Within-population variation in dietary traits: implications for vulnerability and impact of imperiled keystone predators

Within a population of apex predators, differences among individuals can influence both their ecological impact and their vulnerability to threatening processes. Our field studies on a large monitor lizard (Varanus panoptes) in the Australian wet–dry tropics show that diets shift seasonally and depend upon a lizard’s sex and body size. Individuals that had previously been recorded to consume frogs were most at risk following biological invasion by toxic cane toads (Rhinella marina), as were individuals with broad diets during the wet season. As a result, mortality of those individual predators likely reduced predation pressure on other taxa (invertebrates and reptiles) that were frequently consumed by the same lizards that ate frogs, but with less benefit for taxa (e.g., rodents) that were consumed by non-anuran-eating individuals within the predator population. In particular, individuals killed by cane toads often had consumed agamid lizards, a group whose abundance has been reported to increase due to toad-induced mortality of V. panoptes. To understand the vulnerability of apex predators, or the ecological consequences of their extirpation, we need to incorporate the role of variation among individuals in critical ecological traits.