Argentine ants: Patterns of invasion and consequences for northern California grasslands

The Argentine ant, Linepithema humile, is a major pest of mediterranean-type ecosystems worldwide. While it is known to cause local extinctions of many native ants, the indirect effects of losing this native fauna on plant communities is just beginning to receive greater attention. This dissertation examines seasonal patterns of distribution change of the Argentine ant and possible causes of those patterns as well as effects on native ants. The dissertation then examines how Argentine ants may effect plant communities through its displacement of a locally important harvester ant, Messor andrei. Argentine ant workers were more active in the drier months of summer and fall, and activity was correlated with cumulative precipitation the previous winter. No similar relationship existed for native ants. Argentine ant abundance greatly increased in conjunction with macro-weather phenomena such as El Nino. This invasive ant affected native ant abundances differently by species: some species were able to tolerate its presence temporarily. Argentine ants and M. andrei have different diets: Argentine ant workers consume mostly liquid food. In contract, 98% of M. andrei that returned to the nest with any food item were carrying seeds. M. andrei exhibited a selective preference for different species of seeds. Much of the variation in preference was explained by seed weight: M. andrei strongly preferred lighter seeds. Upon the demise of M. andrei, seeds of preferred species should accumulate in the soil. Examination of the plant community and seed bank surrounding M. andrei nests showed that M. andrei altered almost all measures of community composition of the seed bank. Patterns for the plant community were weaker, though showed spatial trends on many measurements of community composition. Evidence for seed-limitation as a causal mechanism for plant spatial patterns was mixed: one ant-preferred species showed limitation and the other did not. Argentine ants would alter plant community composition upon displacing M. andrei in the short term by influencing species composition in areas immediately surrounding M. andrei nests. In the long term, Argentine ants would reduce heterogeneity of the spatial distribution of the plant community and reduce the variance in abundance of certain species.