Using passive acoustic monitoring to investigate the occurrence of invasive Asian house geckos (Hemidacylus frenatus) on an Oceanic Island

Establishing the distributions of invasive species is critical, particularly on island ecosystems that support endemic populations highly susceptible to ecological disruption. However, evaluations of methods capable of efficiently detecting invasive species in remote or inaccessible areas at high temporal and spatial resolutions remain limited. Passive acoustic monitoring (PAM) has proven effective for monitoring a variety of invasive vertebrates, yet its application to monitor reptiles has been largely unexplored. Here we aim to assess the utility of PAM to detect the invasive Asian house gecko ( Hemidactylus frenatus ) and identify environmental variables associated with its occurrence on Christmas Island (Indian Ocean). We searched 29,640 hours of audio recordings collected from May to June 2023 by a network of 81 acoustic recorders deployed across Christmas Island using a semi-automated acoustic detection method to identify the gecko’s ‘multiple chirp’ call. Asian house geckos were detected by 66 recorders, in both disturbed and natural habitats, and were recorded during 61.6% of survey nights at occupied sites. Survey-level detection rates varied markedly across sites, potentially reflecting local abundance, suggesting that future research could enable abundance estimation from acoustic detection rates. A single-season occupancy model revealed areas at higher elevation with taller canopies were associated with gecko absence. Notably, these areas also support the highest abundance of a larger, endemic gecko species, suggesting that competitive exclusion may limit the Asian house gecko’s distribution on Christmas Island. Our findings demonstrate that PAM is an effective, scalable method for detecting an invasive soniferous reptile, and can enhance invasive species monitoring and risk assessment on islands.