Between-mound sharing
To quantify sharing between mounds in the field, we treated select mounds in each site with a stable isotope tracer and quantified its movement into neighboring mounds (Fig 1). Stable isotope tracers employ naturally occurring, non-radioactive forms of biologically relevant elements, such as nitrogen. The heavier isotope of nitrogen (15N) occurs rarely in nature, so by artificially ‘spiking’ a food with an appropriate concentration of this heavy isotope, we can trace the movement of this isotope through consumers and identify the flow of nutrients through an ecosystem (Fry, 2006). We ensured that only the treated mounds had access to the isotope tracer, so if a neighboring untreated mound showed unnaturally high levels of15N, this would indicate an exchange of either workers or resources between the treated and untreated mounds (i.e., no boundaries between mounds).
Sampling was conducted between August and October 2019 in six field sites in Texas, USA (Appendix S1). Habitats ranged from restored grasslands (sites O, A, and B) to mowed fields (sites C, T, and T2). Mounds were used as a proxy for individual nests. We identified three clusters of four to five mounds at each site. One mound within each cluster was selected as the treatment mound. Clusters were separated by at least 50 m within each site to avoid potential sharing between clusters (Fig 1). To determine any effect of distance on resource sharing, untreated mounds within each cluster were between 0.4m – 29.07m from the treated mound, with an average distance of 7.65m ± 0.72m.
Similar to other studies (Goodisman et al., 2007), several fire ant mounds disappeared or moved over the course of the sampling period. As a consequence, we were unable to find three mounds (one mound from site T, one from O, and one from T2) after the treatment period. Each of these were untreated mounds within different clusters, so their removal did not affect the number of clusters analyzed in each location. In total, we sampled from 73 fire ant mounds across six sites, with 12 mounds in Site A, 13 in Site B, 12 in site C, 11 in Site O, 13 in Site T, and 12 in Site T2.