Colony structure, climate and spread in invasive Argentine ants

Many invasive ants are unicolonial and disperse their colonies among many nests that are interconnected by trails. Few studies have explored the spatial organization of unicolonial colonies and how space-use affects the dynamics of spread and invasion success. I investigated the behavior and spatial distribution of invasive Argentine ants, Linepithema humile, at local and landscape scales in the introduced range in northern California and the native range in northeastern Argentina. Observational studies and food sharing experiments indicate that L. humile nests and trails are organized into discrete colony networks ranging in extent from approximately 250 to 850 m 2. Colonies expand and contract seasonally, as the size, spatial pattern, density and location of nests and trails shift. The location of nests is strongly associated with woody plants. Colonies contract in the winter when ants aggregate in a few sites with warm soil temperatures, and expand in the summer when ants disperse to many sites with cool soil temperatures. The same overwintering sites are used year after year. A laboratory experiment showed that L. humile move nests in response to shifts in microclimate. The seasonal pattern of colonies causes the range size of L. humile at the landscape scale to expand and contract with season, and causes seasonal fluctuation in the impact of L. humile on native ants. An ongoing survey at Jasper Ridge Biological Preserve showed that L. humile was more widely distributed in years of high rainfall that were associated with El Nino events. Native ant species decreased in wet years, but their response did not appear to be due to competition with L. humile. An increase in El Nino conditions, predicted by climate change models, is likely to increase the spread of L. humile. In a comparison of L. humile 's colony structure and density at local scales in the native and introduced ranges, I found they were similar, but more ant species coexisted with L. humile in the native range. The invasion success of Argentine ants in the introduced range is probably largely due to differences from the native range in the strength of interspecific competition.