Long-term monitoring reveals size-specific declines in an apex predator (the freshwater crocodile Crocodylus johnstoni) following the invasion of an exotic prey species (the cane toad Rhinella marina) in Australia

Background/Question/Methods

Invasive species are a global conservation concern. While many invasive predators cause population declines in native prey species, empirical evidence linking declines of native predators to invasive prey species is comparatively rare. We used 35 years of standardized spotlight surveys in northern Australia to investigate the effect of an exotic, toxic prey species (the cane toad Rhinella marina) on the abundance of two apex predators: the freshwater crocodile (Crocodylus johnstoni) and the saltwater crocodile (C. porosus).

Results/Conclusions

Our results reveal that the density of large (>1.2 m) freshwater crocodiles increased significantly after commercial hunting was banned in 1964, but that the density of animals >1.5 m did not change substantially following the arrival of cane toads in 2005. In contrast, the density of intermediate-sized crocodiles (0.6 – 1.5 m total length) declined abruptly following toad invasion. Densities of saltwater crocodiles increased over time, but were generally less affected by the arrival of toads. Across all sites and size classes, freshwater crocodile densities declined by 71.3% subsequent to toad arrival. Population declines of this magnitude within approximately one generation place the Daly River freshwater crocodile population in the IUCN category of Endangered. This case study highlights the value of long-term, standardized monitoring in detecting changes in the dynamics of native populations following the establishment of invasive species.