Cane toads reduce the abundance and site occupancy of Merten's water monitor (Varanus mertensi)

Introduced into Australia in 1935, the cane toad (Chaunus [ Bufo] marinus) threatens native vertebrate predators. However, there have been few rigorous quantitative studies on species threatened by this toxic invasive species. This study examines the changes in abundance and proportion of sites occupied by Merten's water monitor (Varanus mertensi) at a site in the Northern Territory following invasion by cane toads. The study was located at Manton Dam Recreation Area, 70 km south of Darwin, and ran for 18 months. Cane toads were first detected at the study site in February 2005, three months after the first survey, and their abundance remained low until February 2006, when an increase was observed. The abundance of V. mertensi declined substantially 8 months after the arrival of cane toads and remained low. The probability of detection of V. mertensi varied considerably within and among surveys, and was higher in the wet season surveys. The proportion of sites occupied by V. mertensi at the start of the study was 0.95 +/- 0.03. Site occupancy remained high for 6 months after the arrival of cane toads, but declined gradually to a low of 0.15 +/- 0.16 within 12 months. There has been demonstrable change in the abundance and proportion of sites occupied by V. mertensi following the colonisation of cane toads, but the population has been able to persist. Monitoring of populations impacted by cane toads may provide unique opportunities to understand processes underlying local extinction and colonisation of native predators following the impact of invasive species.