The effects of spatial food distribution and group size on foraging behaviour in a benthic fish

Animals foraging in heterogeneous environments benefit from information on local resource density because it allows allocation of foraging effort to rich patches. In foraging groups, this information may be obtained by individuals through sampling or by observing the foraging behaviour of group members. We studied the foraging behaviour of goldfish (Carassius auratus) groups feeding in pools on resources distributed in patches. First, we determined if goldfish use sampling information to distinguish between patches of different qualities, and if this allowed goldfish to benefit from a heterogeneous resource distribution. Then, we tested if group size affected the time dedicated to food searching and ultimately foraging success. The decision of goldfish to leave a patch was affected by whether or not they found food, indicating that goldfish use an assessment rule. Giving-up density was higher when resources were highly heterogeneous, but overall gain was not affected by resource distribution. We did not observe any foraging benefits of larger groups, which indicate that grouping behaviour was driven by risk dilution. In larger groups the proportion searching for food was lower, which suggests interactions among group members. We conclude that competition between group members affects individual investments in food searching by introducing the possibility for alternative strategies, such as scrounging or resource monopolisation.     

Trophic cascading effects of predatory fish on leaf litter processing in a Japanese stream

A manipulative field experiment to test for trophic cascading effects of predatory fish on detritus processing by benthic invertebrates was performed in stream channels running through a wetland forest in northern Japan. To control for fish effects on benthic invertebrates, two simple treatments (fish-present and fish-absent) were established for 4 weeks, with two common predatory fish, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin (Cottus nozawae), being introduced into and excluded from stream cages. At the end of experiment, the biomass of the dominant detritivore, an amphipod (Jesogammarus jezoensis), was significantly less in the fish-present treatment (0.56 g m^–2 in dry mass on average) than that in the fish-absent treatment (1.32 g m^–2), there being no significant treatment effect evident for the second-dominant detritivore, coleopteran larvae (Optioservus kubotai). The loss of oak leaves (Quercus crispla) from litter bags in the fish-present treatment (0.31 g week^–1 in dry mass on average) was significantly less than in the fish-absent treatment (0.54 g week^–1). Predator-induced lower biomass and likely lowered foraging activities of the J. jezoensis were responsible for the suppression of litter processing efficiency. In contrast, the standing crop of fine particulate organic matter did not differ significantly between the treatments. The experimental results revealed that the predatory fish had an indirect but significant effect on leaf litter processing in the stream.     

Significance of bacterial biomass in the nutrition of a freshwater isopod (Lirceus sp.)

The quantitative significance of bacterial biomass in the nutrition of detritivores remains equivocal. We have used tritiated thymidine to specifically label stable macromolecules in natural assemblages of sediment-associated and detritus-associated bacteria. This material was presented to the isopod (Lirceus sp.) and incorporation of bacterial biomass measured. The isopod incorporated roughly 1 ng bacterial carbon (mg wet wt.)^-1 h^-1 from leaf discs and about 6 ng mg^-1 h^-1 from sediment. Calculation of grazing rate from changes in cell counts yields grazing rates from 2.3–17.9 ng C mg^-1 h^-1. Even the maximum grazing rate, which is an overestimate of C assimilated, represents only 14.7% of C respired by the isopod.     

Contribution of fungal biomass to the diet of a freshwater isopod (Lirceus sp.)

SUMMARY. 1. The direct contribution of fungal biomass to the carbon needs of a freshwater isopod (Lirceus sp.) was measured using 14C-labelled fungi on leaf material. 14C-glucose was added to fungus-inoculated leaf discs periodically throughout the growth of the fungus. Amount of label incorporated into fungal biomass, radioactivity appearing in Lirceus after feeding on labeled fungus and the standing stock of fungus were used to calculate incorporation of fungal C by the consumer.
2. Incorporation rates ranged from 0.06 to 70 ng fungal C mg wet wt isopod-1h-1. These rates of incorporation represent from 0.05 to 57% of C respired by the isopod.
3. Fungal C does not meet the carbon needs of this consumer and the role of fungi as modifiers of the leaf substrate may be more significant than their role as a food source.     

A flexible model of foraging by a honey bee colony: the effects of individual behaviour on foraging success

This paper develops and explores a model of foraging in honey bee colonies. The model may be applied to forage sources with various properties, and to colonies with different foraging-related parameters. In particular, we examine the effect of five foraging-related parameters on the foraging response and consequent nectar intake of a homogeneous colony. The parameters investigated affect different quantities critical to the foraging cycle--visit rate (affected by g), probability of dancing (mpd and bpd), duration of dancing (mcirc), or probability of abandonment (A). We show that one parameter, A, affects nectar intake in a nonlinear way. Further, we show that colonies with a midrange value of any foraging parameter perform better than the average of colonies with high- and low-range values, when profitable sources are available. Together these observations suggest that a heterogeneous colony, in which a range of parameter values are present, may perform better than a homogeneous colony. We modify the model to represent heterogeneous colonies and use it to show that the most important effect of heterogeneous foraging behaviour within the colony is to reduce the variance in the average quantity of nectar collected by heterogeneous colonies.     

Clock control of foraging in the isopod Scyphax ornatus Dana

Abstract

The terrestrial sand beach isopod Scyphax ornatus Dana exhibits circadian and circa-semilunar activity rhythms when kept in constant conditions in the laboratory. The circadian rhythm restricts activity to the subjective night and can be synchronised to artificial light cycles. The circa-semilunar rhythm is expressed as a cyclic variation in overall activity level, with isopods alternately active on 6–8 consecutive nights and then inactive on the next 6–8 nights. The circa-semilunar rhythm matches a fortnightly cycle in food availability on the isopods’ home beach, but does not appear to be causally related to the lunar cycle, the spring/neap tidal cycle, or to moulting. This and other evidence suggests that the ecologicalrole of the endogenous circa-semilunar rhythm is to allow Scyphax, within the protection of their burrows, to predict nightly foraging opportunities; a new role for such a long-period rhythm.     

Predatory fish sounds can alter crab foraging behaviour and influence bivalve abundance

The risk of predation can have large effects on ecological communities via changes in prey behaviour, morphology and reproduction. Although prey can use a variety of sensory signals to detect predation risk, relatively little is known regarding the effects of predator acoustic cues on prey foraging behaviour. Here we show that an ecologically important marine crab species can detect sound across a range of frequencies, probably in response to particle acceleration. Further, crabs suppress their resource consumption in the presence of experimental acoustic stimuli from multiple predatory fish species, and the sign and strength of this response is similar to that elicited by water-borne chemical cues. When acoustic and chemical cues were combined, consumption differed from expectations based on independent cue effects, suggesting redundancies among cue types. These results highlight that predator acoustic cues may influence prey behaviour across a range of vertebrate and invertebrate taxa, with the potential for cascading effects on resource abundance.     

A general framework for understanding the effects of variability and interruptions on foraging behaviour

A general framework for analysing the effects of variability and the effects of interruptions on foraging is presented. The animal is characterised by its level of energetic reserves, x. We consider behaviour over a period of time [0,T]. A terminal reward function R(x) determines the expected future reproductive success of an animal with reserves x at time T. For any state x at a time in the period, we give the animal a choice between various options and then constrain it to follow a background strategy. The best option is the one that maximizes expected future reproductive success. Using this framework, we show that sensitivity to variability in amount of energy gained is logically distinct from sensitivity to variability in the time at which food is obtained. We also show that incorporating interruptions results in both a preference for variability in time and a preference for a reward followed by a delay as opposed to the same delay before the reward.     

Indirect effects of the algivorous fish Plecoglossus altivelis altivelis on the growth of two insectivorous benthic fish

1. Animals exploiting different resources may nevertheless interact if one species indirectly alters the abundance and distribution of the food of the other. To analyse this indirect effect, we conducted experiments in artificial pools and in the field to investigate the influence of the algivorous fish Plecoglossus altivelis altivelis (known as the ayu) on two species of insectivorous benthic fish, Pseudogobio esocinus esocinus and the goby Gymnogobius petschiliensis .
2. In the pool experiments, algal biomass was not correlated with the number of ayu, but the percentage of blue-green bacteria rose as the number increased. The number of aquatic macroinvertebrates on the upper surface of ceramic tiles placed in the pool bed decreased as the number of ayu increased.
3. Although ayu and the benthic species did not interact directly, the reduction in invertebrate abundance on the upper surface of tiles in the pool reduced the growth rate of the benthic insectivores.
4. In field experiments, the introduction of ayu into habitats with P. esocinus esocinus or G. petschiliensis reduced the growth rate of these benthic fish. In the field experiment that was carried out over 5 years in the G. petschiliensis habitat, the population density of the goby decreased when ayu were stocked.
5. The ayu is a strong interactor or bioengineer in streams, affecting not only benthic algae but also aquatic invertebrates and fishes. We conclude that to predict the outcome of interspecific interactions amongst fishes in streams with high algal production, possible indirect effects must be considered alongside better known direct effects.     

Indirect effects of insect herbivory on leaf gas exchange in soybean

Herbivory can affect plant carbon gain directly by removing photosynthetic leaf tissue and indirectly by inducing the production of costly defensive compounds or disrupting the movement of water and nutrients. The indirect effects of herbivory on carbon and water fluxes of soybean leaves were investigated using gas exchange, chlorophyll fluorescence and thermal imaging. Herbivory by Popillia japonica and Helicoverpa zea (Boddie) caused a 20–90% increase in transpiration from soybean leaflets without affecting carbon assimilation rates or photosynthetic efficiency (Φ_PSII). Mechanical damage to interveinal tissue increased transpiration up to 150%. The spatial pattern of leaf temperature indicated that water loss occurred from injuries to the cuticle as well as from cut edges. A fluorescent tracer (sulforhodamine G) indicated that water evaporated from the apoplast approximately 100 µm away from the cut edges of damaged leaves. The rate of water loss from damaged leaves remained significantly higher than from control leaves for 6 d, during which time they lost 45% more water than control leaves (0.72 mol H₂O per cm of damaged perimeter). Profligate water loss through the perimeter of damaged tissue indicates that herbivory may exacerbate water stress of soybeans under field conditions.     

Density-dependent foraging behaviour of sheep on alpine pastures: effects of scale

Foraging patterns of large herbivores may give important clues as to why their life history varies depending on population density. In this landscape-scale experiment, domestic sheep Ovis aries were kept at high (80 sheep km⁻²) and low (25 sheep km⁻²) population densities during summer in high mountain pastures in Hol, Norway. We predicted an increasing use of less preferred plant species or habitat types with increasing sheep population density. Foraging behaviour was investigated by direct observations of individually marked sheep on different spatial scales, and diet composition was also assessed with microhistological analysis of faecal samples from known individuals. We found that the effects of density on foraging behaviour depended on scale and were only detected at the scale of diet choice. Use of the common grass species Deschampsia flexuosa , which provided the bulk forage (10–65% of the diet), remained constant throughout the season at low densities, but increased significantly over time at high densities. On a coarser spatial scale, neither within vegetation type nor between vegetation types, selection was affected by density, but vegetation type selection differed depending on whether the sheep were grazing or resting. Our study provides evidence of density dependence in foraging behaviour, but only at the finest spatial scale (diet choice).     

Multi-scale effects of resource patchiness on foraging behaviour and habitat use by longnose dace, Rhinichthys cataractae

1. We examined the response of a predatory benthic fish, the longnose dace ( Rhinichthys cataractae ), to patchiness in the distribution of benthic macroinvertebrates on cobbles at three hierarchical spatial scales during summer and autumn 1996, and spring 1997 in a southern Appalachian stream. 2. At the primary scale (four to five individual cobbles separated by <1 m), the intensity of foraging was not correlated with the biomass of benthic macroinvertebrates/cobble, regardless of season. 3. At the secondary scale (i.e. foraging patches <5 m in diameter) we found that benthic macroinvertebrates were patchily distributed in summer, but not in autumn or spring. Concomitantly, in summer, longnose dace foraged on cobbles with a significantly higher biomass of benthic macronvertebrates than nearby, randomly selected cobbles with similar physical conditions (i.e. longnose dace tended to avoid low-prey foraging patches). In contrast, when benthic macroinvertebrates were distributed homogeneously (spring and autumn), dace did not select patches with a significantly higher biomass of benthic macroinvertebrates than that available on randomly selected cobbles. 4. At the tertiary scale (i.e. stream reaches 11–19 m long), the biomass of benthic macroinvertebrates (per cobble per reach) was patchily distributed (i.e. differed significantly among reaches) in all seasons. Among reaches with physical characteristics preferred by longnose dace, (i.e. erosional reaches dominated by cobble/boulder substratum and high current velocity), we detected a significant, positive correlation between the biomass of benthic macroinvertebrates/cobble and longnose dace density in all seasons. 5. Our results demonstrated that both spatial and temporal patchiness in resource availability influenced significantly the use of both foraging patches and stream reaches by longnose dace.     

Predator avoidance costs and habituation to fish chemicals by a stream isopod

1. We examined whether long-term exposure to chemical cues of predatory longear sunfish (Lepomis megalottis) affected growth and rates of leaf processing by the isopod, Lirceus fontinalis, an important facultative shredder in low-order streams in the eastern United States, and whether isopods habituated to sunfish chemicals. 2. Long-term (24 days) exposure to fish chemicals did not affect growth or the extent of leaf processing by isopods in the laboratory. Thus, chemical cues alone may not be important in triggering long-term predator avoidance behaviours that could potentially reduce fitness or affect community processes like Litter breakdown. 3. Isopods exposed to fish chemicals for 3, 9 and 15 days were significantly more active than individuals unexposed to fish chemicals, when transferred to another environment with fish chemicals. This result, coupled with results of the growth experiment, suggests that isopods habituate to fish chemical stimuli and that time to habituation is c. 3 days or less. Because fish chemicals can mislead prey about predator presence, and hence be a non-threatening stimulus, they are probably important in eliciting only short-term antipredatory behaviours by isopods.     

Natural selection on unpalatable species imposed by state-dependent foraging behaviour

Müllerian mimicry is typically thought to arise as a consequence of defended prey species adopting a similar way of signalling their unprofitability, thereby reducing the costs of predator education. Here we consider subsequent selection on the morphology of prey species, in the potentially lengthy period of time when predators are generally aware of the noxious qualities of their prey (and so no further learning is involved). Using a pair of stochastic dynamic programming equations which describe both the toxin burdens of a predator and its energy level, we identified the optimal state-dependent rules that maximize a predator's long-term survivorship, and examined the implications of this behaviour for the evolution of prey morphologies. When palatable prey are in short supply then those prey species which contain relatively low doses of toxins become profitable to consume by hungry predators. Under these conditions, a weakly defended prey could gain selective advantage in the post educational period by resembling a prey species which contained a higher dose of the same or different toxins, although the precise nature of the ecological relationship between model and mimic could either be mutualistic or parasitic depending on how mimic density increases when favoured by selection. Our work formally demonstrates that one does not always need to invoke educational effects to explain why two or more unpalatable species have evolved a similar appearance, or to explain why mimetic similarity among distasteful species is maintained over time. When two species contain high levels of different toxins then they may gain mutual advantage by resembling one another, not only by educating the predator as to their common unprofitability (classical Müllerian mimicry), but also by increasing predator uncertainty as to the specific kind of toxin a prey item contains.     

Evolution of larval foraging behaviour in Drosophila and its effects on growth and metabolic rates

Abstract.  The evolution of foraging in Drosophila melanogaster (Meigen) is studied using outbred populations that had been differentiated using laboratory selection. The foraging behaviour of Drosophila larvae is measured using the foraging path length of 72-h-old larvae. The foraging path length is the distance travelled by foraging larvae over 5 min. Populations of Drosophila selected for rapid development show significantly greater path lengths than their controls. Populations of Drosophila selected for resistance to ammonia and urea in their larval food have shorter path lengths than their controls. Individuals in the ammonia-resistant populations are smaller than those in the control populations, but the size-adjusted metabolic rates are not significantly different. A simple model is proposed suggesting that changes in larval foraging behaviour may be a means for Drosophila larvae to adapt to new environments that require additional maintenance energy. In the ammonia-selected populations, crucial tests of these ideas will have to be conducted in environments with ammonia.     

Influence of sward structure on daily intake and foraging behaviour by horses

The spatial heterogeneity of grasslands determines the abundance and quality of food resources for grazing animals. As plants mature, they increase in mass, which allows greater instantaneous intake rates, but the cell wall concentrations increase too, reducing diet quality. In ruminants, daily intake rates are often constrained by the time needed for the ingesta to pass through the rumen, which is influenced by the rate of digestion. It has been suggested that the digestive constraint should have much less effect on hindgut fermenters such as equids. Horses play an increasing role in the management of grasslands in Europe, but the data on the influence of the heterogeneity of the vegetation on their daily intake and foraging behaviour are sparse. We report here the results of a preliminary study concerning the effects of sward structure on nutrient assimilation and the use of patches of different heights by horses grazing successively a short immature, a tall mature and a heterogeneous pastures (with short and tall swards). Daily nutrient assimilation was higher in the heterogeneous pasture compared to the short (+35%) and the tall (+55%) ones. The digestive constraints may have limited voluntary intake by horses on the tall swards. In the heterogeneous pasture, the mean height used for feeding (6 to 7 cm) by horses was intermediate between the heights used in the short (4 to 5 cm) and tall pastures (22 to 23 cm), and the animals may thus have benefited from both short swards of high quality and tall swards offering a higher instantaneous intake rate.     

Food and foraging behaviour of the Snares fernbird

Abstract

The Snares fernbird (Bowdleria punctata caudata Buller, 1894) is an opportunistic insectivore, its prey ranging greatly in size and type. Birds forage over a wide variety of sites, including forest, tussock, boulder beaches, cliffs, penguin colonies, and floating kelp. Foraging methods vary with the prey sought, and at times birds adopt specific “search images” when hunting certain insects. Details of foods brought to nestlings are given. Four postures used by fernbirds when foraging actively are described.     

Interspecific dominance and the foraging behaviour of juncos

We observed mixed groups of dark-eyed juncos (Junco hyemalis) and grey-headed juncos (Junco caniceps) at baited sites in northern Arizona during the non-breeding season. In interspecific and inter-racial conflicts, J. caniceps dorsalis was dominant to J. caniceps caniceps and to two races of dark-eyed juncos. Junco caniceps dorsalis also fed significantly faster than any of the other juncos. For both species, feeding rates were approximately the same in large and small mixed-species groups, though in larger groups, individual grey-headed juncos won conflicts at a higher rate and individual dark-eyed juncos lost conflicts at a higher rate. Also, dark-eyed juncos fed at a significantly lower rate in groups comprised mostly of grey-headed juncos than in groups of similar size but composed mostly of conspecifics. Residency times and recapture probabilities were similar for the two species, suggesting little difference in over-winter survival.