Analyses of ecological trade-offs typically focus on issues such as vegetative versus reproductive growth. My purpose is to explore the possibility that the nesting behavior of an ant exploits a trade-off to improve foraging efficiency. Western harvester ants (Pogonomyrmex occidentalis), conspicuous residents of shortgrass prairie in eastern Colorado, are especially common in disturbed and eroded areas. Worker P. occidentalis actively clear all vegetation from a perimeter surrounding their large gravel mounds. The reason for maintaining this bare perimeter is unresolved, but it clearly eliminates shading of the mound by vegetation. I measured the effects of shade on harvester ant activity patterns during 2002 and 2003. Application of shade to ant mounds shifted daily activity patterns by lowering ground temperature. Shading in the morning significantly delayed the onset of ant activity. Shading during midday allowed ants to remain active despite dangerously high temperatures that normally force them inside the mound. Thus, by removing vegetative cover, ants may be gaining foraging time early and late in the day, at the expense of midday foraging activity. These results suggest that vegetation removal serves a thermoregulatory function in harvester ants, which could aid in optimization of available foraging time outside the mound. Further work is required to establish whether vegetative removal is a result of selection related to this trade-off, or if the foraging consequences of clearing are coincidental. Since disturbed grassland has lower initial vegetative cover, this finding may also help explain the prevalence of harvester ants in disturbed habitats.
Species 1: Hymenoptera Formicidae Pogonomyrmex occidentalis (western harvester ant)
Keywords: foraging efficiency, thermoregulation
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