UC San Diego

Invader-removal experiments can lend insight into the ecological resilience of biological assemblages and the extent to which they can recover from perturbations. Argentine ant invasions on Santa Cruz Island have disrupted the diversity and functionality of ant assemblages. This thesis sought to quantify changes in native ant species following invader removal in nine experimental plots paired with nine control plots on Santa Cruz Island. Stable isotope analysis (variation in δ15N) was used to estimate relative trophic positions of and resource acquisition by native ant species. Spatiotemporal variation in δ15N was also investigated to assess its relative impact on the trophic niches of twelve native ant species. Stable isotope analysis was conducted on an assemblage of native ants in 2017 and 2019 following the removal of the Argentine ant from Santa Cruz Island conducted by The Nature Conservancy with support from the National Park Service. Soil δ15N and plant δ15N values were positively related to one another. Subsequently, this correlation was used to correct for the δ15N values of native ant species. δ15N values for native ant species did not differ between control and experimental plots. Native ant species also displayed temporal constancy in δ15N values in 2017 and 2019. Overall, there was significant variation in trophic positions among native ant species. It is hypothesized that functional traits of the ant species played a role in niche diversification, and overall competition levels were minimized as a result of these differences. Species richness was lower on experimental plots as the experimental recovery is still ongoing, and additional ant species are expected to re-colonize the plots. The similar trophic positions of ants on invaded and uninvaded plots demonstrated the ability of native ants to recover to their pre-invasion state following successful eradication of the invader.