Prey selection in horned lizards following the invasion of Argentine ants in southern California

Biological invasions can have severe direct and indirect impacts on natural communities and are one of the leading reasons behind the loss of biodiversity. We investigated the indirect effects of Argentine ant (Linepithema humile) invasions on coastal horned lizards (Phrynosoma coronatum) through an examination of prey selection in the field and laboratory. In non-invaded areas, coastal horned lizard diet consists predominately of ants (>94% by prey item in three reserves examined), particularly harvester ants in the genera Pogonomyrmex and Messor. In invaded areas, most native ants were displaced, and remaining horned lizards incorporated more non-ant arthropods and smaller ants into their diet. In addition, Argentine ants were never detected in horned lizard fecal pellets, suggesting that they are not a suitable replacement food source for the native species they are displacing. Analysis of prey selection in uninvaded areas reveals that ant species consumption by adult lizards is correlated with ant body size rather than ant abundance, foraging behavior, or defensive method. Differences in diet between adult and juvenile lizards suggest that a diversity of ant species is necessary to support both age classes. Laboratory experiments, consisting of paired presentations of four native ant species and the Argentine ant, confirmed patterns found in the field. Both adult and juvenile horned lizards preferred native ants to Argentine ants. Patterns of preference suggest that, in addition to L. humile's small size, other factors may contribute to the horned lizard's aversion to this species. Populations of coastal horned lizards have undergone severe declines in recent years. While habitat loss has been the primary concern, the invasion of Argentine ants into their remaining suitable habitat needs to be monitored. Reserves need to be managed to prevent the penetration of Argentine ants in order to minimize their direct and indirect effects on natural communities.