John B. Gentry

Bioenergetics of the Southern Harvester Ant, Pogonomyrmex Badius

The bioenergetics of the southern harvester ant were studied on the Savannah River Project, Aiken, South Carolina. Excavation of hills revealed that density of ants ranged from 4,000 to 6,000 per hill. Labeling ants with p32 indicated that only 10% of the ants in a colony were active above ground during any 2-week period (the limit of recognition of the label) * thus, short-term marking recapture estimates based on above-ground individuals greatly underestimate the size of the colony. There were 27 hills per hectare on the study area. The consumption of oxygen of workers at various temperatures was determined in a simple respirometer, and the temperature at which the ants were living in the field was determined during a years observation at mounds. The energy expense of heat production was calculated from the oxygen consumption and activity- temperature records. Total energy flow (heat production plus tissue growth) was highest in summer and lowest in winter. The annual range was from 14 to 48 kgcal/m2/yr. Tissue growth was estimated to amount to only 0.09 kgcal/m2/yr. Energy flow in this species was higher than that of 2 vertebrate granivores studied in the same field, the old-field mouse and the savannah sparrow.

Response to Predation by Colonies of the Florida Harvester Ant, Pogonomyrmex Badius

The effect of predation (by the systematic removal of forager ants) on the behavior and population structure of Pogonomnyrmex badius colonies was studied during the summer of 1965. For 23 wk, 50 workers per day were removed from one group of 10 colonies and 10 ants per day from another group of 10 colonies. Another 10-colony group served as a control. No effects of removal were observed in the 10/day group. Within a week after removal began, aboveground activity was reduced drastically in many of the 50/day colonies and the removal of a complete sample each day was impossible. Later, colonies underwent extended periods (3 wk or more) of complete inactivity aboveground. The mean dry weight of workers removed at the rate of 10 per day increased over the removal period. The mean dry weight of workers removed at the rate of 50 per day increased for the first 9 wk and then decreased rapidly over the rest of the removal period. As removal continued, craters became smaller and many disappeared; nest entrances were shifted under the over- hang of grass clumps and were covered with dried leaves. Intercolony conflicts became common at most 50/day colony sites. After the 23-wk removal period, five colonies from each treatment group were randomly selected for excavation. Significantly fewer ants were excavated from both the 10/day and 50/day colonies compared to the controls. Significantly more ants altogether were produced (removed plus excavated) by the 10/day and 50/day colonies. However, the increase in the number of ants produced could not immediately counteract the loss of the ants to above- ground removal. At excavation, workers in the 50/day colonies weighed less than those in the control and 10/day group. In each treatment group, workers collected from the top 20 cm of the nest system weighed less than those collected from the remainder of the nest system. Also, in the 10/day group, workers collected at the surface during the removal phase weighed less than those present in the top 20 cm of the nest at excavation. No such difference existed in the colonies of the 50/day group. By midsummer of the.next year (1966), the remaining experimental colonies appeared to be back to normal strength. Although the forager population was greatly reduced, the colonies were able to survive until more foragers could be produced. Such an adaptive strategy is possible because of a highly developed social behavior which results in a well-defined division of labor within the ant colony.

Foraging pattern, colony distribution, and foraging range of the Florida harvester ant Pogonomyrmex badius

This report describes the foraging pattern of the Florida harvester ant Pogonomyrmex badius in a high-density population of colonies. The foraging pattern has both promoted and been influenced by the colony distribution. Pogonomyrmex badius forages from short trails which extend into a surrounding foraging range. Direction of foraging trails is influenced by the location of a colonys near neighbors. Seasonal nest relocations always occur along a foraging trail, usually the main trail. Foraging ranges are not actively defended, but are used almost exclusively by foragers from a single colony. Foraging ranges will be extended into an area abandoned by neighboring foragers, indicating that forager presence may define each colonys range. Colony distribution has remained essentially the same for several years, despite seasonal nest relocations and addition of new colonies. Establishment of trails and exclusive foraging ranges by each colony minimizes encounters with neighboring foragers and guarantees access to available resources; this pattern also promotes maintenance of the existing colony distribution and partitioning of resources.

Number and Variety of Small Mammals on the AEC Savannah River Plant

Small mammal populations were sampled by standard traplines for 12 years on the AEC Savannah River Plant, Aiken, South Carolina. In 86,000 trap nights 12 of 15 potentially trappable species were collected. The variety of species was expressed as species per line and species per number of individuals. The species per number of individuals relationship was used to predict the number of individuals that must be captured to collect the regional species pool of 15 species. The number of individuals required ranged from 360,000 by trapping in only one habitat to 700 by stratified sampling based on the percentage occurrence of each habitat on the total land area. Relative abundance of mammals, as influenced by habitat, year and season, was determined from the trapping data. Peromyscus polionotus and Sigmodon hispidus were the most abundant species. Sigmodon was captured in the greatest variety of habitat at the greatest range in abundance, 0.14 to 16.81 individuals per line. P. polionotus was taken in fewer habitats, but at a relatively constant level of abundance of three mice per line. Broomsedge-vine and lespedeza habitats supported greatest numbers of animals; forest habitats, the least.

A Comparison of Variety and Standing Crop of Vegetation on a One-Year and a Twelve-Year Abandoned Field

Studies of the development of old-field vegetation over 12 years on the AEC Savannah River Plant (SRP), Aiken, South Carolina, USA, showed that an inverse relationship existed between the variety of species and the net production of dry matter of the plant community (ODUM 1960; GOLLEY 1965). After farming was abandoned on the SRP, the annual net production of the natural vegetation during the first year of the old-field sere was about 500 g dry matter/m2/year. In three years production declined to about 300 g/m2/year and stabilized at this level through the seventh year. During these first seven years the community was composed of a collection of plant species dominated by the composites Leptilon canadensis, Haplopappus divaricatus, Gnaphalium purpureum, Heterotheca subaxillaris, and the grass Digitaria sanguinalis. During the forb stage the number of species contributing more than one g/m2/year to net production increased from about 5 to 15. In the eighth year the dominance of the community changed from forbs to grasses. The grass Andropogon virginicus became the most important plant in the community and net production increased to 650 g/m2/year, while the variety of dominant species fell to 8. During the succeeding four years the net production declined to 550 g/m2/year and species variety increased to 14. Since the rate of change within or between seral stages of a community may be influenced by the size of the unvegetated area and the invasion ability of the species growing adjacent to the abandoned field (GOLLEY 1965), the observed inverse oscillation between species variety and net production may not always occur. For instance, in small fields succession may be very rapid, with the

The Effect of Weather on the Winter Activity of Old-Field Rodents

As a part of basic studies of ecological change on the AEC Savannah River Plant Area special attention has been given to the old field ecosystem by a faculty-student team of the University of Georgia. Seasonal and annual changes in the “standing crops” of major plant and animal groups have been followed. For sampling small mammals, extensive use has been made of a standard trapline technic originally developed by Calhoun (1948). The basic trapline unit consists of 20 stations with three traps per station set for three nights. We placed stations 25 feet apart so that small as well as large fields could be sampled (for larger fields more than one unit is often used). Peromyscus polionotus has been, and still is, the most important small mammal in former agricultural fields now abandoned for four growing seasons. Mus musculus, Reithrodontontomys humulis and Cryptotis parva occur in smaller numbers with an occasional Sigmodon hispidus straying into these fields from its normal habitat of older successional stages. Armed with home range data obtained from live-trapping in 1952–54 by Leslie B. Davenport, another member of our team, we have been hopeful of converting numbers per trapline into at least reasonable estimates of animals per acre. Calhoun, Casby and Brant (1955) have recently shown that “space-relative” density can be calculated from ratios between catches of the first two nights, provided the home range is known and the catch on different nights is not greatly affected by the weather. Since Peromyscus in these southern regions reaches a seasonal peak in abundance in winter or early spring (see also McCarley, 1954), sampling should be done then if peak populations of different areas are to be compared (not in late summer or fall as would be the case with many species in the northern United States). …

Response of Wild Rodents to Acute Gamma Radiation

Recent reports have shown that some species of small wild rodents, pocket mice (Perognathus longimembris and Perognathus formosus) (1) and cotton rats (Sigmodon hispidus) (2), given an acute dose of y-radiation have an LD50o(3o) that ranges between 1200 and 1500 r. This level of lethality is considerably higher than that of the laboratory mouse (544 to 665 r) (3) and the laboratory rat (665 r) (4), animals of approximately the same body size as the pocket mouse and cotton rat. Since it has been suggested that resistance to radiation is a criterion of general fitness (5), it might be concluded that wild rodents that are genetically diverse and as adults have passed through a period of rigorous natural selection are more fit and also more resistant than laboratory rodents. Alternatively, these data may represent exceptional species. Our studies have been designed to obtain evidence on these questions of comparative radiation sensitivity. Since a large number of variables such as age, sex, kind and intensity of radiation, previous treatment, length of time in captivity, genetic background, and environment affect lethality and make comparison of the results from different experiments and laboratories difficult, the procedure in these studies has been to collect adult wild small mammals by live-trapping and to compare their response to acute y-radiation with that of domestic mice under both laboratory and field conditions. Where possible, both wild and domestic forms of the same species have been used. In the laboratory experiments the wild mammals must adjust to the unusually stabile laboratory environment; in the field experiments the domestic mammals must adjust to a constantly fluctuating environment. Thus, the experimental design compares wild and domestic forms within a species and between

An Analysis of Elemental Concentrations in Vegetation Bordering a Southeastern United States Coastal Plain Stream

Mineral element concentrations were measured in the component parts of different species of plants from a floodplain community along a coastal plain stream in South Carolina, USA. Frequency distributions of concentrations of mineral elements were positively skewed, although dis- tributions of P. K, Ca and B concentrations tended to be less skewed than those of some trace elements (Na, Al, Cs- 137). Sources of variation in the elemental compositions of the major natural plant species of the floodplain were examined statistically. In herbaceous species (Scirpus cyperinus and Andropogon sp.), 52-56% of the variation in P, Mg, Mo and Na concentrations was accounted for by differences in the chemical composition of stems, leaves, roots and seeds. Seasonal variation in the mineral element composition of leaves from woody species was greater than that in stems. Concentra- tions of K, P. Al, Mo and Sr in leaves were significantly different among seasons in 3 woody species (Alnus serrulata, Myrica cerifera, Salix nigra). Mean concentrations of Ca, Na, Al, B, Mo and Sr in leaves tended to increase from spring to autumn, while K, Mg, N, P. Cu, Zn and Cs-137 concentrations declined. For macroelements (P, Mg, K, Ca and N) and some trace elements (B, Sr, Na and Mo), >50% of the variation in concentrations was accounted for by differences among 4 species (Polygonum punctatum, Scirpus cyperinus, Salix nigra, Sagittaria latifolia) while <12% was ex- plained by differences among 7 locations along the creek. Discriminant function analysis showed that K concentrations, which are low in the floodplain soils, contributed more than did concentrations of 11 other elements to species differences. Concentrations of elements were significantly correlated across species and locations from the floodplain community. A principal components analysis of elemental concentrations in leaves produced 2 components after varimax rotation. Principal com- ponent I was most highly correlated with concentrations of Ca and Sr, but many other elements were loaded on this component.

Interactions of Peromyscus and Mus in a One-Acre Field Enclosure

Peromyscus polionotus (old—field mice) and feral Mus musculus (house mice) have coexisted in a large old—field area at the AEC Savannah River Plant for 12 yr following removal of human habitations and the abandonment of agriculture in the region, but in two replicate experiments a Mus population has failed to survive when the two species were confined to a one—acre enclosure in the field. In the second experiment, as described in this paper, five Mus were introduced into the enclosure in December 1960 and nine Peromyscus in June 1961. Rapid population growth in both species occurred in the fall of 1961 until about 23 of each species were present, a much bigger density than occurred outside the enclosure. A decline in recruitment rate and a continued high disappearance rate in the Mus population resulted in a gradual decline and extinction of this species; whereas the Peromyscus population maintained itself at a level of 20 to 28 individuals. The two species at first occupied separate areas in the enclosur...

Dynamics of an enclosed population of pine mice,Microtus pinetorum

Enclosed populations of pine mice were studied over a 28-month period and compared with populations in surrounding natural areas. The enclosed populations reached densities far greater than any observed in surrounding natural areas. It is suggested that the restriction placed on dispersal by the enclosure walls and the addition of food in the form of trap bait could have allowed the enclosed populations to reach higher densities than normally observed at SRP.