The effects of the invasive thief ant, Solenopsis papuana, on ground-dwelling invertebrates in mesic forests of Hawaiʻi

Invasive predators are often hypothesized to negatively affect native invertebrates, but impacts have not been measured for most invaders. We quantified the effects of Solenopsis papuana, an invasive thief ant that is widespread in Hawaiʻi, on invertebrate communities via an experimental manipulation study. We established six pairs of plots in the Waiʻanae Mountains of Oʻahu, suppressed S. papuana in half of the plots, and measured the responses of invertebrates 6 months and 1 year later. Leaf litter and pitfall sampling revealed that ground-dwelling invertebrate communities were highly invaded, with native species comprising 24% of the taxa and only ~ 1–2% of the individuals captured. Relatively few taxonomic groups increased significantly in abundance following suppression of S. papuana, and responses were generally inconsistent between the two sampling periods. Significant increases in species richness in ant-suppressed plots were more common, particularly in pitfall samples. Most of the significant changes involved non-native species, but several native species also appear to be impacted by S. papuana. Although statistically significant responses were relatively uncommon, nearly all taxonomic groups exhibited non-significant increases in abundance and species richness in response to S. papuana suppression, and there were no significant decreases in either abundance or species richness for any taxonomic group. This suggests that in their current highly invaded states, mesic forest ground-dwelling invertebrate communities are broadly but relatively weakly affected by S. papuana. This conclusion may be tempered by the difficulty of accurately measuring effects on rare species, many of which may now be absent from these communities.