The Effect of Soil Moisture on the Susceptibility to Invasion by the Argentine Ant (Linepithema humile) in Chaparral Communities

The Argentine ant (Linepithema humile) is a successful invasive species found throughout the world. It is commonly associated with urban areas and invades habitats that have favorable conditions, such as adequate moisture levels. Urban areas in the xeric regions of Southern California that use supplemental irrigation create artificially moist habitats that are thought to serve as a permanent refuge for the Argentine ant. Invasion is a three step process of introduction, establishment, and spread; for Argentine ants to invade, they must first be introduced to an area. This study was designed to determine if Argentine ants seasonally spread into habitats that have high soil moisture for only a portion of the year, and to access the role that soil moisture, seasonality, proximity to the urban edge, and proximity to a permanent water source play in determining which areas the Argentine ant will spread into, and where they are likely to successfully invade. This study examined three different habitat types on two reserves that differed in water availability: mesic (permanently wet), xeric (upland areas without a permanent water source), and seasonally moist (intermittently wet). Sampling cups were set out in each habitat during the wet and dry season for a period of two years. Measurements were taken on Argentine ant abundance, native ant species abundance, distance to the urban edge, and distance to a permanent water source. Argentine ants were found in mesic sites throughout the study period in both wet and dry seasons. No Argentine ants were found at the xeric sites in either wet or dry seasons. Out of six seasonally moist sites, one that was in close proximity to permanent water had Argentine ants year round, while the other seasonally moist sites had no Argentine ants. Distance to a permanent water source was the best predictor for Argentine ant abundance, followed by soil moisture. Seasonal spreading was not observed in any habitat type. Based on these findings, intermittent water does not seem to be a risk factor for Argentine ant invasion, but permanent water is a risk factor for invasion. Dry-season urban runoff will potentially lead to increased Argentine ant invasions in this region.