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

Abstract

The effect of predation (by the systematic removal of forager ants) on the behavior and population structure of Pogonomyrmex 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 overhang of grass clumps and were covered with dried leaves. Intercology 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 aboveground 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.