The impact of invasive toads (Bufonidae) on monitor lizards (varanidae): an overview and prospectus

Toads of the amphibian family Bufonidae possess toxins that can be lethal to monitor lizards (family Varanidae) when ingested. In Australia, populations of several species of monitors (Varanus spp.) have been heavily impacted via direct poisoning by invasive cane toads (Rhinella marina). It has been suggested that the invasion of Asian black-spined toads (Duttaphrynus melanostictus) through the Indonesian archipelago could have equally devastating effects on other monitors, especially the iconic Komodo dragon, Varanus komodoensis. We review published information to evaluate the risk posed by invasive toads to varanids worldwide, and identify critical gaps in knowledge (e.g., specific varanid lineages not yet tested for the genetic signature of toxin resistance; mechanisms that buffer toad impacts even on physiologically susceptible predators; toxicity of D. melanostictus). We conclude that population-level impacts of invasive toads are unlikely to be significant for most yet-to-be-affected species of monitors, including V. komodoensis. Nonetheless, the likely spread of D. melanostictus through southern Australia may affect monitors (e.g., heath monitor V. rosenbergi) that are not currently imperiled by cane toads and, more generally, wildlife managers need to evaluate potential impacts of these toxic invaders on vulnerable native predators.