In social insects, colonies as well as individuals have evolving life histories. Identification of the life history tactics of a social insect requires data on colony attributes and their development. To this end a full range of fire ant (Solenopsis invicta) colony sizes was sampled and censused on seven dates throughout 1 yr. Data included: mound volume; the number, dry masses, and fat contents of sexual and worker adults and immatures; stratified nest temperatures; worker distribution within the nest throughout the year; duration of the pupal stages; and respiration rates. Analysis showed:1. Colonies reached their annual maximum population size in midwinter and their maximum biomass in spring. During the spring sexual production period they declined to a midsummer minimum. Calculations showed that the magnitude of this decline increased with colony size. During January to July, worker mortality exceeded natality, causing colony decline, while from July to December, natality predominated, causing growth.2. Mound volume was closely related to the total mass of ants in the colony, and varied with season paralleling the mass of ants.3. The mean size and variability of workers, and the percent major workers, increased with colony size and changed over the year.4. The fat content (percent fat) of workers increased with worker size and colony size. Worker percent fat was lowest in summer after sexual production, climbed immediately to the annual maximum and then declined gradually through winter and spring.5. Although sexual male and female pupae were close in mean dry mass (2.55 mg and 3.10 mg, respectively), males gained only 6% during adult maturation while females gained 290%. Females gained fat more rapidly than lean tissue causing their percent fat to increase from 31% to 49%. Mean mass of male and female sexual adults did not change with colony size.6. The cost of worker maintenance declined from nearly 100% of total colony cost in winter to 46% in late spring when brood production peaked. 7. Production rates peaked in spring, with colonies investing 50% of their daily production in sexuals. This peak production was not sustained through the summer, and was probably fueled by stored worker fat. Worker production dominated in the latter part of the summer. All measures of production rate as well as total annual production increased with colony size, but most did so less rapidly than colony size, resulting in a declining efficiency of production and a declining natality rate.8. The percent of annual production invested in sexuals increased sharply in colonies of between 20 000 and 50 000 workers, then remained at ~ 33% for the remainder of colony growth, showing that the transition from the ergonomic to the reproductive stages is sharp, and that colonies must grow in order to produce more sexuals.9. Many quantitative colony attributes were related to one another by differential growth, and can thus be seen as isometric or allometric measures. Rules of relative growth may thus constrain the possible combinations o...

Biological invasions are often closely associated with human impacts and it is difficult to determine whether either or both are responsible for the negative impacts on native communities. Here, we show that human activity, not biological invasion, is the primary driver of negative effects on native communities and of the process of invasion itself. In a large-scale experiment, we combined additions of the exotic fire ant, Solenopsis invicta, with 2 disturbance treatments, mowing and plowing, in a fully crossed factorial design. Results indicate that plowing, in the absence of fire ants, greatly diminished total native ant abundance and diversity, whereas fire ants, even in the absence of disturbance, diminished some, but not all, native ant abundance and diversity. Transplanted fire ant colonies were favored by disturbance. In the absence of disturbance and on their own, fire ants do not invade the forest habitats of native ants. Our results demonstrate that fire ants are ''passengers'' rather than ''drivers'' of ecological change. We propose that fire ants may be representative of other invasive species that would be better described as disturbance specialists. Current pest management and conservation strategies should be reassessed to better account for the central role of human impacts in the process of biological invasion.community organization ͉ competition ͉ disturbance ͉ exotic species ͉ pest management

No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Summary. Social organization is generally assumed to increase colony efficiency and survival; however, little quantitative information is available to support this assumption. Polymorphism is an important aspect of labor division in colonies of the fire ant, Solenopsis invicta. Our objective was to investigate the effect of fire ant polymorphism on brood production efficiency. We set up standardized polymorphic colonies with a full range of worker sizes and artificial monomorphic colonies that contained only small, medium or large workers respectively. Polymorphic colonies produced brood at about the same rate as colonies composed of only small workers ( Fig. 2A). Colonies composed of only medium workers produced about 30% less brood, and colonies composed of only large workers produced little or no brood at all. This pattern was independent of colony size; however, smaller colonies (0.75 g, live weight) produced almost twice as much brood per gram of workers as larger colonies (3.0 g). Additional experiments revealed that the size of workers in the artificial monomorphic colonies affected all stages of brood rearing. Large workers not only inhibited the development of early and late instar larvae (Fig. 4), but also reduced the queen's oviposition rate (Fig. 3). Brood production efficiency on an energetic basis was determined by dividing the grams of brood produced per unit time by the energetic costs expended for the maintenance and production of each worker size class. Worker maintenance costs were estimated from respiration, while production costs were determined from the caloric content of worker tissue divided by their average longevity. Worker respiration per milligram body weight decreased about 40% as body size increased (Fig. 5). Large workers lived about 50% longer than small workers (Fig. 6) and contained 9% more energy per milligram of tissue (Fig. 7). Energetic efficiency in polymorphic colonies was approximately 10% higher than in colonies composed of only small workers (Fig. 9). In other words, when food supplies are limiting, polymorphism may offer a slight advantage in brood production.

No abstract

The architecture of the subterranean nests of the Florida harvester ant, Pogonomyrmex badius, was studied through excavation and casting. Nests are composed of two basic units: descending shafts and horizontal chambers. Shafts form helices with diameters of 4 to 6 cm, and descend at an angle of about 15–20° near the surface, increasing to about 70° below about 50 cm in depth. Superficial chambers (< 15 cm deep) appear to be modified shafts with low angles of descent, and are distinct from deeper chambers. In larger nests, they have a looping, connected morphology. Chambers begin on the outside of the helix as horizontal-floored, circular indentations, becoming multi-lobed as they are enlarged. Chamber height is about 1 cm, and does not change with area. Chamber area is greatest in the upper reaches of the nest, and decreases with depth. Vertical spacing between chambers is least in the upper reaches and increases to a maximum at about 70 to 80% of the maximum depth of the nest. The distribution of chamber area is top-heavy, with about half the total area occurring in the top quarter of the nest. Each 10% depth increment of the nest contains 25 to 40% less area than the decile above it, no matter what the size of the nest.Nests grow by simultaneous deepening, addition of new chambers and/or shafts and enlargement of existing chambers. As a result, the vertical spacing between chambers is similar at all nest sizes, and the relative distribution of chamber area with relative nest depth did not change during colony growth (that is, the size-free nest shape was the same at all colony sizes). Total chamber area increased somewhat more slowly than the population of workers excavating the nest. The branching of shafts was consistently shallow (< 40 cm), somewhat more so in large nests than small. Large colonies rarely had more than 4 shaft/chamber series. Each new series contributed less to the total chamber area because its chambers were smaller. Incipient colonies were usually 40 to 50 cm deep while mature colonies were commonly 2.5 to 3.0 m deep.Workers captured near the top of a mature nest (and therefore older) and penned in escape proof enclosures, excavated larger nests than did young workers captured from the bottom of the nest. Most of this difference was due to a larger fraction of older workers engaging in digging, rather than an increase in their rate of work. All ages of workers produced similar top-heavy nests. When different ages of workers from different levels of a mature colony were allowed to re-assort themselves in a vertical test apparatus buried in the soil, older workers moved upward to assume positions in the upper parts of the nest, much as in the colonies from which they were taken. The vertical organization of workers based on age is therefore the product of active movement and choice. A possible template imparting information on depth is a carbon dioxide gradient. Carbon dioxide concentrations increased 5-fold between the surface and the depths of the nest. A preference of young workers for hi...

Many species of ants excavate complex, species-typical nests in soil. The basic structural units of many nests are descending tunnels connecting flattened, generally horizontal chambers of oval to lobed outline. The species-typical structure of many nests results from variation in the size, shape, number and arrangement of these basic elements. Nest architecture can be rendered by filling subterranean nests with a thin slurry of orthodontal plaster, then excavating and reconstructing the hardened cast. Photographs of such nest casts of nine species of ants from northern Florida show the range and type of variation of architecture. Preservation conditions under which ant nests could form complex trace fossils are discussed, and reports of such traces reviewed. The images presented in this paper will help to alert trace fossil specialists to the potential range of appearance of such nest fossils. ß

No abstract