COS 18-1 - Impacts of yellow crazy ants and ant eradication on the seabirds and terrestrial crabs of a Polynesian atoll

Abstract: Invasive species pose a major threat to biodiversity, especially on islands, where the identification and eradication of problematic invaders represents an important management tool. Yellow crazy ants (Anoplolepis gracilipes) are a highly invasive “tramp” ant species that spray formic acid to subdue their prey. Seabirds and terrestrial crabs, the two faunal groups that dominate atoll ecosystems, may both be affected by the presence of this invader. However, the extent to which these ants influence these key groups, and how quickly native communities can recover post-eradication, remains largely unknown. Here we sought to quantify the impact of yellow crazy ant presence and abundance on the density of both ground-nesting seabirds and terrestrial crabs by establishing 70 plots (r=5m²) across fully invaded, partially invaded, and uninvaded (control) islets on Tetiaroa atoll (French Polynesia). Within each plot we quantified ant, crab, and seabird abundances both before and after the initiation of an ant eradication program in which we reduced ant abundances by deploying hydrogel crystals saturated with sugar water and insecticide. Despite relatively low ant abundances during the pre-eradication monitoring period, we found that yellow crazy ant abundance was negatively associated with the density of both ground-nesting seabirds (predominantly brown noddies, Anous stolidus) and the dominant terrestrial crab species (the strawberry hermit crab, Coenobita perlatus). Brown noddy chicks raised within invaded habitats were more likely to exhibit physical deformities and spent more time preening, shaking, and foot stamping than chicks in uninvaded habitats. However, nest monitoring data at 360 noddy nests demonstrates that egg-hatching success was unaffected by ant presence, potentially because the strawberry hermit crabs that avoid invaded areas were also one of the most important egg predators at our monitoring sites. Their exclusion from invaded areas may indirectly benefit nesting noddies. Post-eradication monitoring demonstrated a rapid, >99% decrease in ant abundance 10-days post-baiting, and showed that non-target impacts of the baiting itself on crab communities were relatively minimal, with visible mortality confined to two of the most terrestrial crab species present. Eradication programs both provide conservation benefits and a unique experimental context for investigating the effects of invaders on native fauna. Quantifying both the benefits and non-target impacts of baiting programs is informative to future managers facing similar issues with invasive species.