The lethal and sub-lethal effects of the invasive yellow crazy ant (Anoplolepis gracilipes) on the terrestrial hermit crabs (Coenobita spp.) of Tokelau

Abstract

Invasive ants can inflict a range of sub-lethal and lethal effects on the resident fauna of invaded areas. The yellow crazy ant, Anoplolepis gracilipes (Smith), is a highly aggressive invasive ant species now widespread throughout islands in the Pacific Ocean. Anoplolepis gracilipes has been shown to impose substantial sub-lethal and lethal effects on a range of crustacean species. However, their effect on the widespread, abundant, and ecologically important terrestrial hermit crab species (species in the genus Coenobita) has not been investigated. Anoplolepis gracilipes was first reported in Tokelau in 1934, but populations have recently undergone rapid growth. It is considered one of the world's worst invasive species and, despite achieving high densities on Tokelau, it continues to coexist with the three resident terrestrial hermit crab species; Coenobita brevimanus (Dana, 1852), C. perlatus (H. Milne Edwards, 1837) and C. rugosus (H. Milne Edwards, 1837). In this study I; 1) assess for sub-lethal and lethal effects of A. gracilipes on resident terrestrial hermit crab species of Tokelau, 2) examine if the diet of the resident terrestrial hermit crabs differs between A. gracilipes invaded and uninvaded islets using carbon and nitrogen stable isotope analysis, and 3) plot the global distribution of A. gracilipes and Coenobita species to show where my results are applicable. Anoplolepis gracilipes imposes a range of sub-lethal effects on the terrestrial hermit crabs of Tokelau, excluding them from the interior of islets and forcing them to feed at a significantly lower trophic level on invaded islets. However, there was no evidence that A. gracilipes and the terrestrial hermit crabs temporally partition foraging. Field and laboratory experiments showed that A. gracilipes could not kill healthy hermit crabs in their shells but could kill hermit crabs out of their shells swiftly. Mapping the global distribution of A. gracilipes and the 15 species of terrestrial hermit crab showed these species inhabit tropical and sub-tropical areas worldwide, but are concentrated in the Pacific region where ~58% of sites included in this study are occupied by A. gracilipes and at least one species of terrestrial hermit crab. These results allow us to make predictions about how an invasion by A. gracilipes may affect the resident terrestrial hermit crab species of an area, and aid in the conservation of invertebrate biodiversity.