Variation in Argentine ant (Linepithema humile) trophic position as a function of time

The ecological effects of species introductions can change over time, but an understanding of how and why they do remains hindered by the lack of long-term data sets that permit investigation into underlying causes. We employed stable isotope analysis to estimate how trophic position changes as a function of time for the Argentine ant, a widespread, abundant, and ecologically disruptive introduced species. Previous research at a site in southern California (Rice Canyon, San Diego Co.) found that Argentine ant δ¹⁵N values were higher at the leading edge of invasion than at those same sites in years subsequent to invasion (i.e., after the invasion front had advanced). To assess if a reduction in relative trophic position over time is a typical feature of Argentine ant invasions, we expanded the temporal and spatial scale of sampling and measured δ¹⁵N values of the Argentine ant at three locations with a known or inferred history of invasion: Rice Canyon (the location of the original study), 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 significant increase in Argentine ant δ¹⁵N values. At the two other locations, Argentine ant δ¹⁵N values were independent of time since invasion (Sacramento River Valley) or position relative to the invasion front (San Nicolas Island). These findings suggest that post-invasion reductions in trophic position may not be a general phenomenon or could reflect transitory ecological processes that require finer-scale temporal sampling than was possible to achieve in the present study. Our findings are nonetheless consistent with the results of recent studies, which found that the effects of Argentine ant invasions persist over decadal time scales.