Group-level dynamics in harvester ants: young colonies and the role of patrolling

The flexibility of colony organization in the red harvester ant, Pogonomyrmex barbatus, was investigated in the field in older (about 5 years) and younger (about 2 years) colonies. Perturbations affecting nest maintenance work, foraging and patrolling were made singly and in various combinations. In both younger and older colonies, events that directly affected only one activity caused changes in the numbers of ants engaged in other activities. An increase in patrolling early in the activity period caused foraging to decrease later on. Combined perturbations show that in both younger and older colonies, all three activities were linked by complex, synergistic interactions: the response to a perturbation affecting one activity itself varied, depending on the states of the other activities. The results suggest a more strongly homeostatic response to perturbations in older colonies than in younger ones: older colonies appeared to respond more consistently to single perturbations, and to decrease foraging less in combined perturbations, than did younger ones. Individual workers live only about a year. Thus similarities and differences in the flexibilities of colonies 3 or more years apart are organizational rather than a result of the accumulated experience of particular individuals. Ontogenetic changes in a colony's response to environmental stress may be ecologically important.