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Desert horned lizards Phrynosoma platyrhinos consumed the solitary-foraging seed- harvester ant, Pogonomyrmex californicus, 10 to 100 times more often than expected based upon its relative abundance in the environment. The group-foraging seed-harvester ants, Pogonomyrmex ru- gosus and Veromessor pergandei, were correspondingly underrepresented in the horned lizard's natural diet. In a series of laboratory prey choice tests, where prey availabilities were held constant and equal, these preferences were confirmed: P. californicus was eaten more often than P. rugosus which was eaten more often than V. pergandei. These preferences correspond to differences in ant aggressive behavior which, in turn, correspond to differences in ant foraging-group size and foraging method. Colonies of solitary-foraging ants have small foraging groups and are generally incapable of mobbing a lizard, but they are also difficult to find. Colonies of group-foraging ants have large to massive foraging groups and vigorously mob lizards. Such aggressiveness is probably effective by increasing the risk of exposure of a lizard to predators of its own; the indirect costs of preying upon these ant species, normally assumed constant over all prey types for most predators, are variable here. Differences in experience with ant aggressive behavior probably account for differences in prey preferences found among adult lizards captured from the same area. Ant-species-specific predation by Phrynosoma may be an important ecological force structuring the Mohave Desert seed-harvesting ant community.

Nests of the desert seed-harvester ant Pogonomyrmex rugosus are located preferentially near shrubs in some microhabitats at a Mohave Desert site, while nests of the same species are located preferentially away from shrubs at a Sonoran Desert site. At the Mohave site, nearest neighboring shrubs to ant nests show greater mortality than expected at random; this effect is reduced at the Sonoran site. The ants appear to be the cause of shrub mortality; workers defoliate shrubs and make no apparent use of severed leaves or stems. Shrub viability at the Mohave site is dependent upon distance from a P. rugosus nest and size of that nest. In the Mohave Desert, shrubs may initially provide valuable shade to starting colonies, yet later in colony ontogeny represent a significant competitor for water.