Live Fast, Die Young: Life History Traits of an Apex Predator Exacerbate the Ecological Impact of a Toxic Invader

Abstract

We studied a population of large varanid lizards (yellow-spotted monitors Varanus panoptes) on a floodplain in tropical Australia. Growth records from radio-tracked lizards show that despite their large adult body sizes (to > 7 kg in males), these lizards attained sexual maturity at less than 1 year of age and rarely lived for more than 2 years (females) or 4 years (males), even before mortality increased due to the arrival of toxic cane toads (Rhinella marina). This is a “faster” life history than has been reported for other species of large monitors. Growth was especially rapid in males during the wet season. The low survivorship prior to toad invasion was due to predation by pythons; communal nesting by female varanids may render them especially vulnerable. The life history of yellow-spotted monitors requires high feeding rates, favouring the evolution of “risky” tactics such as consuming novel prey items (such as cane toads); and the combination of high abundance (> 20 adult lizards per square kilometre) and high feeding rates (> 9.9 kg of prey per lizard per annum) means that these giant lizards play a critical role in energy and nutrient flow within the floodplain ecosystem. As a result, foodwebs with the yellow-spotted monitor as an apex predator are more vulnerable to disruption by cane toads than is the case in other parts of the toad's invasive range, where the varanid species affected by toads have “slower” life histories.