Thermoregulatory trade-offs result from vegetation removal by a harvester ant

Abstract.  1. Analyses of ecological trade-offs help to explain how organisms balance competing demands. Harvester ants ( Pogonomyrmex occidentalis ), are conspicuous residents of shortgrass prairie in western North America; worker P. occidentalis actively clear all vegetation from the immediate vicinity of their large gravel mounds. This study is based on the prediction that vegetation clearing yields a thermal trade-off by increasing soil temperatures; during cool periods the resulting increase in soil temperature opens new time windows for activity, while during hot periods the soil temperature is more likely to exceed the maximum thermal tolerance for this species. To test the hypothesis that daily and seasonal trade-offs in ant activity result from vegetation removal, the effects of experimentally applied shade on activity patterns were measured.
2. Harvester ant activity correlated highly with ground temperature; experimental shading of ant mounds shifted daily activity patterns by lowering ground temperature. Shading in the morning significantly delayed the onset of ant activity by preventing solar warming of the mound. In contrast, mid-day experimental shading prevented elevation of ground temperatures to above 50 °C and allowed ants to remain active when lethally high temperatures would normally force them inside the mound.
3. A model derived from field data predicted surface ground temperature (and therefore ant activity) based on air temperature and solar radiation, under conditions of sun and shade. For each of six seasons modelled, shade removal yielded a net gain of activity time. These results indicate that vegetation removal by harvester ants produces an advantageous thermoregulatory effect by helping to maximise activity time.