Curbing catastrophe: ecology and conservation of the Yellow-spotted Monitor (Varanus panoptes) in tropical Australia

Abstract

Worldwide, one of the most catastrophic impacts of invasive species is to imperil populations of apex predators, thereby inducing trophic cascades that can substantially modify ecosystem function. Management strategies generally focus on controlling the invader; but we can, instead, manipulate the behavioural response of vulnerable native taxa to render them more resilient to the invader’s arrival. In Australia, large varanid lizards (“goannas”) are fatally poisoned when they ingest invasive cane toads (Rhinella marina). As a result, the spread of cane toads has caused catastrophic population declines in these important predators. My PhD investigated the potential for an adaptive learning mechanism, Conditioned Taste Aversion (CTA), to conserve populations of tropical Australia’s largest lizard, the Yellow-spotted Monitor (Varanus panoptes). Ahead of the cane toad invasion, I exposed free-ranging goannas to small, live, non-lethal toads to assess the influence of training on lizard survival post-invasion. After cane toads arrived, trained goannas had significantly higher survival than their untrained (control) counterparts. I investigated mechanisms underpinning CTA to clarify how best to apply it in the field; for example, Yellow-spotted Monitors develop stronger aversions to live prey than sausages laced with chemicals, and an individual’s behavioural phenotype influences the effectiveness of CTA. I documented a behavioural syndrome that correlates with aspects of an individual’s ecology, life history and microevolutionary fitness. Furthermore, some individuals are inherently toad-averse; therefore some goannas (or populations) may not need CTA training. This behavioural diversity demonstrates how inter-individual variation can shape a population’s response to a novel pressure. Identifying and exploiting pre-existing behavioural mechanisms to deal with new challenges holds great promise for amplifying the tools available for conservation efforts globally.