Foraging by Male and Female Solitary Bees with Implications for Pollination

Both male and female solitary bees visit flowers for rewards. Sex related differences in foraging efficiency may also affect their probability to act as pollinators. In some major genera of solitary bees, males can be identified from a distance enabling a comparative foraging-behavior study. We have simultaneously examined nectar foraging of males and females of three bee species on five plant species in northern Israel. Males and females harvested equal nectar amounts but males spent less time in each flower increasing their foraging efficiency at this scale. The overall average visit frequencies of females and males was 27.2 and 21.6 visits per flower per minute respectively. Females flew shorter distances increasing their visit frequency, relative foraging efficiency and their probability to pollinate. The proportion of conspecific pollen was higher on females, indicating higher floral constancy and pollination probability. The longer flights of males increase their probability to cross-pollinate. Our results indicate that female solitary bees are more efficient foragers; females seem also to be more efficient pollinators but males contribute more to long-distance pollen flow.     

Foraging strategies of insects for gathering nectar and pollen, and implications for plant ecology and evolution

The majority of species of flowering plants rely on pollination by insects, so that their reproductive success and in part their population structure are determined by insect behaviour. The foraging behaviour of insect pollinators is flexible and complex, because efficient collection of nectar or pollen is no simple matter. Each flower provides a variable but generally small reward that is often hidden, flowers are patchily distributed in time and space, and are erratically depleted of rewards by other foragers. Insects that specialise in visiting flowers have evolved an array of foraging strategies that act to improve their efficiency, which in turn determine the reproductive success of the plants that they visit. This review attempts a synthesis of the recent literature on selectivity in pollinator foraging behaviour, in terms of the species, patch and individual flowers that they choose to visit.

The variable nature of floral resources necessitate foraging behaviour based upon flexible learning, so that foragers can respond to the pattern of rewards that they encounter. Fidelity to particular species allows foragers to learn appropriate handling skills and so reduce handling times, but may also be favoured by use of a search image to detect flowers. The rewards received are also used to determine the spatial patterns of searches; distance and direction of flights are adjusted so that foragers tend to remain within rewarding patches and depart swiftly from unrewarding ones. The distribution of foragers among patchy resources generally conforms to the expectations of two simple optimal foraging models, the ideal free distribution and the marginal value theorem.

Insects are able to learn to discriminate among flowers of their preferred species on the basis of subtle differences in floral morphology. They may discriminate upon the basis of flower size, age, sex or symmetry and so choose the more rewarding flowers. Some insects are also able to distinguish and reject depleted flowers on the basis of ephemeral odours left by previous visitors. These odours have recently been implicated as a mechanism involved in interspecific interactions between foragers.

From the point of view of a plant reliant upon insect pollination, the behaviour of its pollinators (and hence its reproductive success) is likely to vary according to the rewards offered, the size and complexity of floral displays used to advertise their location, the distribution of conspecific and of rewards offered by other plant species, and the abundance and behaviour of other flower visitors.     

Context-dependent specialisation drives temporal dynamics in intra- and inter-individual variation in foraging behaviour within a generalist bird population

Individual niche variation is common within animal populations, and has significant implications for a wide range of ecological and evolutionary processes. However, individual niche differences may also temporally vary as a result of behavioural plasticity. While it is well understood how niche variation is affected by changes in resource availability, comparatively little is known about the extent to which individual niche differences may vary within the annual cycle due to internal drivers. Here, we assess how time- and energy-constraints imposed by incubating and brood rearing affect inter- and intra-individual variation in the foraging behaviour of lesser black-backed gulls, a generalist seabird with strong individual niche variation. To this end, we compared daily foraging trips of 22 breeding and 23 non-breeding GPS-tracked adult gulls from two colonies in the Southern Bight of the North Sea over the course of the breeding season. We find that breeding birds, unlike non-breeding ones, did indeed alter their foraging behaviour during the breeding season. Both sexes reduced their searching effort by increasingly revisiting earlier foraging locations, allowing for shorter and more frequent foraging trips. Breeding females also showed pronounced shifts in their habitat use and strongly specialised on urbanised foraging habitats throughout the breeding season. Hence, while individual variation in habitat use remained largely consistent within non-breeders and in breeding males, individual variation among breeding females almost completely disappeared. Female lesser black-backed gulls are on average smaller, and therefore often outcompeted by males for the most profitable food sources. The temporal specialisation on spatially reliable anthropogenic food sources during breeding hence suggests a complex interplay between intrinsic competitive constraints, resource reliability and shifting time- and energy budges in shaping temporal dynamics in individual niche variation within our study population.     

Pollen Harvest by Stingless Bee Foragers (Hymenoptera,Apidae, Meliponinae)

Data on pollen load capacity and flower constancy are discussed for nine stingless bee species. The foragers present high levels of flower constancy and often visit only one flower type (on average 97% of bee foragers), rarely a few flower types (on average 3% of bee foragers), during the same foraging trip. The latter foragers exhibit no tendencies for choosing similar sources, related either to flower type or to pollen type. Pollen load capacity (the ratio between pollen load weight/worker weight) decreases as forager body weight increases, so it is larger in smaller stingless bees species and smaller in larger ones. Nevertheless, it seems that specific load capacity also depends on pollen types.     

Proboscis length and resource utilization in two Uruguayan bumblebees: Bombus atratus Franklin and Bombus bellicosus Smith (Hymenoptera: Apidae)

Bumblebees (Bombus sp.) are eusocial insects with an annual life cycle whose use as pollinator of crops has gained great importance in modern agriculture. Several authors have found that resource use in Bombus species is usually based on the correlation between the proboscis length of the bumblebees and the corolla depth of the flowers. The aim of this study was to determine proboscis length of Bombus atratus and B. bellicosus, two Uruguayan bumblebees, and verify the resource exploitation testing two cultivated species, the red clover and the bird's foot trefoil. Bumblebee foraging activity was recorded in two culture conditions: in a red clover and bird's foot trefoil mixed meadow, and in contiguous plots of these legumes, and the proboscis length of collected foragers was determined. Both species of bumblebees exploited red clover and bird's foot trefoil although they did it in different proportions in all instances tested. The results indicated that the choice of the resources in B. atratus and B. bellicosus was influenced by their proboscis length. Bombus atratus has a longer proboscis and preferably visited red clover, possibly obtaining nectar easier and faster than B. bellicosus, which has a shorter proboscis. Bombus bellicosus used both resources without any clear preference.     

Long-term species, sexual and individual variations in foraging strategies of fur seals revealed by stable isotopes in whiskers

Background

Individual variations in the use of the species niche are an important component of diversity in trophic interactions. A challenge in testing consistency of individual foraging strategy is the repeated collection of information on the same individuals.

Methodology/Principal Findings

The foraging strategies of sympatric fur seals (Arctocephalus gazella and A. tropicalis) were examined using the stable isotope signature of serially sampled whiskers. Most whiskers exhibited synchronous δ¹³C and δ¹⁵N oscillations that correspond to the seal annual movements over the long term (up to 8 years). δ¹³C and δ¹⁵N values were spread over large ranges, with differences between species, sexes and individuals. The main segregating mechanism operates at the spatial scale. Most seals favored foraging in subantarctic waters (where the Crozet Islands are located) where they fed on myctophids. However, A. gazella dispersed in the Antarctic Zone and A. tropicalis more in the subtropics. Gender differences in annual time budget shape the seal movements. Males that do not perform any parental care exhibited large isotopic oscillations reflecting broad annual migrations, while isotopic values of females confined to a limited foraging range during lactation exhibited smaller changes. Limited inter-individual isotopic variations occurred in female seals and in male A. tropicalis. In contrast, male A. gazella showed large inter-individual variations, with some males migrating repeatedly to high-Antarctic waters where they fed on krill, thus meaning that individual specialization occurred over years.

Conclusions/Significance

Whisker isotopic signature yields unique long-term information on individual behaviour that integrates the spatial, trophic and temporal dimensions of the ecological niche. The method allows depicting the entire realized niche of the species, including some of its less well-known components such as age-, sex-, individual- and migration-related changes. It highlights intrapopulation heterogeneity in foraging strategies that could have important implications for likely demographic responses to environmental variability.     

Accumulating wing damage affects foraging decisions in honeybees (Apis mellifera L.)

Abstract.  1. Nectar-foraging honeybees ( Apis mellifera ) on lavender ( Lavandula stoechas ) appear to forage so as to maximise net energy return from foraging bouts; however, evidence from other studies suggests that foraging has a detrimental effect on survival, due at least in part to physiological deterioration of the flight mechanism. But foragers also acquire wing damage during foraging, which may increase foraging effort and reduce foraging lifespan.
2. The accumulation of damage over time and its effects on foraging flight and flower choice were studied in the field using a system in which the criteria for flower preference by foragers was known from previous work. Wing damage accumulated exponentially over time and resulted in foragers becoming less choosy about the flowers they visited.
3. Damage added experimentally contributed independently to the effect on choosiness. Effects of wing damage (natural and added experimentally) were also independent of those of a relative measure of age, which related in an inconsistent way to changes in foraging preferences.     

Why are there so many species of bumble bees at Dungeness?

WILLIAMS, P. H., 1989. Why are there so many species of bumble bees at Dungeness? Dungeness is unique in the British Isles in that it has more species of bumble bees than any other locality. Three ideas about what governs the number of species at a locality are examined by locking at patterns of flower visits at Dungeness in comparison with those at Shoreham, a species-poor locality also in Kent. The species of bumble bees that are present at Dungeness but absent from Shoreham show no association in their distributions among 2 km grid-squares in Kent with the species of food-plants that they prefer at Dungeness, nor is there any correlation between the diversity of bees and diversity of food-plants at Dungeness and Shoreham. From the information available, Dungeness is most likely to have more species of bumble bees because it has a particularly high density of the more nectar-rich flowers that bumble bees can use. Bumble bees feed most profitably from deep flowers because these contain more nectar than shallow flowers, although direct access to deeper flowers is ultimately limited by the length of each bee's proboscis. The distribution of worker proboscis lengths among species in the species-pool in Kent is clumped about a median of 7.9 mm. The best foraging conditions for the maximum number of species should be provided when flowers of similar depths are present in sufficiently large numbers for all foragers to make near-optimal flower choices. Although there is no difference in median between the distributions of the bees' proboscis lengths and the depths of the flowers they use at Dungeness, at Shoreham the flower depths used are shorter than the proboscis lengths. Among the food-plants at Dungeness, high densities of Teucrium scorodonia and Echium vulgare are likely to be especially important.     

Frequency–dependent pollinator foraging in polymorphic Clarkia xantiana ssp. xantiana populations: implications for flower colour evolution and pollinator interactions

Under many circumstances pollinators are expected to practice positive frequency–dependent foraging in colour-polymorphic plant populations. Theory suggests, however, that competition for floral resources might favor negative frequency–dependent foraging by some pollinator species, possibly contributing to the maintenance of flower colour variation by negative frequency–dependent selection. We addressed this idea with pollination studies of the California annual plant Clarkia xantiana ssp. xantiana (Onagraceae), which is polymorphic for the presence of conspicuous petal spots and is pollinated by several specialist bee species. At the level of entire pollinator assemblages, we did not detect significant fixed flower colour preferences or frequency–dependent foraging. Three species of specialist bee pollinators, however, showed contrasting forms of frequency–dependent foraging. The most widespread species, Hesperapis regularis (Melittidae) exhibited positive frequency dependence. Two other common species, Lasiglossum pullilabre (Halictidae) and Ceratina sequoiae (Apidae), preferred to visit whichever morph (unspotted or spotted) was locally in the minority. All three species were found to be effective at transferring C. xantiana pollen; H. regularis appeared most effective. Our findings suggest that a mixture of positive and negative frequency–dependent selection on flower colour occurs in C. xantiana , with the form and intensity of selection varying in space and time with pollinator assemblages. Negative frequency–dependent selection via pollination dynamics may play a larger role in maintaining genetic variation in flower colour than was previously thought. Our results also suggest an unappreciated form of niche partitioning among specialist pollinators. Genetic polymorphism in flower colour may sometimes facilitate pollinator coexistence.     

Temporal stability of niche use exposes sympatric Arctic charr to alternative selection pressures

There is now strong evidence that foraging niche specialisation plays a critical role in the very early stages of resource driven speciation. Here we test critical elements of models defining this process using a known polymorphic population of Arctic charr from subarctic Norway. We test the long-term stability of niche specialisation amongst foraging predators and discuss the possibility that contrasting foraging specialists are exposed to differing selection regimes. Inter-individual foraging niche stability was measured by combining two time-integrated ecological tracers of the foraging niche (each individual’s δ¹³C and δ¹⁵N stable isotope (SI) signatures and their food borne parasite fauna) with a short-term measure of foraging niche use (stomach contents composition). Three dietary subgroups of predators were identified, including zooplankton, gammarid and benthivore specialists foragers. Zooplanktivorous specialists had muscle low in δ ¹³C, a high abundance of parasites transmitted from pelagic copepods, a smaller head, longer snout and a more slender body-form than gammaridivorous specialist individuals which had muscle more enriched in δ ¹³C and high abundance of parasites transmitted from benthic Gammarus. Benthivorous individuals were intermediate between the other two foraging groups according to muscle SI-signals (δ¹³C) and loadings of parasites transmitted from both copepods and Gammarus. The close relationship between subgroups identified by stomach contents, time-integrated tracers of niche use (SI and parasites) and functional trophic morphology (niche adaptations) demonstrate a long-term temporally stable niche use of each individual predator. Differential habitat use and contrasting parasite communities and loadings, show differential exposure to different suites of selection pressures for different foraging specialists. Results also show that individual specialisation in trophic behaviour and thus exposure to different suites of selection pressures are stable over time, and thus provide a platform for disruptive selection to operate within this sympatric system.     

Adaptive foraging behaviour of individual pollinators and the coexistence of co-flowering plants

Although pollinators can play a central role in determining the structure and stability of plant communities, little is known about how their adaptive foraging behaviours at the individual level, e.g. flower constancy, structure these interactions. Here, we construct a mathematical model that integrates individual adaptive foraging behaviour and population dynamics of a community consisting of two plant species and a pollinator species. We find that adaptive foraging at the individual level, as a complementary mechanism to adaptive foraging at the species level, can further enhance the coexistence of plant species through niche partitioning between conspecific pollinators. The stabilizing effect is stronger than that of unbiased generalists when there is also strong competition between plant species over other resources, but less so than that of multiple specialist species. This suggests that adaptive foraging in mutualistic interactions can have a very different impact on the plant community structure from that in predator–prey interactions. In addition, the adaptive behaviour of individual pollinators may cause a sharp regime shift for invading plant species. These results indicate the importance of integrating individual adaptive behaviour and population dynamics for the conservation of native plant communities.     

Effects of recent experience on foraging decisions by bumble bees

The temporal and spatial scales employed by foraging bees in sampling their environment and making foraging decisions should depend both on the limits of bumble bee memory and on the spatial and temporal pattern of rewards in the habitat. We analyzed data from previous experiments to determine how recent foraging experience by bumble bees affects their flight distances to subsequent flowers. A single visit to a flower as sufficient to affect the flight distance to the next flower. However, longer sequences of two or three visits had an additional effect on the subsequent flight distance of individual foragers. This suggests that bumble bees can integrate information from at least three flowers for making a subsequent foraging decision. The existence of memory for floral characteristics at least at this scale may have significance for floral selection in natural environments.     

Not only hedgerows,but also flower fields can enhance bat activity in intensively used agricultural landscapes

Agri-environment schemes (AES) have become important tools for farmland biodiversity conservation, providing suitable habitats, resources, and connectivity within the agricultural landscapes. Bats are rarely studied in relation to AES effectiveness in contrast to birds, even though their presence and activity as biological control agents on insects, especially pest species, can be important for agricultural crops. While the role of hedgerows for bat occurrence and activity, as well as for their prey's diversity and abundance has been widely studied, the role of other AESs such as flower fields remains unclear. We monitored the activity of the main functional groups (edge, narrow and open space foragers) using ultrasound recorders, as well as potential prey abundances using light traps, across 35 study sites representing different AES in Central Germany from late July to September 2018. The sampled AES consisted of annual flower fields, mixed flower fields (with annual and biennial vegetation), perennial flower fields (sown every 5 years), hedgerows (surrounded by meadows and agricultural fields), and were compared to winter wheat (control) in a balanced design. Bat activity over hedgerows increased threefold for edge space foragers and sevenfold for narrow space foragers compared to wheat fields. Compared to wheat fields, narrow space forager activity increased fourfold over perennial flower fields, threefold over annual and twofold over mixed flower fields. This group's activity over hedgerows also increased almost threefold compared to mixed flower fields. However, the number of feeding buzzes and prey abundance did not differ significantly between AES. We detected foraging group-specific differences in bat activity between the studied AES. Thus, to promote bats in agricultural landscapes and to ensure their biological control services, it is important to establish more AES, such as hedgerows and flower fields, to increase their diversity and connectivity in intensively used agricultural landscapes.     

The use of field–based social information in eusocial foragers: local enhancement among nestmates and heterospecifics in stingless bees

Abstract. 1. Foragers of social insects can be guided to profitable food sources by social information transfer within the nest. This study showed that in addition to such an information-centre strategy, social information in the field also plays an important role in individual foraging decisions. The effect of the presence of a nestmate on individual decision-making on where to forage was investigated in six species of stingless bee that differ in their recruitment system. Some species preferred to feed close to a nestmate (local enhancement) whereas other species actively avoided landing close to a nestmate. The term local inhibition is introduced for this avoidance behaviour.
2. Local enhancement and local inhibition were species specific but were not related to the species' recruitment system.
3. Local enhancement and local inhibition were affected by the individual's experience with the food source. Newly recruited foragers of Trigona amalthea showed local enhancement whereas experienced foragers showed local inhibition.
4. These individual decision-making rules explained accurately the spatial distribution of recruited nestmates: foraging groups of T. amalthea , which shows local inhibition, were more dispersed than foraging groups of Oxytrigona mellicolor , which shows local enhancement.
5. The effect of heterospecifics on stingless bee flower choice was investigated for 18 species combinations. Landing decisions were influenced significantly by the aggressiveness and the body size of the resident bee. Larger and more aggressive heterospecifics were avoided, whereas in some cases less aggressive bees acted as an attraction cue.     

Chemical and behavioural studies of the trail-following pheromone in the leaf-cutting ant Atta opaciceps,Borgmeier (Hymenoptera: Formicidae)

To understand the significance of the trail pheromone used in chemical communication of the leaf-cutting ants Atta opaciceps we investigated, under laboratory conditions, the trail-following behaviour of different castes. We observed a clear behavioural discrimination of conspecific venom gland extract of foraging ants from those of other species. Additionally, we determined the pheromone composition of A. opaciceps venom gland secretion using a two-dimensional gas chromatography coupled with mass spectrometry. Chemical analyses revealed the presence of three nitrogen-containing compounds, identified as 2,5-dimethylpyrazine, 3-ethyl-2,5-dimethylpyrazine and methyl 4-methylpyrrole-2-carboxylate (M4MPC). Four different bioassays performed with workers from different castes of A. opaciceps suggested that the trail pheromone elicits the trail-following behaviour in conspecifics of all castes, but the foragers respond more strongly to their own pheromone than to that of other castes (gardeners, generalists and soldiers). In addition, A. opaciceps foragers follow the trails made with the venom gland extracts of the unrelated Acromyrmex subterraneus subterraneus foragers as well as they follow the trails made with their own venom gland extract. M4MPC was identified to be the most abundant and the most behaviourally active component of the venom gland extract of A. opaciceps foragers.     

The Allometry of Bee Proboscis Length and Its Uses in Ecology

Allometric relationships among morphological traits underlie important patterns in ecology. These relationships are often phylogenetically shared; thus quantifying allometric relationships may allow for estimating difficult-to-measure traits across species. One such trait, proboscis length in bees, is assumed to be important in structuring bee communities and plant-pollinator networks. However, it is difficult to measure and thus rarely included in ecological analyses. We measured intertegular distance (as a measure of body size) and proboscis length (glossa and prementum, both individually and combined) of 786 individual bees of 100 species across 5 of the 7 extant bee families (Hymenoptera: Apoidea: Anthophila). Using linear models and model selection, we determined which parameters provided the best estimate of proboscis length. We then used coefficients to estimate the relationship between intertegular distance and proboscis length, while also considering family. Using allometric equations with an estimation for a scaling coefficient between intertegular distance and proboscis length and coefficients for each family, we explain 91% of the variance in species-level means for bee proboscis length among bee species. However, within species, individual-level intertegular distance was a poor predictor of individual proboscis length. To make our findings easy to use, we created an R package that allows estimation of proboscis length for individual bee species by inputting only family and intertegular distance. The R package also calculates foraging distance and body mass based on previously published equations. Thus by considering both taxonomy and intertegular distance we enable accurate estimation of an ecologically and evolutionarily important trait.     

Role of morphological variables of the visitor butterfly species in relation to their foraging behaviour on Lantana camara: Implication for conservation

The study of foraging behaviour of butterflies has been an important focus among the lepidopterists for years. Although established as invasive weed plant, the role Lantana camara as an important host plant as well as a food plant for the butterfly species is now well-known. The present study aims to interpret the role of morphological variables of the visitor butterflies in relation to their foraging behaviour on Lantana camara. Butterflies with longer proboscis were more efficient in foraging on flowers of Lantana camara than the butterflies with shorter ones, when the species containing proboscis same as or longer than their body length were excluded. Longer relative proboscis length (RPL) have selective advantage for foraging over Lantana camara up to a certain point (RPL = 0.881) and after that larger relative proboscis length often obstructs foraging behaviour as the handling time increases and these butterflies face more resistance against incoming nectar than the others.The residuals (body weight-wing span) also act as important factor.The handling time of the butterflies on Lantana camara shows a negative relationship with the residuals. The findings suggests that in case of Lantana camara as a nectaring plant butterfly species with longer proboscis have selective advantage for exploitation of the resource than the species with shorter ones and plantation of Lantana camara may be useful for conserving the long tongued butterfly species.     

Production of greenhouse tomatoes (Lycopersicon esculentum) using Nannotrigona perilampoides, Bombus impatiens and mechanical vibration (Hym.: Apoidea)

Importation of exotic bumblebees for greenhouse pollination may be restricted in México, thus making it necessary to evaluate the potential of native species as pollinators in enclosures. We studied the foraging activity and fruit production of tomato using one colony of Nannotrigona perilampoides (NP) and one colony of Bombus impatiens (BI) in greenhouses with ≈1000 plants. Mechanical vibration (MV) was included as a test treatment. The foraging activity was measured as the number of flowers visited within 5 min, the time spent on a flower collecting pollen, the number of visits that a flower received and the duration of a foraging trip. BI collected pollen more rapidly, visited more flowers within 5 min and did more visits per flower when compared with NP that also lasted longer in their trips. Significant correlations were found between environmental variables and the number of bees entering the hive and the number of bees on the flowers. For NP, the highest correlation was found for light intensity whilst in BI a negative effect of environmental temperature was detected. Regarding the quantity of fruit, BI resulted in higher fruit set when compared with NP, but the latter performed similarly to MV. However, the weight of the fruit and seed number was significantly higher for BI when compared with NP, and this was higher than for MV. Our results demonstrate that at the densities of tomato plants tested, one colony of BI was more efficient pollinator when compared with NP. We suggest that pollination efficiency of NP could have been limited by a reduced number of foragers on the plants at a given time and their limited flight range when compared with BI. Therefore, it will be necessary to evaluate if increasing densities of colonies of NP could improve tomato yield in tropical greenhouses.     

Influence of environmental and colony factors on the initial commodity choice of foragers of the stingless bee Plebeia tobagoensis (Hymenoptera, Meliponini)

Novice foragers of social bees have to decide what food commodity to collect when they start foraging for the first time. In this decision making process two types of factors are involved: internal factors (the response threshold) and external factors (environmental and colony conditions). In this study we will focus on the importance of two external factors, pollen storage level and information from experienced foragers about food availability in the field, on the initial commodity choice of foragers of the stingless bee species Plebeia tobagoensis. We also studied the effect of the initial choice of individuals on their subsequent foraging career. This study was performed in a closed greenhouse compartment, where food availability and colony condition could be controlled. Information on food availability in the field from experienced foragers and pollen storage level both greatly influenced the initial commodity choice of individuals, with more choices for the commodity communicated by experienced foragers or lacking in storage. The initial choice of foragers is of importance for their future foraging career, although a substantial proportion of foragers did switch between food commodities. Because of the ability of novice foragers to become flexibly distributed over foraging tasks, social bees are able to react to changes in their environment without directly having to decrease foraging effort devoted to other foraging tasks. This, in combination with individual flexibility during foraging careers makes it possible for colonies of P. tobagoensis to forage efficiently in an ever-changing environment. Received 7 November 2005; revised 12 January 2006; accepted 16 February 2006.     

A field assessment of optimal foraging in ants: trail patterns and seed retrieval by the European harvester ant Messor barbarus

Summary: The ant Messor barbarus is a major seed predator on annual grasslands of the Mediterranean area. This paper is an attempt to relate the foraging ecology of this species to resource availability and to address several predictions of optimal foraging theory under natural conditions of seed harvesting.¶Spatial patterns of foraging trails tended to maximise acquisition of food resources, as trails led the ants to areas where seeds were more abundant locally. Moreover, harvesting activity concentrated on highly frequented trails, on which seeds were brought into the nest in larger numbers and more efficiently, at a higher mean rate per worker.¶The predictions of optimal foraging theory that ants should be more selective in both more resource-rich and more distant patches were tested in the native seed background. We confirm that selectivity of ants is positively related to trail length and thus to distance from the nest of foraged seeds. Conversely, we fail to find a consistent relationship between selectivity and density or species diversity of seed patches. We discuss how selectivity assessed at the colony level may depend on factors other than hitherto reported behavioural changes in seed choice by individual foragers.