The effects of spatially heterogeneous prey distributions on detection patterns in foraging seabirds

Many attempts to relate animal foraging patterns to landscape heterogeneity are focused on the analysis of foragers movements. Resource detection patterns in space and time are not commonly studied, yet they are tightly coupled to landscape properties and add relevant information on foraging behavior. By exploring simple foraging models in unpredictable environments we show that the distribution of intervals between detected prey (detection statistics) is mostly determined by the spatial structure of the prey field and essentially distinct from predator displacement statistics. Detections are expected to be Poissonian in uniform random environments for markedly different foraging movements (e.g. Lévy and ballistic). This prediction is supported by data on the time intervals between diving events on short-range foraging seabirds such as the thick-billed murre (Uria lomvia). However, Poissonian detection statistics is not observed in long-range seabirds such as the wandering albatross (Diomedea exulans) due to the fractal nature of the prey field, covering a wide range of spatial scales. For this scenario, models of fractal prey fields induce non-Poissonian patterns of detection in good agreement with two albatross data sets. We find that the specific shape of the distribution of time intervals between prey detection is mainly driven by meso and submeso-scale landscape structures and depends little on the forager strategy or behavioral responses.     

Unidirectional prey-predator facilitation: apparent prey enhance predators' foraging success on cryptic prey

Food availability can strongly affect predator-prey dynamics. When change in habitat condition reduces the availability of one prey type, predators often search for other prey, perhaps in a different habitat. Interactions between behavioural and morphological traits of different prey may influence foraging success of visual predators through trait-mediated indirect interactions (TMIIs), such as prey activity and body coloration. We tested the hypothesis that foraging success of stream-dwelling cutthroat trout (Onchorhyncus clarki) on cryptically coloured, less-active benthic prey (larval mayfly; Paraleptophebia sp.) can be enhanced by the presence of distinctly coloured, active prey (larval stonefly shredder; Despaxia augusta). Cutthroat trout preyed on benthic insects when drifting invertebrates were unavailable. When stonefly larvae were present, the trout ate most of the stoneflies and also consumed a higher proportion of mayflies than under mayfly only treatment. The putative mechanism is that active stonefly larvae supplied visual cues to the predator that alerted trout to the mayfly larvae. Foraging success of visual predators on cryptic prey can be enhanced by distinctly coloured, active benthic taxa through unidirectional facilitation to the predators, which is a functional change of interspecific interaction caused by a third species. This study suggests that prey-predator facilitation through TMIIs can modify species interactions, affecting community dynamics.     

Optimal foraging of sticklebacks on swarming prey

Hungry sticklebacks, Gasterosteus aculeatus, preferentially attacked the densest region of a swarm of water fleas, but with decreasing attack readiness they increasingly preferred less dense regions. Such a hunger dependent change in feeding preference has not yet been dealt with by optimal foraging theory. A model, which assumes that high swarm densities provide high feeding rates (because of small inter-prey distances) but also high costs of confusion, predicts that a predator should always choose the lowest prey density in which it can achieve a feeding rate sufficient to satisfy its hunger. Some predictions of the model were experimentally verified. Hungry fish have a higher feeding rate in a high prey density than in a lower density and less hungry fish have a higher rate in a low density than in a high density.     

The effects of human presence, flock size and prey density on shorebird foraging rates

Animals may alter their foraging behaviour in the presence of humans because they perceive humans as potential predators. In this study I determined whether people caused shorebirds to reduce feeding rates at a stopover site in coastal British Columbia, Canada. I controlled for prey density and flock size because these variables may influence both the foraging rates as well as the effect of human disturbance on feeding efficiency. Semipalmated plovers decreased feeding rates when there were more people on the beach (multiple regression: F_1,15=5.86, b=0.59, P=0.029, R²=37.6%). For least sandpipers, the effect of human densities on feeding rates depended on flock size (F_1,21=5.97, P=0.023) and amphipod availability (F_1,21=4.98, P=0.037). This study demonstrated the importance of measuring subtle behavioural changes in foraging rates along with key ecological variables in order to assess the true impact of human disturbance on migratory shorebirds.     

Influence of prey foraging posture on flight behavior and predation risk: predators take advantage of unwary prey

Foraging in animals is often associated with characteristicbody postures, such as the head-down posture. When foragingconflicts with the ability to detect predators or to flee, individualsmay incur a greater risk of mortality to predation than otherwise.Here we investigate the influence of different foraging postures(horizontal versus nose-down body posture) on the ability ofindividuals to respond to approaching predators and on the riskof mortality to predation in the guppy (Poecilia reticulata).Individuals engaged in nose-down foraging were assumed to beable to visually scan a smaller area for predators and to escapeless effectively due to their body posture, and thus are morevulnerable to stalking predators than horizontally foragingones. In a first experiment, we separately exposed nonforaging,horizontally foraging, and nose-down foraging guppies to anapproaching cichlid fish predator model. Nonforaging guppiesreacted sooner to and initiated flight further away from theapproaching model than did foraging fish collectively, and horizontallyforaging individuals responded sooner to the model than nose-downforaging ones. Comparing all test guppies, nose-down foragingindividuals were the most likely not to exhibit any responseto the predator model. When presented with a simultaneous choiceof two guppies behind a one-way mirror, individual blue acaracichlid (Aequidens pulcher), a natural predator of the guppy,preferred to attack foraging guppies over nonforaging ones andnose-down foraging guppies over horizontally foraging individuals.In a final experiment with free-swimming cichlids and guppies,we demonstrated that individual risk of predation for guppiesforaging nose down was greater than for guppies foraging horizontally,and both were at greater risk than nonforaging guppies. Thislatter result is consistent with the above differences in theguppy's responsiveness to approaching predators depending ontheir foraging behavior, and with the finding that cichlid predatorspreferred fish that were less likely to show any response tothem. Our results therefore indicate that the ability to respondto approaching predators and the risk of mortality to predationin the guppy is strongly influenced by their foraging activity,and in particular their foraging posture, and that cichlid predatorspreferentially select less wary and more vulnerable guppies.[BehavEcol 7: 264–271 (1996)]     

Ontogenetic changes in behavioral and kinematic components of prey capture strikes in a praying mantis

Evolutionary Ecology - Juveniles within a species are often faced with similar ecological pressures on performance as adults, such as obtaining food and escaping predation, but with the potential...     

Overwintering strategy of Yunnan snub-nosed monkeys: adjustments in activity scheduling and foraging patterns

Temperate forests are characterized by pronounced climatic and phenological seasonality. Primates inhabiting such environments experience prolonged resource scarcity and low ambient temperatures in winter and are expected to adjust time allocation and foraging behavior so as to maintain their energy balance. We analyzed the activity scheduling of a group of Yunnan snub-nosed monkeys (Rhinopithecus bieti) based on data collected over 20 months in the high-altitude (>3000 m) Samage Forest, Baimaxueshan Nature Reserve, PRC. The forest consists of evergreen conifers and oaks and deciduous broadleaf trees. The diet varied seasonally, with young leaves preferentially exploited in spring and fruits in summer. The monkeys subsisted on readily available fallback resources (mainly lichens) in winter [Grueter et al. in (Am J Phys Anthropol 140:700–715, 2009)]. We predicted that this switch to a relatively low-quality diet would prompt an increase in feeding effort and decrease in moving effort. We found that the monkeys spent significantly more time feeding in winter than in the other seasons. The monthly time devoted to feeding was also negatively correlated with temperature and positively with percentage of lichens in the diet. Time spent on moving did not vary among seasons or with temperature, but day-journey length was found to be longer on hotter days. Time spent resting was lower in winter and under colder conditions and was also negatively correlated with time spent feeding, indicating that resting time is converted into feeding time during times of ecological stress. These results indicate a strong effect of seasonality on time allocation patterns, constraints on inactivity phases, and the prevalence of an energy-conserving foraging strategy in winter, when costs of thermoregulation were high and the availability of preferred food was low.     

Scale-dependent hierarchical adjustments of movement patterns in a long-range foraging seabird

Foraging animals are expected to adjust their path according to the hierarchical spatial distribution of food resources and environmental factors. Studying such behaviour requires methods that allow for the detection of changes in pathways' characteristics across scales, i.e. a definition of scale boundaries and techniques to continuously monitor the precise movement of the animal over a sufficiently long period. We used a recently developed application of fractals, the changes in fractal dimension within a path and applied it to foraging trips over scales ranging across five orders of magnitude (10 m to 1000 km), using locations of wandering albatrosses (Diomedea exulans) recorded at 1 s intervals with a miniaturized global positioning system. Remarkably, all animals consistently showed the same pattern: the use of three scale-dependent nested domains where they adjust tortuosity to different environmental and behavioural constraints. At a small scale (ca. 100 m) they use a zigzag movement as they continuously adjust for optimal use of wind; at a medium scale (1-10 km), the movement shows changes in tortuosity consistent with food-searching behaviour; and at a large scale (greater than 10 km) the movement corresponds to commuting between patches and is probably influenced by large-scale weather systems. Our results demonstrate the possibility of identifying the hierarchical spatial scales at which long-ranging animals adjust their foraging behaviour, even in featureless environments such as oceans, and hence how to relate their movement patterns to environmental factors using an objective mathematical approach.     

The regulation of foraging activity in red harvester ant colonies

Behavioral plasticity in social insects is intriguing because colonies adjust to environmental change through the aggregated responses of individuals. Without central control, colonies adjust numbers of workers allocated to various tasks. Individual decisions are based on local information from the environment and other workers. This study examines how colonies of the seed-eating ant Pogonomyrmex barbatus adjust the intensity of foraging in an arid environment where conspecific neighbors compete for foraging area. The main question is how foragers decide whether to leave the nest. Patrollers search the area before foragers emerge. Removal experiments show that the return of the patrollers stimulates the onset of foraging, and later, the rate at which foragers return affects the rate at which foragers continue to leave the nest. Foraging activity is less sensitive to changes in the rate of returning foragers than to changes in the rate of returning patrollers. These results suggest that whether a colony forages at all on a given day depends on conditions detected early by patrollers but that once foraging begins, the intensity of foraging does not track, on an hourly timescale, how quickly foragers can find food.     

Biomass recycling: a key to efficient foraging by fungal colonies

Using an existing fungal growth model that captures the physiological processes of vegetative growth and development of a fungal colony, and in particular incorporates, for the first time, a recycling of biomass mechanism, we explore the effects of recycling in various environmental contexts. Here we test whether resource density thresholds exist, below which finite colony expansion occurs, in three dimensions based on the number of randomly removed resource sites. We then test the effect of recycling on resource density thresholds. Modelled soil structure, derived from experiments, is combined with the fungal growth model. The effect of recycling on foraging efficiency is investigated for resource distributed homogeneously and heterogeneously throughout the modelled soil structure. The simulated results show that resource density thresholds do exist in three dimensions and that the recycling mechanism decreases the threshold value. Our results indicate that recycling promotes persistence and a recycling mechanism is crucial for those fungi that reside in a resource patchy and limited environment.     

Seasonal changes in the behavioral effects of neuroleptics on white rats

Summarized results of the experiments (conducted in 1981-1984) demonstrate seasonal rhythms of some behavioural effects (catalepsy and depression of locomotor activity) of haloperidol (0.5 mg/kg) and levomepromazine (5 mg/kg) in white rats. In intact rats neuroleptics were more effective in depressing high than low motor activity. Catalepsy induced by single administration of neuroleptics was more pronounced in spring and autumn months. A certain negative correlation exists between seasonal variations of neuroleptic catalepsy and the speed of monoamine (dopamine and serotonin) metabolism in the brain of intact rats.     

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.     

A comparison of the effects of prey and non-prey neighbours on foraging rates of Epischura nevadensis (Copepoda: Calanoida)

SUMMARY. 1. Adults of the calanoid copepod, Epischura nevadensis , aggregate in situ near the thermocline in Lake Tahoe, California-Nevada. together with adults of another species of calanoid copepod, Diaptomus tyrelli and juveniles of both species. With a series of laboratory predation and algal clearance trials, we show that foraging rates of adult E. nevadensis are determined not only by the density of co-occurring potential prey (small copepods), but also by the presence of co-occurring non-prey neighbours (large, adult copepods). These effects occur at densities and in zooplankton assemblages found naturally, emphasizing the ecological importance of neighbours other than prey on zooplankton feeding.
2. Neighbours are distinguished primarily by size. Although predation rates increase linearly with the densities of small copepods. both algal clearance and predation rates decrease in the presence of large copepods. We also show, with a field predation experiment using small enclosures, that adults are size selective within species and that Diaptomus are selected over conspecifics of the same size.
3. We hypothesize that by reducing foraging rates in the presence of large zooplankton. E. nevadensis avoids predators and reduces predation risk at the cost of reduced energy consumption.     

Birds and neotropical mistletoes: effects on seedling recruitment

Two groups of neotropical mistletoes differ in fruit structure and consequently in the manner their seeds are deposited by avian frugivores. Birds defecate seeds of one group in clumps, but deposit seeds of the other group singly by regurgitation. A significant proportion of adult plants of both species were found growing alone, indicating a higher mortality with seed clumping.     

Seasonal changes in invertebrate drift: effects of declining summer flows on prey abundance for drift-feeding fishes

Hydrobiologia - To evaluate the consequences of declining summer discharge for drift abundance and energy flux to drift-feeding fish, we collected monthly drift samples from April to September in...     

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.