The population dynamics of the yellow crazy ant Anoplolepis gracilipes (Hymenoptera: Formicidae) on a tropical island in Malaysia

The yellow crazy ant (Anoplolepis gracilipes) is a widespread tropical invasive species that causes ecological damage in introduced habitats; their negative impacts are especially prominent on island ecosystems. We examined abiotic and biotic effects on a tropical island (Lang Tengah Island on the East Coast of Malaysia) that experiences annual monsoonal disturbance and hosts two distinct populations of yellow crazy ants. To understand how biotic factors affect A. gracilipes, we conducted ant‐ant interaction bioassays to examine the extent of aggression levels between populations. We tested the abiotic monsoonal rain effect by employing passive (pitfall traps) and active (quantifying ground foragers on marker crossings) sampling, across pre‐and post‐monsoon. The two populations of A. gracilipes exhibited aggression toward each other. Furthermore, the onset of monsoonal rains reduced A. gracilipes abundance and foraging activity. Our findings suggest that A. gracilipes populations are sensitive to monsoonal rains and long‐term population fluctuation data are urgently needed to examine the effect of monsoonal rains on this invasive ant's expansion dynamics on island ecosystems. The two populations of A. gracilipes on Lang Tengah Island exhibited aggression toward each other. The onset of monsoonal rains reduced A. gracilipes abundance and foraging activity. A. gracilipes populations on this island are sensitive to monsoonal rains and experience population fluctuation.