Interactions between native lizards and introduced mammals in New Zealand's dryland ecosystems

Introduced predators have had devastating impacts for native prey globally, contributing to high rates of species extinction and endangerment, particularly within island ecosystems. New Zealand is one of these island systems where native fauna, including a high diversity of endemic lizard species, are highly threatened. The heightened vulnerability of New Zealand’s fauna to introduced predators is often attributed to prey-naivety, which results from a lack of experience with mammalian predators over evolutionary time-scales. In addition to introduced predators, New Zealand’s native species are threatened by habitat degradation, introduced prey species, and synergies between these co-occurring threats. Effective conservation in heavily modified ecosystems with multiple invaders is an ongoing challenge globally, with management actions often having unintended consequences. Understanding the interactions between native lizards and introduced mammalian predators, is therefore essential to ensuring New Zealand’s biodiversity is conserved effectively and efficiently. This study focused on understanding the extent to which two key threats; (i) prey naivety and (ii) hyperpredation, contribute to the heightened vulnerability of lizards in New Zealand’s dry grassland habitats (drylands). This included assessing how current conservation management strategies, including (i) mammalian-predator exclusion and (ii) rabbit control, were impacting lizard populations. The first aim of the study is to understand the extent to which New Zealand lizards remain naïve of the threat posed by introduced mammalian predators. To do this we tested whether McCann’s skink (Oligosoma maccanni), a ‘non-threatened’ dryland lizard species, responded to cat cues by reducing basking frequency or increasing their preference for basking sites closer to refuge. We found that skinks from a population with a high-density of introduced mammals reduced basking frequency when exposed to cat cues, whereas skinks from a low-predator population, sourced from within a fenced reserve where mammalian predators had been excluded for 3-4 skink generations, did not. The low-predator population also failed to discriminate the cues of an avian predator, despite having evolutionary and ecological experience with this predatory threat. These results suggest that at least one species of New Zealand lizard has rapidly gained the ability to recognise the threat posed cats, a species introduced ~150 years ago. However, they also suggest that the behavioural response of skinks is flexible, and that predator avoidance behaviours can be rapidly lost when predation pressure is relaxed. The second aim is to better understand if and how an introduced prey species, in this case rabbits (Oryctolagus cuniculus), facilitates hyperpredation of native prey. Hyperpredation occurs when rabbits support higher densities of a shared generalist predator, which increases predation pressure on alternative prey species, particularly when rabbits become less abundant. Specifically, we are interested in whether rabbit control is an effective mechanism for reducing the impacts of ferret (Mustela furo) predation on native lizards and invertebrates in New Zealand’s drylands. To do this we analysed changes in ferret densities and per-capita dietary intake in response to a large-scale rabbit control operation, in the period up to 18 months post-control. Following rabbit control, ferret densities declined but remaining ferrets increased their per capita consumption of native prey. This resulted in an overall decrease in predation pressure for invertebrates, but an increase in predation pressure for lizards. These results indicate that reducing rabbits by over 80% in this system fails to protect lizards in the short-term period following rabbit-control, and that lizard conservation would benefit from the simultaneous control of predators. This research demonstrates that prey-naivety is dynamic, and that evolutionary isolation from mammalian predators does not prevent New Zealand’s lizards from recognising and attempting to avoid the risk associated with novel predator cues. This research also highlights how established rabbit populations and short-term fluctuations in rabbit abundance, increase native species’ vulnerability to introduced predators. Finally, this research demonstrates that current management strategies aimed at protecting vulnerable native species, can have unintended consequences. This includes increasing the naivety of prey through conservation fencing and predator exclusion, as well as increasing the predation pressure on native prey species through suppressing introduced prey populations. To understand the vulnerability of native species, and ensure to conservation actions are effective, research must continue to unravel the spatial and temporal complexity of species interactions within invaded ecosystems