UC San Diego

The ecological effects of species introductions can change in magnitude over time, but how and why such impacts exhibit temporal variation remains incompletely understood. In this study we used stable isotope analysis to estimate how trophic position changes as a function of the stage of invasion for the Argentine Ant (Linepithema humile), a widespread, abundant, and ecologically disruptive invader. Previous studies in southern California found that the trophic position of the Argentine ant was higher at the leading edge of invasion than at sites invaded several years earlier. To assess if a reduction in relative trophic position over time is a common feature of ant invasions, we expanded the temporal and spatial scale of sampling and estimated the relative trophic position of Argentine ants along three invasion chronosequences: Rice Canyon (San Diego Co. CA), the Sacramento River Valley (Yolo and Solano Cos., CA), and San Nicolas Island (Ventura Co., CA). Resampling Rice Canyon in 2019, 16 years after the original survey, revealed a surprising increase in Argentine ant trophic position. At the two other invasion chronosequences, the trophic position of the Argentine ant did not change with the stage of invasion. These findings suggest that changes in relative trophic position associated with invasion processes may reflect short-term responses, which are not evident in long-term sampling efforts. These findings also point to the potential value of historical data and repeated, annual sampling.