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.     

Semi-claustral colony founding in the seed-harvester ant Pogonomyrmex californicus: a comparative analysis of colony founding strategies

The evolution of queens that rear their first brood solely using body reserves, i.e. fully claustral, is viewed as a major advance for higher ants because it eliminated the need for queens to leave the nest to forage. In an apparently unusual secondary modification, the seed-harvester ant Pogonomyrmex californicus displays obligate queen foraging, i.e. queens must forage to garner the resources necessary to survive and successfully rear their first brood. I examined the potential benefits of queen foraging by comparing ecological and physiological traits between P. californicus and several congeners in which the queen can rear brood using only body reserves. The primary advantage of foraging appears to lie in providing the queens of P. californicus with the energy to raise significantly more brood than possible by congeners that use only body reserves; the workers reared in the first brood were also heavier in mass than that predicted by their head width. Other correlates of queen foraging in P. californicus relative to tested congeners included a significantly lower total fat content for alate queens, a small queen body size, and a low queen to worker body mass ratio. Queens also forage in several other well-studied species of Pogonomyrmex, suggesting the possibility that queen foraging may be more common than previously thought in higher ants.     

The effects of interspecific interactions on resource use and behavior in a desert ant

We examined how interspecific competition in ants affects resource use and behavior. To test how neighboring Myrmecocystus colonies influence the desert ant Aphaenogaster cockerelli, we placed temporary enclosures around Myrmecocystus spp. colonies and recorded the resources collected by A. cockerelli and the numbers of A. cockerelli ants engaged in various tasks outside the nest. When neighbors were enclosed, A. cockerelli colonies collected a significantly higher proportion of termites and significantly less plant matter than when neighbors were active. The numbers of A. cockerelli ants engaged in foraging behavior and nest maintenance work increased when Myrmecocystus colonies were enclosed. Interspecific interactions thus can affect the behavior and resource use of A. cockerelli colonies and may influence colony fitness.     

Worker size polymorphism and ethological role of sting associated glands in the harvester ant Messor barbarus

Summary: Though harvester ants are closely similar in ecology, species differ in their worker size polymorphism as well as in the glandular source of their trail pheromones and defensive compounds. In the harvester ant Messor barbarus, we find that the recruitment trail pheromone is located in the Dufour gland, while defence-alarm substances are produced in the poison gland. We also investigated how the glandular development and the ethological response to these abdominal glands are related to worker body size. For both glands, M. barbarus workers show monophasic and nonisometric growths with slopes of allometric regression lines lower than 1. The highest trail-following response is elicited by the Dufour gland secretion from media workers, responsible for most foraging activities in M. barbarus. Aggressive behaviour is more frequently observed in the presence of poison gland secretions from medium and large-sized workers. Differences between species and between worker size classes in the ethological role of sting associated glands are discussed in relation to the foraging ecology and defensive characteristics of harvester ants.     

Nest relocation and encounters between colonies of the seed-harvesting ant Messor andrei

Summary: Nest relocation in ants may be an attempt to escape areas of high competition. Encounters between colonies have been suggested to be the proximate cause of nest relocation. I examined the relation between nest relocation and encounters in a population of the seed-harvesting ant Messor andrei, a native of the western United States. Over 80% of colonies relocated their nest site during a year. Colonies moved up to ten times in a year, and new nest-sites were more distant from their nearest neighbour. However, encounters did not precipitate nest relocation. Relocation was not more likely to occur directly after an encounter, and colonies that moved did not experience more encounters than other colonies. Other possible cues for nest relocation, including predation, disease, microclimatic effects, and local resource depletion, are discussed.     

How plants shape the ant community in the Amazonian rainforest canopy: the key role of extrafloral nectaries and homopteran honeydew

Ant-plant interactions in the canopy of a lowland Amazonian rainforest of the upper Orinoco, Venezuela, were studied using a modified commercial crane on rails (Surumoni project). Our observations show a strong correlation between plant sap exudates and both abundance of ants and co-occurrence of ant species in tree canopies. Two types of plant sap sources were compared: extrafloral nectaries (EFNs) and honeydew secretions by homopterans. EFNs were a frequent food source for ants on epiphytes (Philodendron spp., Araceae) and lianas (Dioclea, Fabaceae), but rare on canopy trees in the study area, whereas the majority of trees were host to aggregations of homopterans tended by honeydew-seeking ants (on 62% of the trees examined). These aggregations rarely occurred on epiphytes. Baited ant traps were installed on plants with EFNs and in the crowns of trees from three common genera, including trees with and without ant-tended homopterans: Goupia glabra (Celastraceae), Vochysia spp. (Vochysiaceae), and Xylopia spp. (Annonaceae). The number of ant workers per trap was significantly higher on plants offering one of the two plant sap sources than on trees without such resources. Extrafloral nectaries were used by a much broader spectrum of ant species and genera than honeydew, and co-occurrence of ant species (in traps) was significantly higher on plants bearing EFNs than on trees. Homopteran honeydew (Coccidae and Membracidae), on the other hand, was mostly monopolised by a single ant colony per tree. Homopteran-tending ants were generally among the most dominant ants in the canopy. The most prominent genera were Azteca, Dolichoderus (both Dolichoderinae), Cephalotes, Pheidole, Crematogaster (all Myrmicinae), and Ectatomma (Ponerinae). Potential preferences were recorded between ant and homopteran species, and also between ant-homopteran associations and tree genera. We hypothesize that the high availability of homopteran honeydew provides a key resource for ant mosaics, where dominant ant colonies and species maintain mutually exclusive territories on trees. In turn, we propose that for nourishment of numerous ants of lower competitive capacity, Philodendron and other sources of EFNs might be particularly important.     

Host-plant selection,diet diversity,and optimal foraging in a tropical leafcutting ant

Summary A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.     

The role of resource dispersion in promoting the co‐occurrence of dominant and subordinate ant species

Dominant competitors govern resource use in many communities, leading to predictions of local exclusion and lower species diversity where dominant species are abundant. However, subordinate and dominant species frequently co‐occur. One mechanism that could facilitate resource sharing and co‐occurrence of dominant and subordinate competitors is fine‐scale resource dispersion. Here, we distributed 6 g of a food resource into 1, 2, 8, 32 or 64 units in small 0.40 m² areas centred on nests of the dominant ant Monomorium sydneyense. We tested three hypotheses. First, we hypothesized that the species richness and abundance of foraging ants would increase with increasing resource dispersion. Accordingly, species richness doubled and total ant abundance was two orders of magnitude higher in high resource dispersion treatments. Secondly, we hypothesized that increasing resource dispersion would reduce competitive interactions such as resource turnover events and lower the probability of food resources being occupied. Substantial support for this hypothesis was observed. Finally, we tested the hypothesis that the foraging time of each species would be proportional to the relative abundance of each species solely in high resource dispersion treatments. Expected and observed foraging times were statistically similar for only the dominant ant M. sydneyense. The subdominant Pheidole rugosula increased its foraging time much more than was expected, while two subordinate ants showed no relationship between observed and expected times. Thus, while increasing resource dispersion significantly increased overall species richness, this increase in co‐occurrence did not correlate with a significant increase in foraging time for the two subordinate species. Rather, changes in resource dispersion appeared to benefit only the subdominant species. Inter‐site variation appeared more important for other subordinate species indetermining co‐occurrence and foraging time. Multiple mechanisms facilitate co‐occurrence and resource sharing in this community, and probably in most other communities.     

Effect of leaf toughness on the susceptibility of the leaf-cutting ant Atta sexdens to attacks of a phorid parasitoid

Summary: Because the size of Atta spp. along foraging trails is partly determined by the characteristics of the plants harvested, and considering that parasitic phorid flies are attracted mostly to large individuals, we hypothesized that plant toughness affects the susceptibility of Atta spp. to these parasitoids. To test this hypothesis, we evaluated parasitism rates of the phorid Neodohrniphora sp. and its effect on Atta sexdens (L.) foragers in a laboratory colony. We manipulated forager size by alternating tough (Anthocephalus chinensis, Rubiaceae) and tender (Rosa chinensis, Rosaceae) plants given to the colony. Ants foraging on tough leaves were larger than ants foraging on soft leaves, and there was a significant reduction in forager size for both plants when the colony was exposed to Neodohrniphora sp. However, there were no relative differences on forager size between the two plants after the introduction of the parasitoid. The lack of response of Neodohrniphora sp. to the increase in ant size when the colony was given tough leaves may be attributed to the unusually large number of suitable hosts in a laboratory colony. However, large foragers are much less abundant in the field, in which case shifts in the size of the workforce triggered by different substrates could affect the incidence of parasitism.     

Parasitism and biology of Myrmosicarius grandicornis (Diptera, Phoridae) in relationship to its host, the leaf-cutting ant Atta sexdens (Hymenoptera, Formicidae)

Summary. The parasitic phorid Myrmosicarius grandicornis Borgmeier is commonly found around nest entrances of the leaf-cutting ant Atta sexdens (L.) in Brazil, but there is no information about the importance of this fly for A. sexdens. We evaluated the parasitic capability of female M. grandicornis collected in the field and released in laboratory nests of A. sexdens and compared ants' foraging rhythm before and after M. grandicornis were released. We also determined biological characteristics of the parasitoid. Presence of M. grandicornis elicited foragers to abandon their loads and return to the nest, an effect previously described for the phorid Neodohrniphora sp., which is a more abundant A. sexdens parasitoid. Both phorid species occur at the same foraging trails during part of the year and attack ants of different size classes. Therefore, they may have a combined effect on the reduction of A. sexdens foraging. Myrmosicarius grandicornis pupariated inside hosts' head capsules, which are detached from their bodies and deposited in the nest's refuse pile. Adults emerged through the hosts' mouth cavity about a month after oviposition. The characteristics of M. grandicornis biology suggest that this parasitoid completes its development inside A. sexdens nests.     

Harvester ant response to spatial and temporal heterogeneity in seed availability: pattern in the process of granivory

The influence of temporal and spatial heterogeneity in seed availability on the foraging behaviour of the harvester ant Messor arenarius was studied in an arid shrubland in the Negev Desert, Israel. The study investigated the implications of behavioural responses to heterogeneity in seed availability for the seed predation process and the potential for feedback effects on vegetation. Vegetation and seed rain were monitored across two landscape patch types (shrub patches and inter-shrub patches) in 1997. Shrub patches were shown to have higher plant and seed-rain density than inter-shrub patches. Patch use and seed selection by M. arenarius foragers were monitored through the spring, summer and autumn of 1997. After a pulse of seed production in the spring, the ants exhibited very narrow diet breadth, specialising on a single annual grass species, Stipa capensis. At this time, ants were foraging and collecting seeds mainly from inter-shrub patches. In the summer, diet breadth broadened and use of shrub patches increased, although the rate of seed collection per unit area was approximately equal in the two patch types. The increase in the use of shrub patches was due to colony-level selection of foraging areas with relatively high shrub cover and an increase in the use of shrub patches by individual foragers. In the autumn, a pulse of seed production by the shrub species Atractylis serratuloides and Noaea mucronata led to a reduction in diet breadth as foragers specialised on these species. During this period, foragers exhibited a large increase in the proportion of time spent in shrub patches and in the proportion of food items collected from shrub patches. The seasonal patterns in foraging behaviour showed linked changes in seed selection and patch use resulting in important differences in the seed predation process between the two landscape patch types. For much of the study period, there was higher seed predation pressure on the inter-shrub patches, which were of relatively low productivity compared with the shrub patches. This suggests that the seed predation process may help maintain the spatial heterogeneity in the density of ephemeral plants in the landscape.     

Foraging rate of ants collecting honeydew or extrafloral nectar, and some possible constraints

ABSTRACT.

• 1 In a given ant species, the number of ants collecting honeydew in an aphid colony or extrafloral nectar on a plant is proportional to the productivity of the colony or plant. Thus, the number of ants per resource unit and the ingestion rate per ant are constant for a species.
• 2 Mean number of ants per resource unit and ingestion rate per ant differed considerably between the investigated species. The ingestion rate increases with the body size of the species and decreases with an increase of the mean number of ants per resource unit.
• 3 Ingestion rates were higher in ants foraging singly at the resource than in ants foraging in the normal way in a group.
• 4 It is suggested that the ingestion rate per ant is reduced below a maximum level by the number of ants present per resource unit because a certain number of ants is needed to defend the resource against alien ants. Small species need more individuals for this purpose than large species, and consequently suffer a larger reduction of their ingestion rate.

     

Crab regulation of cross-ecosystem resource transfer by marine foraging fire ants

Permeability of boundaries in biological systems is regulated by biotic and/or abiotic factors. Despite this knowledge, the role of biotic factors in regulating resource transfer across ecosystem boundaries has received little study. Additionally, little is known about how cross-ecosystem resource transfer affects source populations. We used experiments, observations and stable isotopes, to evaluate: (1) the proportion of intertidal-foraging black fire ant (Solenopsis richteri) diet derived from marine sources, (2) how black fire ant cross-ecosystem resource transfer is altered by the dominant bioengineer in the intertidal, a burrowing crab (Neohelice granulata), (3) the top-down impact of these terrestrial ants on a marine resource, and (4) the effect of marine resources on recipient black fire ants. We found that more than 85% of the black fire ant diet is derived from marine sources, the number of intertidal foraging ants doubles in the absence of crab burrows, and that ants cause a 50% reduction in intertidal polychaetes. Also, ant mound density is three times greater adjacent to marine systems. This study reveals that cross-ecosystem foraging terrestrial ants can clearly have strong impacts on marine resources. Furthermore, ecosystem engineers that modify and occupy habitat in these ecosystem boundaries can strongly regulate the degree of cross-ecosystem resource transfer and resultant top down impacts.     

Behavioral interactions of the invasive Argentine ant with native ant species

Summary: The Argentine ant, Linepithema humile, has invaded many areas of the world, displacing native ants. Its behavior may contribute to its competitive success. Staged and natural encounters were observed at food resources in the field, between Argentine ants and eight ant species native to northern California. There was no relation between the frequency of aggression by any ant species and the outcome of encounters, though Argentine ants were more likely than ants of native species to behave aggressively. When an ant of one species initiated an encounter of any kind with an ant of another species, the ant that did not initiate was likely to retreat. This was true of all species studied. Most encounters between ants were initiated by Argentine ants. Thus the native species tended to retreat more frequently than Argentine ants. Interactions between Argentine ants and native species at food resources, causing ants of native species to retreat, may help Argentine ants to displace native species from invaded areas.     

Variation in resource size distribution around colonies changes ant–parasitoid interactions

The distribution of resources within habitats affects species abundance, richness and composition, but the role of resource distribution in species interactions is rarely studied. In ant communities, changes in resource distribution within habitats may influence behavioral interactions because many ant species are specialized to efficiently harvest a subset of available resources. This study investigates whether interactions between the behaviorally dominant host ant Pheidole diversipilosa and its specialist parasitoid (Phoridae: Apocephalus orthocladus) depend on resource size distribution around the colony. Using in situ foraging arenas to manipulate parasitoid abundance and resource size distribution around colonies, we tested whether variation in resource size distribution allows P. diversipilosa to alter its foraging behavior in ways that lessen the impact of parasitoid attack. P. diversipilosa colonies do not lower the impact of parasitoid attack by increasing the number of workers foraging individually on small and widely dispersed resources. However, the presence of multiple large resources allows colonies to temporarily redistribute soldier ants from resources patrolled by parasitoids to other resources not patrolled by parasitoids, and to maintain soldier abundance at levels found in the absence of parasitoids. These results highlight the importance of placing behavioral interactions within the context of variation in resource distribution.     

Variation in the effectiveness of biotic defence: the case of an opportunistic ant-plant protection mutualism

Benefits to plants in facultative ant protection mutualisms are highly variable. This allows examination of the sources of this variation and the mechanisms by which ants protect plants. We studied opportunistic interactions between ants and an extrafloral nectary-bearing vine, Dioscorea praehensilis, during 3 different years. Variation in plant protection among years was striking. Several factors affected the effectiveness of the biotic defence. Stems recently emerged from the underground tuber were self-supporting, contacting no other plants and encountering few foraging ants. Stems then became lianescent, and contact with supporting plants greatly increased ant recruitment. Both species and number of ant workers influenced the effect of ants on the major herbivore, the chrysomelid beetle Lilioceris latipennis. Protective actions included limitation of oviposition (reduction in the number of eggs laid on the plant) and predation, leading to increased larval mortality. The probability of successful predation was strongly dependent on larval size. If temporarily low ant-patrolling activity allows larvae to grow beyond a critical size, their mechanical (thick integument) or chemical (plant-derived compounds in a fecal shield) defences become more effective against ants. Secondary metabolites derived from the host plant thus appear to be important for the anti-predator mechanisms of this beetle, being necessary for its survival and reproduction on a host plant that actively recruits ants as a biotic defence against herbivores.     

The spatial scale of seed collection by harvester ants

Colonies of the seed-eating ant, Pogonomyrmex barbatus, compete with neighboring colonies for foraging areas. In a conflict over foraging area, what is at stake? This depends on how resources are distributed in time and space: if certain regions consistently provide particularly nutritious seed species, or especially abundant seeds, such regions will be of greater value to a colony. During the summer, seeds were taken from returning foragers in colonies located in 4 different vegetation types. There was no relation between the vegetation currently growing in the foraging area, and the species of seeds collected by ants. During the summer, ants collect mostly seeds produced in previous seasons and dispersed by wind and flooding. In 1991, colonies in all vegetation types collected mostly Bouteloua aristidoides; in 1992, Eriastrum diffusum and Plantago patagonica. There was no relation between colony density and numbers of seeds collected. Seed species collected by ants were compared in different colonies, and on different foraging trails within a colony. The results show that seed patches are distributed on the scale of distances between nests, not the smaller scale of different foraging trails of one colony. It appears that colonies are competing for any space in which to search for seeds, not competing for certain regions of consistently high value.     

A seasonal natural history of the ant, Odontomachus brunneus

A north Florida population of Odontomachus brunneus, a species of ponerine ants, was studied for a one-year period to determine the annual cycle of reproduction and colony growth, including the foraging biology and seasonal changes in nest architecture. The life cycle of O. brunneus is strongly seasonal. Colonies produce brood for 6 months and are broodless for 6 months. Alates are produced in mixed broods at the beginning of each season, consuming much of the colony’s energy reserves. These reserves recover slowly through foraging during the summer’s worker production, and rapidly after brood production ceases in October. The foraging population was estimated to average 77% (SD 22) of the workforce. This proportion was not related to colony size and female alates were also found to forage. Nest architecture was found to change seasonally, with winter nests being more than twice as deep as the average summer nest.     

Larval niche selection and evening exposure enhance adoption of a predacious social parasite, Maculinea arion (large blue butterfly), by Myrmica ants

Larvae of the butterfly genus Maculinea feed briefly on a foodplant before being adopted as social parasites into Myrmica ant nests. Each Maculinea species typically survives only with a single Myrmica species, yet the eggs are laid across the overlapping territories of 3-5 Myrmica species and several other ants. The ability of Maculinea arion - a 'predatory' species of Maculinea - to influence its adoption into host Myrmica colonies was studied for the first time in the field. Some earlier reports, involving captive non-host ants, suggested that larvae of the predatory Maculinea follow ant trails into host nests or wander some distance from their foodplant before being discovered and (after a long interaction) carried away by Myrmica foragers. No dispersal from foodplants occurred in wild Maculinea arion larvae. Instead, they increased by >100-fold their probability of being discovered and adopted by Myrmica spp., rather than by ants of other genera, by exposing themselves in the micro-niche occupied by Myrmica foragers at their time of day of peak foraging. Despite a complex, hour-long interaction with Myrmica workers before being carried to the nest, Maculinea arion did not enhance its adoption by host species of Myrmica. Eggs were laid without bias in Myrmica sabuleti (host) and Myrmica scabrinodis (non-host) territories; larval survival on Thymus was the same in both ants' territories; larvae waited to be found beneath their foodplant rather than seek their host; Myrmica sabuleti and Myrmica scabrinodis foraged in the same vertical and temporal niches, and had the same probability of discovering larvae; both ants behaved identically after finding larvae and took the same time to adopt them; and the ratio of wild larvae taken into Myrmica sabuleti or Myrmica scabrinodis nests was the same as the distribution between these ants of Thymus, eggs and pre-adoption larvae.     

Prey depletion by the foraging of the Eurasian treecreeper, Certhia familiaris, on tree-trunk arthropods

We examined to what extent breeding insectivorous treecreepers, Certhia familiaris, affect the abundance and the mean size of their prey population on the surface of tree trunks. In order to determine foraging pressure on tree trunks, we observed the parent birds' foraging behaviour in marked squares (25Ꭱ m) at a short (10 m) and long distance (90 m) from the nest, when the nestlings were near fledging. Immediately after fledging, we measured the remaining food abundance on a sample of tree trunks near to and far from the nest. All arthropods longer than 1 mm were collected and identified to family level and their length was measured. Treecreepers foraged for significantly less time on trunks far from the nest than on trunks close to the nest (5 s vs 186 s). Consequently, treecreepers were found to deplete food abundance on trunks close to the nest. During the nesting period, parent birds removed almost twice as many spiders and other arthropods (excluding Formica ants) from the tree trunks close to the nest than from those further away. The size distribution of arthropods was the same between frequently and seldom-used tree trunks, suggesting that arthropod consumption by treecreepers was not size selective. Our direct measurements on the abundance of arthropods provide rare evidence for the ability of predators to deplete their food resources to a large extent.