Impacts of Anoplolepis gracilipes (yellow crazy ant) on invertebrate communities in Nu’utele, Samoa

Abstract

Ant invasions frequently cause ecological disturbances on both island and continental ecosystems all over the world. One of the most devastating impacts of an invasion by invasive ants has been reported on Christmas Island, where Anoplolepis gracilipes (yellow crazy ant) is responsible for the collapse of the rainforest ecosystem. Negative impacts of A. gracilipes invasion have been reported from Tokelau, Seychelles and Christmas Island. The preservation of native biodiversity in Samoa is limited to two offshore islands; Nu’ulua and Nu’utele. In 2006, A. gracilipes was recorded in Nu’utele and justifying management for this invasive ant has proven difficult due to the knowledge gap of impacts on Nu’utele. The aims of this research are to: 1) determine how A. gracilipes activity and abundance compare elsewhere, 2) determine the optimal time to bait, and 3) determine how ground and arboreal invertebrates (inclusive of hermit crabs) are being influenced by A. gracilipes. Card counts, tuna baiting and pitfall traps showed A. gracilipes activity and abundance on Nu’utele was greatest in the dry season but was much lower in comparison to Christmas Island and Tokelau. Analysis of nest size and composition revealed the presence of reproductives (males & queens) from wet season sampling only, therefore baiting should be applied in the dry season before the onset of the rain. Pitfall trapping and foliage beats illustrated that A. gracilipes: 1) coexisted with smaller sized, generally exotic, ant species; 2) was significantly impacting on hermit crab populations particularly in the dry season; 3) resulted in a decrease in the abundance of macro-invertebrate Araneae; and 4) caused a complete absence of macroinvertebrate Lepidoptera from trees. Macro-invertebrates Isopoda and Homoptera were more abundant in the infested site. The findings from the present study are consistent with what is observed elsewhere. Though ant activity and abundance of A. gracilipes on Nu’utele is much lower than on Christmas Island and Tokelau, it is clear from results in the present study that A. gracilipes are significantly influencing the invertebrate communities and therefore justifies management of this invasive species.