Simulation of topographic and daily variation in colony activity of Pogonomyrmex occidentalis (Hymenoptera : Formicidae) using a soil temperature model

The relationships between soil temperature and foraging activity of ground- dwelling ants have been widely studied, but variation in activity among ant colonies has hindered the integration of foraging activity across time and space. We developed time-integrated simulations of colony foraging activit y by linking estimates of the temperature-activity responses of ant colonie s to soil temperature model. In the field, we measured the relationship bet ween soil surface temperature and foraging activity by 29 colonies of the w estern harvester ant, Pogonomyrmex occidentalis Cresson, located at 3 topog raphic positions within a shortgrass steppe ecosystem. The temperature-acti vity relationships estimated by logistic regression were then coupled with a soil temperature model to predict the number of foraging hours per day du ring June and July 1996. Soil temperature was a significant predictor of co lony foraging probability, whereas topographic position affected overall fo raging probability or shifted peak activity to higher or lower temperatures . These tempelature-activity relationships resulted in greater simulated fo raging times for colonies on slopes than on uplands during June, but upland colonies had greater foraging times during July. Simulated foraging times for ant colonies on uplands also were the least sensitive to changes in soi l temperature because of variation in vegetation cover, air temperature, an d solar radiation. We suggest that greater densities of ant colonies on upl ands are partly because of a favorable microclimate for P. occidentalis in addition to soil characteristics, nest maintenance, or proximity to mating sites.