Simulation of Topographic and Daily Variation in Colony Activity ofPogonomyrmex occidentalis(Hymenoptera: Formicidae) Using a Soil Temperature Model

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Abstract
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 activity by linking estimates of the temperature–activity responses of ant colonies to a soil temperature model. In the field, we measured the relationship between soil surface temperature and foraging activity by 29 colonies of the western harvester ant, Pogonomyrmex occidentalis Cresson, located at 3 topographic positions within a shortgrass steppe ecosystem. The temperature–activity relationships estimated by logistic regression were then coupled with a soil temperature model to predict the number of foraging hours per day during June and July 1996. Soil temperature was a significant predictor of colony foraging probability, whereas topographic position affected overall foraging probability or shifted peak activity to higher or lower temperatures. These temperature-activity relationships resulted in greater simulated foraging 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 soil temperature because of variation in vegetation cover, air temperature, and solar radiation. We suggest that greater densities of ant colonies on uplands are partly because of a favorable microclimate for P. occidentalis in addition to soil characteristics, nest maintenance, or proximity to mating sites.