Deborah M. Gordon

Community disassembly by an invasive species

Invasive species pose serious threats to community structure and ecosystem function worldwide. The impacts of invasive species can be more pervasive than simple reduction of species numbers. By using 7 years of data in a biological preserve in northern California, we documented the disassembly of native ant communities during an invasion by the Argentine ant. In sites without the Argentine ant, native ant communities exhibit significant species segregation, consistent with competitive dynamics. In sites with the Argentine ant, native ant communities appear random or weakly aggregated in species co-occurrence. Comparisons of the same sites before and after invasion indicate that the shift from a structured to a random community is rapid and occurs within a year of invasion. Our results show that invasive species not only reduce biodiversity but rapidly disassemble communities and, as a result, alter community organization among the species that persist.

Behavioral Flexibility and the Foraging Ecology of Seed-Eating Ants

Seed-eating ants are part of a guild of granivorous desert species that compete for food. This study examines the factors that influence the intensity, location, or temporal pattern of foraging in the red harvester ant, Pogonomyrmex barbatus. I consider behavioral flexibility on three time scales: year to year, day to day, and hour to hour. On a yearly time scale, there does not appear to be any simple relation between high intensities of foraging and survival rate. Colony-specific differences in foraging activity persist from one year to the next, but survivorship is not higher in colonies more likely to forage. Distributions of nests change from one year to the next and are usually underdispersed or clumped. Colonies live for 15-20 yr. Colony behavior is more stable, and more likely to avoid intraspecific conflict, in older colonies (>5 yr old) than in younger ones (2 yr old). Mortality rates of 2-yr-old colonies are similar to those of older ones. Other results suggest that a colonys competitive status is determined by its behavioral flexibility in changing conditions, on daily and hourly time scales. Each day that a colony forages, it chooses a few foraging routes out of a larger set of available ones. This decision is made early each day by the patrollers, a distinct group of workers, before the foragers emerge from the nest. The choice of foraging trails is influenced by interactions with neighboring colonies. The extent to which neighboring colonies forage on trails that intersect, and engage in long-term conflict, depends on colony age and food availability. When there is a food incentive, pairs of neighboring younger colonies are more likely than older pairs to continue using foraging trails that overlap. On an hourly time scale, the intensity of foraging depends on the rate of food intake and on the activities of workers, such as patrollers, engaged in other tasks besides foraging.

Founding, Foraging, and Fighting: Colony Size and the Spatial Distribution of Harvester Ant Nests

This study examines how the spatial distribution of nests is related to the behavioral interactions of conspecific neighbors in a population of the seed-eating ant, Pogonomyrmex barbatus. Colonies live for 15-20 yr, reaching reproductive age and a stable size at =:5 yr. Spatial distributions were measured for 6 yr (1988-1993) in a population of -::250 colonies of known age. The probability that a I -yr-old colony occurs in a given location is related to the distance to, and ages of, its five nearest conspecific neighbors. One-year-old colonies are most likely to occur near small, 2- and 3-yr-old colonies. Neighboring colonies encounter each other when foragers of both colonies search the same area on the same day. The probability of an encounter between two colonies decreases with the distance between their nests. For colonies of all ages, encounters are most likely with their larger neighbors, -5 yr old. Encounters are more likely if there was an encounter the previous day, and this effect can overwhelm the effect of distance. Two-yr-old colonies are more likely than colonies of other ages to lose ground in repeated encounters with a particular neighbor. Though encounters are frequent, their costs are low: few of the foragers on a trail that meets a neighbors actually encounter an ant of the neighboring colony, and interaction with a neighboring colony does not increase the typical duration of a foraging trip. If foragers of two colonies do meet, most fights are brief, with few resulting in injury or death. These results suggest that the cost of conspecific neighbors searching the same ground for seeds may be greater than the cost of behavioral interaction itself. Exploitative com- petition may have more important effects than interference competition on founding colony survival and thus on the spatial distribution of nests.

Dynamics of task switching in harvester ants

Abstract Harvester ants, Pogonomyrmex barbatus , were marked when engaged in one of four activities outside the nest: foraging, patrolling, nest maintenance and upkeep of the colony refuse pile. In undisturbed older colonies, each activity was done by a distinct group of workers. In undisturbed younger colonies, nest maintenance workers were likely to forage. Further experiments examined the conditions under which workers of each task group engaged in other activities. Marked workers were observed in the course of perturbations that increased the numbers engaged in one activity. In response to perturbations, most workers would switch tasks to forage, and nest maintenance workers were likely to switch to other tasks. Previous work showed that changes in the numbers of ants engaged in one activity alter the numbers engaged in other activities. The experiments with marked individuals described here show that task switching among exterior workers does not account for the observed responses to perturbations. This means that within a time scale of several hours, one worker group receives information about events affecting other worker groups.

Genetic basis for queen–worker dimorphism in a social insect

Eusocial insects are characterized by reproductive division of labor, cooperative brood care, and the presence of a sterile worker caste. It is generally accepted that caste determination, including the differentiation of females into sterile workers and reproductive queens, is determined by environmental factors. In contrast, we find that in the red harvester ant, Pogonomyrmex barbatus, an individuals genotype at a particular microsatellite locus predicts its caste. We propose that this microsatellite locus is in tight linkage disequilibrium with at least one locus that plays an important role in caste determination. We call this the caste locus. We hypothesize that the system of caste determination we observe segregates the population into two distinct genetic lineages, each of which has distinct alleles at the microsatellite locus and also has distinct alleles, we propose, at caste. Workers are the offspring of parents from different lineages, and are thus heterozygous at caste, whereas queens are the offspring of parents from the same lineage, and are, therefore, homozygous at caste. This mode of caste determination has important consequences for the evolution of multiple mating by females and for control of the sex ratio and reproductive allocation in social insect colonies.

HARVESTER ANTS UTILIZE CUTICULAR HYDROCARBONS IN NESTMATE RECOGNITION

Cuticular hydrocarbons appear to play a role in ant nestmate recognition, but few studies have tested this hypothesis experimentally with purified hydrocarbon extracts. We exposed captive colonies of the harvester ant Pogonomyrmex barbatus to small glass blocks coated with whole cuticular lipid extracts and the purified hydrocarbon portion of extracts from nestmate and nonnestmate workers. As an estimate of agonistic behavior, we measured the proportion of ants in contact with blocks that flared their mandibles. Blocks coated with cuticular extracts from nonnestmates were contacted by more workers in one of two experiments and elicited higher levels of aggression in both experiments than blocks bearing extracts from nestmates. The cuticular hydrocarbon fraction of extracts alone was sufficient to elicit agonistic behavior toward nonnestmates. The results demonstrate that harvester ants can perceive differences in cuticular hydrocarbon composition, and can use those differences in nestmate recognition.

Effects of social group size on information transfer and task allocation

SummarySocial animals exchange information during social interaction. The rate of interaction and, hence, the rate of information exchange, typically changes with density and density may be affected by the size of the social group. We investigate models in which each individual may be engaged in one of several tasks. For example, the different tasks could represent alternative foraging locations exploited by an ant colony. An individuals decision about which task to pursue depends both on environmental stimuli and on interactions among individuals. We examine how group size affects the allocation of individuals among the various tasks. Analysis of the models shows the following. (1) Simple interactions among individuals with limited ability to process information can lead to group behaviour that closely approximates the predictions of evolutionary optimality models, (2) Because per capita rates of social interaction may increase with group size, larger groups may be more efficient than smaller ones at tracking a changing environment, (3) Group behaviour is determined both by each individuals interaction with environmental stimuli and by social exchange of information. To keep these processes in balance across a range of group sizes, organisms are predicted to regulate per capita rates of social interaction and (4) Stochastic models show, at least in some cases, that the results described here occur even in small groups of approximately ten individuals.

What is the function of encounter patterns in ant colonies

Abstract Abstract. This study explores how patterns of brief antennal contacts may function in the organization of ant colonies. In the course of antennal contact, an ant can determine whether another is a nestmate. The first part of the work was comparative, and showed that three ant species ( Solenopsis invicta, Myrmica rubra, and Lasius fuliginosus ) differ in the frequency, location and context of antennal contact. The next part examined whether ants might use the rate of contacts with nestmates as a cue to nestmate density. Density was varied experimentally, and variation in contact rate was measured. If encounters between ants were the result of purely random collisions, the principles of Brownian motion suggest numbers of contacts would increase quadratically with numbers of ants. The results show that in undisturbed conditions, contact rate was not random. Instead ants ( L. fuliginosus ), regulated contact rate. Ants aggregated more when density was low, which keeps contact rate up, and avoided contact with nearby ants when density was high, which keeps contact rate low. One ant responds to another, and thus can decide whether to engage in contact, at a distance of 1 · 2 cm. Next, ants were exposed to workers from another colony. In these disturbed conditions, contact rates increased. The magnitude of the increase depended on proportions, not numbers of non-nestmates present, suggesting that contact rate may be a cue to nestmate density.

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.

Task‐specific expression of the foraging gene in harvester ants

In social insects, groups of workers perform various tasks such as brood care and foraging. Transitions in workers from one task to another are important in the organization and ecological success of colonies. Regulation of genetic pathways can lead to plasticity in social insect task behaviour. The colony organization of advanced eusocial insects evolved independently in ants, bees, and wasps and it is not known whether the genetic mechanisms that influence behavioural plasticity are conserved across species. Here we show that a gene associated with foraging behaviour is conserved across social insect species, but the expression patterns of this gene are not. We cloned the red harvester ant (Pogonomyrmex barbatus) ortholog (Pbfor) to foraging, one of few genes implicated in social organization, and found that foraging behaviour in harvester ants is associated with the expression of this gene; young (callow) worker brains have significantly higher levels of Pbfor mRNA than foragers. Levels of Pbfor mRNA in other worker task groups vary among harvester ant colonies. However, foragers always have the lowest expression levels compared to other task groups. The association between foraging behaviour and the foraging gene is conserved across social insects but ants and bees have an inverse relationship between foraging expression and behaviour.