Changes In Argentine Ant 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. To assess if a reduction in relative trophic position over time is a typical feature of 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 site 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 signicant 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 ndings suggest that post-invasion reductions in trophic position may not be a general phenomenon or could reect transitory ecological processes that require ner-scale temporal sampling than was possible to achieve in the present study. Our ndings are nonetheless consistent with the results of recent studies, which found that the effects of Argentine ant invasions persist over decadal time scales.


Introduction
The ecological effects of biological invasions can vary with time. Understanding the basis of this variation requires long-term data sets and quantitative information about the form and magnitude of trophic interactions between an invader and the species with which it interacts (Strayer et al. 2006, Strayer 2012. Changes in competitive interactions, nutritional demands, and resource availability may contribute to trophic shifts observed in invading species over time. Isolating the underlying causes of this type of variation can reveal insights into evolutionary responses of both the invader and the native species with which it interacts (Strayer 2012). The lack of comprehensive data limits an understanding of why invasion impacts vary with time (Simberloff andGibbons 2004, Strayer et al. 2006).
Introduced ants represent a highly disruptive force in ecosystems worldwide because of their abundance and generalist diet (Holway et al. 2002, Lach 2010. A recent global meta-analysis of invasive terrestrial invertebrates, for example, found that ant invasions were associated with reduced animal abundance and diversity (Cameron et al. 2016). While the ecological effects of ant invasions can persist over decadal time scales (Menke et al. 2018, Achury et al. 2021), some populations of introduced ants have declined in abundance with concomitant reductions in invasion impacts (Lester and Gruber 2016). Progress towards identifying the causes of such declines hinges on understanding how introduced ants interact with other species, especially those that they consume. However, quantifying the diets of introduced ant species is complicated by the fact that they are omnivores that feed extensively on liquid foods ranging from honeydew to hemolymph.
Stable isotope analysis can provide insights into the diets of introduced ants and how resource assimilation varies across space. For example, comparisons of native and introduced populations can suggest causes of niche expansion in the introduced range (Tillberg et.

Methods
We collected Argentine ant samples between late August and late September 2019 at three locations in California: Rice Canyon, Chula Vista, San Diego Co., the lower Sacramento River Valley, Yolo and Solano Cos., and San Nicolas Island, Ventura Co. (Table 1; Appendix 1). At Rice Canyon, we collected ants at nine coastal sage scrub sites sampled in previous studies (Suarez et al. 1998 At all three locations, 50 to 100 Argentine ant workers were collected by beating perennial vegetation with a beating net. Workers were placed immediately in 95% ethanol and then stored at -20° C before processing. At each site, perennial plant material was collected to estimate δ15N values for primary producers (as in Gibb et al. 2014, Roeder andKaspari 2017). The plant species collected differed among the three study locations but were standardized across sites at each location (Table 1). At each site, leaves from three different individuals of each plant species were collected and placed in paper envelopes. Plants sampled at each site were always within 20 m of where ants were collected. After collection plant samples were air dried and then stored at -20° C.

Discussion
We expanded the temporal and spatial scale of previous sampling efforts to assess how the trophic ecology of ant invasions changes as a function of time since invasion. At three different locations in California, Argentine ant δ15N values either increased (one site) or did not change (two sites) as a function of time. These ndings suggest that post-invasion reductions in trophic position, as observed in Tillberg et al. (2007), could be evident at some sites but not others or that they re ect transitory ecological processes that require ner-scale temporal sampling than that possible to achieve in the present study. the most likely form of prey depletion at these sites given that comparisons of non-ant arthropod richness and abundance from the lower Sacramento Valley (Holway 1998) and the Channel Islands (Hanna et al. 2015) have revealed no differences between sites with and without the Argentine ant.
Progress towards understanding how the ecological effects of invasions change over time will typically require quantitative information about the diets of introduced species. Stable isotope analysis is useful in the attribution of plant-based versus animal-based dietary inputs. However, the diets of introduced ants are diverse and encompass living prey, carrion, and liquid exudates from plants and honeydew producing insects. The results of stable isotope analysis can be misinterpreted when the trophic positions of known herbivores and predators are compared to those of ant species that act in part as predators but that also assimilate carrion of diverse origin (Holway & Cameron 2021). Caution thus seems warranted in the interpretation of spatial and temporal comparisons of invasion impacts based solely on stable isotope data.
Declarations Table   Table 1. Sampling locations, dates of sampling, number of statistical blocks (and total number of sites), and plant species collected at each study location. Coordinates for sites at each location are listed in Appendix 1.