applied to understand the emergence of food‐web structure through a consumer‐centric consideration of diet choice. However, the resource‐centric viewpoint, where species adjust their behaviour to reduce the risk of predation, has not been considered. We develop a mechanistic model that merges metabolic theory with OFT to incorporate the effect of predation risk on diet choice to assemble food webs. This ‘predation‐risk‐compromise’ (PR) model better captures the nestedness and modularity of empirical food webs relative to the classical optimal foraging model. Specifically, compared with optimal foraging alone, risk‐mitigated foraging leads to more‐nested but less‐modular webs by broadening the diet of consumers at intermediate trophic levels. Thus, predation risk significantly affects food‐web structure by constraining species’ ability to forage optimally, and needs to be considered in future work.     

Search strategies of tyrant flycatchers

Aspects of searching behaviour among free-living South American flycatchers (Aves: Tyrannidae) are compared quantitatively. Flycatchers forage with stationary searching periods, followed either by an attempted prey capture (sally) or a ‘give-up’ flight to a new perch. Search times are proportional to body size within each of three categories of foraging behaviour: aerial hawking, sally-gleaning, and perch-gleaning. Over the family as a whole, search times are directly proportional to the size of the visual field scanned during the search. Intraspecific variations in search times are caused by local variations in prey density or visual complexity of the habitat. Between foraging modes, differences in searching and movement patterns are related to prey dispersion characteristics. Aerial hawkers regularly return to favoured perches, but foliage gleaners, which reduce the resources surrounding a perch by sallying only once, rarely return to a perch. In contrast to aerial hawkers, foliage gleaners appear to follow an organized scanning procedure on each perch, by searching nearby surfaces before they examine more distant prey substrates. Throughout the family, the median flight distance after a perch is abandoned is approximately twice the median search radius. Comparisons of search time distributions preceding sallies with those preceding give-up flights suggest that there is no single, optimal give-up time in a given habitat. Foliage-gleaning species appear to assess the amount of search time each perch warrants, presumably based on the degree of complexity of the search area. They either sally at prey before that time, or give-up when the allotted time has elapsed.     

How phylogeny and foraging ecology drive the level of chemosensory exploration in lizards and snakes

The chemical senses are crucial for squamates (lizards and snakes). The extent to which squamates utilize their chemosensory system, however, varies greatly among taxa and species’ foraging strategies, and played an influential role in squamate evolution. In lizards, ‘Scleroglossa’ evolved a state where species use chemical cues to search for food (active foragers), whereas ‘Iguania’ retained the use of vision to hunt prey (ambush foragers). However, such strict dichotomy is flawed as shifts in foraging modes have occurred in all clades. Here, we attempted to disentangle effects of foraging ecology from phylogenetic trait conservatism as leading cause of the disparity in chemosensory investment among squamates. To do so, we used species’ tongue‐flick rate (TFR) in the absence of ecological relevant chemical stimuli as a proxy for its fundamental level of chemosensory investigation, that is baseline TFR. Based on literature data of nearly 100 species and using phylogenetic comparative methods, we tested whether and how foraging mode and diet affect baseline TFR. Our results show that baseline TFR is higher in active than ambush foragers. Although baseline TFRs appear phylogenetically stable in some lizard taxa, that is a consequence of concordant stability of foraging mode: when foraging mode shifts within taxa, so does baseline TFR. Also, baseline TFR is a good predictor of prey chemical discriminatory ability, as we established a strong positive relationship between baseline TFR and TFR in response to prey. Baseline TFR is unrelated to diet. Essentially, foraging mode, not phylogenetic relatedness, drives convergent evolution of similar levels of squamate chemosensory investigation.     

Locomotor capacity and foraging behaviour of kalahari lacertid lizards

Closely related lacertid lizards (Eremias, Nucras) in the Kalahari desert differ in patterns of foraging behaviour. Some species are relatively sedentary (‘sit-and-wait’) whereas others are more active (‘widely-foraging’) predators. We determined whether whole-animal locomotor capacities (cruising endurance on a treadmill, initial speed and maximum burst speed in a racetrack, and sprint endurance in a torus-shaped track) correlated with interspecific differences in foraging behaviour. Two of three widely-foraging species had greater cruising endurance, graater sprint endurance, but lower burst speed than did a sit-and-wait species. However, the two species that sprinted quickly also had limited endurance, and vice versa. Pre-feeding negatively influenced endurance but not sprint capacity. Theoretical models of foraging behaviour should recognize that ectotherms have limited endurance, that there can be a trade-off between speed and endurance, and that pre-feeding can reduce some aspects of locomotor capacity.     

Varied tastes: home range implications of foraging‐patch selection

Despite evidence of home range behaviour across many taxa, the mechanisms underlying the development of home ranges are still unknown. Recently, models have been developed to explore these mechanisms for both territorial and non‐ territorial species. One such model for a generic forager suggests animal memory and optimal foraging theory as underlying mechanisms driving forager movement and the development of stable home ranges. Although this is a promising model for ungulate home range development, assumptions of the model have yet to be evaluated. Using GPS relocation data from two populations of elk, we explored how foraging patch selection might influence the structure and development of home ranges in elk Cervus elaphus. During the summer growing season, we identified and sampled foraging patches used by elk. Points along elk paths not used for foraging were sampled identically for comparison. We contrasted ‘patch’ and ‘nonpatch’ data points, to identify foraging selection differences across herd, sex and season using a combination of directly sampled and remotely sensed covariates. In general, elk selected patches with higher biomass, cover, slope and lower traffic on the nearest road. These patch‐selection results speak directly to differences between foraging areas and other areas used by elk and demonstrate that both physiographic and anthropocentric features influence foraging patch selection. Our results offer insight as to what defines a valuable foraging patch for elk and how these patches might influence the development and structure of home ranges in a free‐ranging ungulate.     

The Effect of Moonlight on Scopoli's Shearwater Calonectris diomedea Colony Attendance Patterns and Nocturnal Foraging: A Test of the Foraging Efficiency Hypothesis

Moonlight is known to affect the nocturnal behaviour and activity rhythms of many organisms. For instance, predators active at night may take advantage from increased visibility afforded by the moon, while prey might regulate their activity patterns to become less detectable. Many species of pelagic seabirds attend their colony only at night, in complete darkness, avoiding approaching their nest sites under moonlight. This behaviour has been most often interpreted as an antipredator adaptation (‘predation avoidance’ hypothesis). However, it may also reflect a lower foraging efficiency during moonlit nights (‘foraging efficiency’ hypothesis). Indeed, moonlight may reduce prey availability because preferred seabird prey is known to occur at higher depths in moonlit nights. Using high‐accuracy behavioural information from data loggers, we investigated the effect of moonlight on colony attendance and at‐sea nocturnal foraging in breeding Scopoli's shearwaters Calonectris diomedea. We found that birds departing for self‐feeding trips around the full moon performed longer trips than those departing around the new moon. On nights when the moon was present only partly, nest burrow entrances took place largely in the moonless portion of the night. Moreover, contrary to predictions from the ‘foraging efficiency’ hypothesis, nocturnal foraging activity increased according to moonlight intensity, suggesting that birds increased their foraging activity when prey became more detectable. This study strengthens the idea that colony attendance behaviour is strictly controlled by moonlight in shearwaters, which is possibly related to the perception of a predation risk.     

The foraging of New Zealand honeyeaters

Abstract

New Zealand has three species of honeyeaters, all of which feed on nectar, fruit, and ‘insects’. There is disagreement between published data and those becoming available from long-term studies on the relative proportion of these items in the diet. The effect of factors such as body size, dominance status, degree of movement, and time of year on diet and foraging behaviour are outlined, and predictions of differences between species and between sexes are made. A brief comparison of foraging in relation to the flora is made between New Zealand and Australian species.     

Old park trees as habitat for saproxylic beetle species

Very old trees harbour a diverse fauna of saproxylic insects, many of which are classified as threatened due to the scarcity of this kind of habitat. Parks, which often contain many old trees, are therefore considered to be important sites for this fauna. However parks are intensively managed and dead wood is often removed. Therefore this study compares if the saproxylic beetle fauna in parks is as diverse as it is in more natural stands. Eight ‘Park’ sites at manor houses around lake Mälaren, Sweden were compared with trees in wooded meadows: eight grazed sites, here termed ‘Open’, and 11 sites regrown with younger trees, termed ‘Re-grown’. The comparison was made on lime trees (Tilia spp.): one of the most frequent tree species in old parks which host a diverse beetle fauna. Beetles were sampled with window traps, which in total caught 14,460 saproxylic beetles belonging to 323 species, of which 50 were red-listed. When comparing all saproxylic species, ‘Park’ sites had significantly fewer species than ‘Open’ sites. However, for beetles in hollow trees and for red-listed species there was no significant difference, the number in ‘Park’ being intermediate between ‘Open’ and ‘Re-grown’. Species composition differed between sites, but only marginally so. Therefore, the conclusion is that old park trees on average are as valuable for faunal diversity as trees in more natural sites. Large conservation benefits can be obtained by combining cultural and conservational values in the management of park habitats.     

Optimal foraging as the criteria of prey selection by two centrarchid fishes

The nature of prey selection by two centrarchids (white crappie and bluegill) is presented as a model incorporating optimal foraging strategies. The visual field of the foraging fish as represented by the reactive distance is analysed in detail to estimate the number of prey encounters per search bout. The predicted reactive distances are compared with experimental data. The energetic cost associated with fish foraging behaviour is calculated based on the sequence of events that takes place for each prey consumed. Comparisons of the relative abundance of prey species and size categories in the stomach to the lake environment indicated that both white crappie and bluegill (length < 100 mm) strongly select prey utilising an energy optimization strategy. In most cases, the fish exclusively selected large Daphnia ignoring evasive prey types (Cyclops, Diaptomids) and small cladocera. This selectivity is the result of fish actively avoiding prey with high evasion capabilities even though they appear to be high in energetic content and having translated this into optimal selectivity through capture success rates. The energy consideration and visual system, apart from the forager's ability to capture prey, are the major determinants of prey selectivity for large-sized bluegill and white crappie still at planktivorous stages.     

Measuring fruit patch size for three sympatric indonesian primate species

Food availability is one of the basic factors affecting primate density and socioecology, but food availability is difficult to assess. Two different ways to obtain accurate estimates of food availability have been proposed: using phenology data or using the behaviour of animals. Phenology data can be refined by only including trees that are large enough to be used; including (potential) tree species in which by the concerned primate species forage; or including (fruiting) trees of these species that actually produce fruit. Alternatively, the sizes of the actually visited trees (foraging trees) give an estimate of fruit availability. These measures are compared for three sympatric primate species at the Ketambe Research Station, Sumatra, Indonesia: the Thomas langur, the long-tailed macaque and the orangutan. The sizes of fruiting trees and the foraging trees are larger than the potential trees. The sizes of the potential trees and of the fruiting trees are similar for the three primate species. This, however, is not reflected in the use of trees: the langurs forage on average in trees of similar size to those producing fruit, whereas the macaques and orangutans forage in trees larger than those producing fruit. The use of trees does not necessitate a different cut off point of included dbhs for the three compared primate species. The use of trees of different sizes, however, may be regulated by food competition. This indicates that sympatric primates make different foraging decisions and that behavioural measures of food availability will be less reliable.     

Producers and scroungers: A general model and its application to captive flocks of house sparrows

Many forms of interaction within and between species appear to be based on ‘scrounger’ individuals or species exploiting a limited resource provided ‘producers’. A mathematical model is presented which shows whether or not scroungers are maintained in a group, depending on their frequency and the group size. Some of the predictions of the model were tested in captive flocks of house sparrows Passer domesticus L. Here the scroungers obtained most of their food (mealworms) by interaction and the producers found most of their food by actively foraging: the pay-off to each type was measured as mealworm capture rate. Neither type changed strategy opportunistically in response to instantaneous flock composition but, not surprisingly, scroungers fared better when one of more producers were present. However, scrougers did much worse than expected when greatly outnumbered by producers, perhaps because producers then found the available food very quickly.     

The influence of flock size and geometry on the scanning behaviour of spotted turtle doves,Streptopelia chinensis

Abstract The negative correlation between the time individuals spend scanning the environment for predators and group size is usually explained by the benefit of corporate vigilance. However, this negative correlation may be explained additionally in terms of the ‘dilution effect’ and ‘selfish herd geometry’. Our experimental investigation of the scanning behaviour of free-living spotted turtle doves foraging at different shaped feeders revealed that flock geometry influenced individual scanning rates. The time spent scanning declined with group size less rapidly among birds foraging in linear flocks than among those foraging in more two-dimensional flocks. These results were not confounded by aggressive behaviour, and indicate that the benefits of foraging in groups include the so-called selfish herd geometry.     

Frequency-dependent predation, crypsis and aposematic coloration

Frequency-dependent predation may maintain or prevent colour pattern polymorphisms in prey, and can be caused by a variety of biological phenomena, including perceptual processes (search images), optimal foraging and learning. Most species are preyed upon by more than one predator species, which are likely to differ in foraging styles, perceptual and learning abilities. Depending upon the interaction between predator vision, background and colour pattern parameters, certain morphs may be actively maintained in some conditions and not in others, even with the same predators. More than one kind of predator will also affect stability, and only slight changes in conditions can cause a transition between polymorphism and monomorphism. Frequency-dependent selection is not a panacea for the explanation of variation in animal colour patterns, although it may be important in some systems.     

The town Crepis and the country Crepis: How does fragmentation affect a plant–pollinator interaction?

In fragmented habitats, one cause of the decrease of plant diversity and abundance is the disruption of plant–animal interactions, and in particular plant–pollinator interactions. Since habitat fragmentation acts both on pollinator behaviour and plant reproduction, its consequences for the stability of such interactions are complex. An extreme case of habitat fragmentation occurs in urbanised areas where suitable habitat (in the present study small patches around ornamental trees) is embedded in a highly unsuitable environment (concrete matrix). Based on simple experiments, we ask whether pollinators can adapt their foraging behaviour in response to the amount of available resources (flowers) in the fragments and their isolation, as predicted by the optimal foraging theory. To do so we analysed the effect of fragmentation on the behaviour of pollinators visiting Crepis sancta (L.) Bornm. (Asteraceae), which forms large populations in the countryside and patchy populations in urban environments. More precisely we studied pollinator visitation rates, capitulum visit durations, capitulum search durations and capitulum size choice. Pollinators chose larger capitula in both types of populations and their foraging behaviour differed between the two population types in three ways: (1) pollinator visits were lower in urban fragmented populations, perhaps due to the lower accessibility of urban patches; (2) capitulum visit durations were longer in urban fragmented populations, a possible compensation of energy lost during flights among patches; and (3) capitulum search durations where longer in urban fragmented populations, which may represent an increase in capitulum prospecting effort. We discuss the possible impacts of such differences for plant population functioning in the two types of populations.     

Resource partitioning among three woodpecker species Dendrocopos spp. during the breeding season

Food composition, prey size utilization and foraging behaviour of three sympatric woodpecker species ( Dendrocopos major, D. medius, D. minor ) were studied in an oak forest near Budapest during the breeding season in 1983 and 1984. Considering these three aspects of feeding, the great spotted woodpecker is a generalist species. Food composition of this species resembled the arthropod supply on the bark of trees more than those of the other two species. The bark of the trees seems to be a relatively unproductive microhabitat in the breeding season, so woodpecker species use, to different degrees, the food supply of the foliage as well. The food and the foraging behaviour of the middle spotted woodpecker show that this species feeds on prey living both on barks and in the foliage; it occupies-an intermediate position between the great and the lesser spotted woodpeckers. Prey size did not correlate with predator size suggesting that woodpeckers adapted not to the summer resources but rather the winter ones.     

An evaluation of hypotheses to explain the pattern of sap feeding by the yellow-bellied glider,Petaurus australis

Yellow-bellied gliders on occasion extensively incise the trunks of eucalypts to feed on sap but in doing so show strong preference for the species and number of trees utilized. Three hypotheses that may account for aspects of this behaviour were examined. They were: (i) sap feeding is related to the incidence of rain; (ii) sap feeding is influenced by the sap flow idiosyncracies of trees; and (iii) sap feeding is influenced by the availability of alternative food resources. Sap feeding at two sites occurred independent of ‘wet’ or ‘dry’ nights. Sap flow (measured as an index) in two species of eucalypt was highly variable among trees and among sample periods. This is the only hypothesis that accounts for the selectivity of the trees incised. Sap feeding at one site coincided with periods of high sap flow. However, at another site, sap feeding did not occur on measured trees despite their often high sap flow. At such times, flowering trees were abundant and gliders fed extensively on nectar. Thus, elevated sap flow may provide the necessary precondition conducive for sap feeding but may be ignored when other food types are available.     

Context-dependent navigation in a collectively foraging species of ant, Messor cephalotes

More than 100 years of scientific research has provided evidence for sophisticated navigational mechanisms in social insects. One key role for navigation in ants is the orientation of workers between food sources and the nest. The focus of recent work has been restricted to navigation in individually foraging ant species, yet many species do not forage entirely independently, instead relying on collectively maintained information such as persistent trail networks and/or pheromones. Harvester ants use such networks, but additionally, foragers often search individually for food either side of trails. In the absence of a trail, these ‘off-trail’ foragers must navigate independently to relocate the trail and return to the nest. To investigate the strategies used by ants on and off the main trails, we conducted field experiments with a harvester ant species, Messor cephalotes, by transferring on-trail and off-trail foragers to an experimental arena. We employed custom-built software to track and analyse ant trajectories in the arena and to quantitatively compare behaviour. Our results indicate that foragers navigate using different cues depending on whether they are travelling on or off the main trails. We argue that navigation in collectively foraging ants deserves more attention due to the potential for behavioural flexibility arising from the relative complexity of journeys between food and the nest.     

The effect of prey density on foraging mode selection in juvenile lumpfish: balancing food intake with the metabolic cost of foraging

1. In many species, individuals will alter their foraging strategy in response to changes in prey density. However, previous work has shown that prey density has differing effects on the foraging mode decisions of ectotherms as compared with endotherms. This is likely due to differences in metabolic demand; however, the relationship between metabolism and foraging mode choice in ectotherms has not been thoroughly studied. 2. Juvenile lumpfish Cyclopterus lumpus forage using one of two modes: they can actively search for prey while swimming, or they can 'sit-and-wait' for prey while clinging to the substrate using a ventral adhesive disk. The presence of these easily distinguishable foraging modes makes juvenile lumpfish ideal for the study of foraging mode choice in ectotherms. 3. Behavioural observations conducted during laboratory experiments showed that juvenile lumpfish predominantly use the 'cling' foraging mode when prey is abundant, but resort to the more costly 'swim' mode to seek out food when prey is scarce. The metabolic cost of active foraging was also quantified for juvenile lumpfish using swim-tunnel respirometry, and a model was devised to predict the prey density at which lumpfish should switch between the swim and cling foraging modes to maximize energy intake. 4. The results of this model do not agree with previous observations of lumpfish behaviour, and thus it appears that juvenile lumpfish do not try to maximize their net energetic gain. Instead, our data suggest that juvenile lumpfish forage in a manner that reduces activity and conserves space in their limited aerobic scope. This behavioural flexibility is of great benefit to this species, as it allows young individuals to divert energy towards growth as opposed to activity. In a broader context, our results support previous speculation that ectotherms often forage in a manner that maintains a minimum prey encounter rate, but does not necessarily maximize net energy gain.     

Learning to forage in a regenerating patchy environment: Can it fail to be optimal?

The behaviour of animals foraging along closed traplines of regenerating patches of food has been simulated using a learning rule that determines when an animal should leave the patch at which it is currently feeding to search for another one. The rule causes the animal to stay at the patch as long as it is feeding faster than it remembers doing. The foraging behaviour of one animal, and of two or more animals together, feeding in traplines containing patches of the same and of differing types has been simulated, and in all cases the foraging behaviour generated by the rule allowed the animals to exploit the food very efficiently. The learning model is also responsible for indirect social interactions among animals sharing the same trapline because the feeding of each animal reduces the availability of food for the others. This causes a population of animals to disperse themselves, on average, among patches of food according to the ideal free distribution. The relationship between the learning model and conventional optimal foraging models is examined and it is shown that it is pointless to try to account for learned behaviour in the context of optimal foraging theory.     

Feeding ecology and foraging behaviour of impala Aepyceros melampus in Lake Mburo National Park, Uganda

Intersexual and seasonal variation in foraging behaviour of impala (Aepyceros melampus), was studied in the Lake Mburo National Park, Uganda. There was a moderate seasonal difference in foraging efficiency (as measured by ‘acceptable food abundance’), with a minimum in dry season and a maximum in Rainy season. The variation between sexes was more distinct with a pronounced minimum in time spent browsing of males in early wet season. By distinguishing between feeding time spent grazing and feeding time spent browsing the seasonal variation was confirmed. The proportion of foraging time spent feeding (expressed as ‘food ingestion rate’) showed an inverse pattern with a maximum in the late dry season (75.5%), decreasing values throughout the Rainy season and a minimum in early dry season (57.8%). Differences between sexes were explained in terms of reproductive demands and seasonal balance in terms of moderate climate throughout the year. Impala foraging patterns in the bimodal tropics (two Rainy seasons) is discussed and compared with unimodal tropics. The findings are matched against current ideas on optimal foraging.