Foraging Ecology of Seed-Harvesting Ants, Pheidole spp. in a Chihuahuan Desert Ecosystem

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Abstract
Seasonal activity, factors affecting foraging activity and forage selection were studied in 3 spp. of Pheidole (Hymenoptera: Formicidae) on a watershed in the southern New Mexico Chihuahuan Desert. Pheidole spp. exhibited a seasonal pattern in numbers of active colonies with most colonies active in July coincident with onset of summer rains and seed drop by annual plants. P. militicida occurred only on the deeper soils of the lower watershed while P. rugulosa and P. xerophila occurred on the entire watershed. The 3 Pheidole spp. were active only at soil temperatures between 15-35.degree. C and were most active at sunrise. Analysis of factors affecting foraging intensity using a stepwise discriminant function and regression showed evaporation and soil surface temperature to be the strongest influences and seed availability to rank second. The factors included in the analysis accounted for < 50% of the variance in all 3 spp. P. militicida collected mostly forb seeds while P. xerophila used primarily grass seeds, especially the seeds of fluff grass Erioneuron pulchellum.