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 and Gibbons 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. al 2007, Wilder et al. 2011, Suehiro et al. 2017); introduced populations of the red imported fire ant appear less carnivorous compared to native populations, presumably because of relaxed interspecific competition for carbohydrate resources in their introduced range (Wilder et al. 2011). Introduced ant populations also exhibit spatial variation in estimated trophic position at a variety of scales (Menke et al. 2010, Wilder et al. 2011, Balzani et al. 2021), including among colonies from the same location. Roeder and Kaspari (2017), for example, found that the estimated trophic position of individual red imported fire ant colonies varies from primary consumers to higher order carnivores even within a single population in Oklahoma.
Temporal variation in the estimated trophic position of introduced ants has received less attention compared to spatial variation. Temporal comparisons are important, however, given that (i) invasion impacts can change as a function of time since establishment (Strayer et al. 2006, Strayer 2011), and (ii) introduced ant populations can decrease in abundance over time (Lester and Gruber 2016). By following the spread of an expanding Argentine ant invasion front over an eight-year period at a site (Rice Canyon) in southern California, Tillberg et al. (2007) found that the δ15N values for this invader were higher at the leading edge of invasion than at those same sites in the years subsequent to invasion. Two possible (and non-mutually exclusive) mechanisms might explain this pattern: (i) prey depletion, and (ii) enhanced availability of carbohydrate resources, such as honeydew from Hemiptera. Prey depletion could result from the elimination of native ants (that succumb to the Argentine ant from both competition and predation) as the invasion proceeds (Naughton et al. 2020), whereas access to hemipteran honeydew could increase if densities of honeydew-producing insects themselves increase as a result of being tended by the Argentine ant. Diet manipulation studies demonstrate that Argentine ant colonies provided honeydew-producing aphids have lower δ15N values than those fed animal-based diets (Menke et. al 2010) as well as increased worker survival, worker activity, and colony growth (Shik et. al 2012).
Here, we assess the generality of post-invasion reductions in trophic position for the Argentine ant (Linepithema humile). Specifically, we used stable isotope analysis (based on N) to investigate how the relative trophic position of the Argentine ant changes with time since invasion at three different locations in California: the Rice Canyon transect previously sampled in Tillberg et. al (2007), an invasion chronosequence in the Sacramento Valley (Menke et al. 2018), and replicate transects on San Nicolas Island that were each oriented perpendicular to expanding invasion fronts (Boser et al. 2018). By expanding the spatial and temporal sampling considered by Tillberg et. al (2007), our main objective is to test whether or not ant invasions typically exhibit reductions in trophic position in the years subsequent to invasion.