Acute impacts of invasive toads on the population demography of a native predator in tropical Australia

If an animal’s size, age and/or sex influence its vulnerability to an invasive species, the arrival of such an invader can cause rapid changes in the population demography of an affected species. We studied free-ranging varanid lizards (Yellow-spotted monitors, Varanus panoptes) at a site in tropical Australia during the influx of fatally toxic cane toads, Rhinella marina. Mortality was inferred from shifts in population structure, as well as the survival rates (time to death) of 107 radio-tracked lizards. Of 57 deaths whose cause was unambiguous, 32 were due to fatal poisoning by ingesting a cane toad; the other 25 lizards were consumed by pythons. Size and age structure shifted between years, such that the population post-invasion was dominated by smaller, younger lizards, and by females rather than males. Radiotelemetric monitoring confirmed that survival rates were reduced more in males than in females in the post-toad year, with males most at risk late in the dry-season, when food was scarce and females were nesting rather than foraging. Pythons disproportionately consumed larger female lizards during the nesting season. Toad-induced poisoning of adult male varanids (which are larger and bolder than females) likely produced a population that was more resilient to toad impact, but less easily surveyed by conventional techniques.