Variation in Argentine ant trophic position as a function of stage of invasion

David Holway, Evelyne Baratelli and Ida Naughton, Division of Biological Sciences, University of California, San Diego, La Jolla, CA, Chadwick V. Tillberg, Department of Biology, Linfield University, McMinnville, OR, Sean B. Menke, Department of Biology, Lake Forest College, Lake Forest, IL, Andrew V. Suarez, Department of Animal Biology and Department of Entomology, University of Illinois at Urbana-Champaign, Urbana, IL

Background/Question/Methods - The ecological effects of species introductions can change in magnitude over time, but how and why these impacts vary with time remains incompletely understood. In this study we investigate how trophic position changes as a function of stage of invasion for the Argentine Ant, which is a widespread, abundant and ecologically disruptive invader. By employing stable isotope analysis, past work conducted in southern California found that the estimated trophic position of the Argentine ant was higher at the leading edge of invasion than it was where this species had invaded several years before. This finding suggests that over time the Argentine ant either (i) depletes high trophic level resources, or (ii) builds up populations of honeydew-producing insects. Here, we expanded the temporal and spatial scale of sampling to assess whether or not a reduction in trophic position over time is a common feature of ant invasions.
Results/Conclusions - Resampling the site of the original study (Rice Canyon, San Diego Co. CA) after 16 years revealed a significant increase in Argentine ant trophic position. This result was contrary to our prediction, but we can’t determine if this increase is restricted to just the Argentine ant or if it reflects long-term environmental changes affecting the entire site. We broadened the spatial scale of sampling by considering two different invasion chronosequences: Sacramento River Valley (Yolo and Solano Cos., CA) and San Nicolas Island (Ventura Co., CA). At both of these chronosequences, Argentine ant trophic position did not change over time. These findings suggest that the original observation that trophic position declined with time since invasion may reflect a short-term response, or may not represent a general phenomenon. Our approach illustrates the value of historical data to examine long-term effects of biological invasions.