A project undertaken at The University of Canberra, and supervised by Dr Simon Clulow.
Invasive species pose a major threat to biodiversity and it is critical to understand how some native species can deal with this threat to inform management of more vulnerable species. While native species could, in the long-term, adapt to invaders through natural selection, the emergence and spread of behavioural innovations via behavioural flexibility and social learning could allow more rapid adaptive responses. This suggests there is potential for novel conservation strategies that exploit social networks to introduce adaptive behavioural innovations that can mitigate the negative impacts of invasive species.
In northern Australia, the invasive cane toad (Rhinella marina) has decimated populations of some native predators through lethal poisoning because toads are highly toxic and Australian species are naïve to this defence. While the evolution of toxin-resistance or behavioural avoidance may be the only long-term hope for many native predators, there is evidence that the rapid emergence and spread of behavioural innovations through behavioural flexibility and social learning may not only enable some native species to avoid lethal poisoning, but to utilise toads as a major food resource. For example, native water rats have been documented flipping toads on their back to consume non-toxic organs and tissues, while avoiding the toxic glands in the skin and neck. Birds, including Torresian crows, have also been observed flipping and consuming the non-toxic organs of cane toads in different ways. This suggests that in animal species with sufficiently complex cultures, behavioural innovations could emerge and spread rapidly via social learning, potentially turning a lethal toxic invader into a novel and abundant food resource. However, the mechanisms by which behavioural innovations emerge and spread, and whether induced behaviours result in long-term cultural adaptation is an open question.
In this project, we used interactions between Torresian crows and invasive cane toads in northern Australia as a model system to investigate the rapid emergence, spread and establishment of behavioural innovations via flexibility and social learning that could allow some native species to overcome and even thrive in the face of a novel invader.
Our project had three specific objectives:
(1) Quantify the sequence of population changes that occur across the toad invasion front in the Torresian crow. Does a highly intelligent predator with social learning, such as the crows, experience population crashes followed by rapid recovery? How rapidly do crow populations recover?
(2) Study the emergence and spread of behavioural innovations that enable crows to avoid lethal poisoning and potentially utilise toxic prey as a food resource. How quickly do adaptive innovations emerge and spread?
(3) Determine the role of social learning in the emergence and spread of adaptive innovations. Does social learning facilitate faster spread of innovations?
Key Findings:
Our data revealed that crow populations typically decline in the first year following cane toad arrival but show signs of recovery within the subsequent year, suggesting they are initially vulnerable to toxic poisoning but have mechanisms for rapid adaptation to the toxic invader.
Our experiments involving captive crows demonstrated that toad-naïve crows can rapidly learn to safely interact with toxic prey in at least some individuals through behavioural flexibility, suggesting this as a likely mechanism for rapidly adapting to toads. Toad-exposed crows in areas ~10 years since toad invasion showed rapid and more consistent safe handling behaviours, suggesting these innovations have established in crow populations in relatively short timeframes. Finally, preliminary results from social learning experiments suggest crows exposed to demonstrator crows with knowledge of safe handling techniques, acquire those safe handling behaviours more rapidly than those without, suggesting social transmission plays a key role in accelerating the spread of adaptive behaviours within populations.
Together, our findings suggest that behavioural flexibility and social learning are probable mechanisms for enabling some culturally complex, more intelligent native species to rapidly adjust and overcome sudden environmental change, such as invasive species. Conclusion and Significance: Our research contributes to our understanding of ways in which animals respond to biological invasions in short timeframes and offers insights for conservation strategies aimed at enhancing resilience in native populations.