Bioenergetics of the Western Harvester Ant in the Shortgrass Plains Ecosystem

The bioenergetics of Pogonomyrmex occidentalis (Cresson) was studied on the IBP grassland intensive study site in northeast Colorado. Energy flow values were determined by estimating the energy required per colony for production of ant tissue and for respiratory energy loss. Extrapolation of the energy values per colony to colony densities in the differentially grazed pastures resulted in seasonal estimates of population energy flow values for western harvester ant populations in ungrazed, lightly grazed, moderately grazed, and heavily grazed pastures. Excavation of 11 colonies resulted in estimates of 2032 worker ants and 315 winged reproductives produced per colony each year. This resulted in an estimate of 46.81 kcal per colony required for secondary production per year. Respiratory energy losses were estimated from measurements of CO2 production in relation to field temperatures, as determined from measurements conducted on 2 colonies established in the laboratory. Annual energy requirements for respiration were estimated at 418.42 kcal per colony. Energy flow values (tissue production + heat production) ranged from 1.45 kcal per m3 per year in the moderately grazed pasture to 0.14 kcal per m2 per year under heavy grazing pressure. The energy flow values for P. occidentalis are much lower than those calculated for P. badius (Latreille). The major area of difference occurs in estimates made of energy required for respiration.