Fear for conservation: deterring an invasive nest predator

Abstract

Fear may affect predator-prey interactions by eliciting behavioural and physiological responses in prey. This has consequences for wider trophic interactions and ecosystem function. Amid a global species extinction crisis, driven partially by invasive species and compounded by the loss of apex predators, the application of fear as a management tool to improve conservation outcomes in disturbed landscapes is receiving growing interest. However, the influence of fear on prey is contested as it is rarely tested experimentally in field-based studies. Intense predation pressure from introduced sugar gliders (Petaurus breviceps) in Tasmania is the leading cause of population declines of the critically endangered swift parrot (Lathamus discolor), and they also affect other more common species. I experimentally simulated the presence of an apex predator (owls) in swift parrot breeding regions and analyzed: i) sugar glider nest predation in real and artificial nest experiments and ii) the body condition of the nestlings of an abundant species, the tree martin (subsp. Petrochelidon nigricans nigricans), as a proxy for the impact owl broadcasts have on nesting birds. I broadcast predatory owl calls for seven nights in the vicinity of cavity-nesting birds in an array of nest boxes in December 2018 and compared predation rates to a control group of nest boxes. Swift parrots were unable to be used in this study so I included other cavity-nesting bird species (tree martins, starlings Sturnus vulgaris and an owlet-nightjar Aegotheles cristatus) as proxies for swift parrots. Nest predation was not affected by owl broadcasts, indicating that even if a landscape of fear is induced, it does not affect sugar glider predatory behaviour. I replicated the experiment in a 14-day artificial nest experiment at two study sites in February 2019. Owl broadcasts resulted in a slightly lower predation probability (0.15 ± 0.09 SE) than control groups (0.30 ± 0.10). However, there was no difference between models, and ultimately, owl broadcasts did not affect patterns of predation. To ascertain whether owl broadcasts negatively impacted on nesting birds, I conducted a control-impact experiment on nesting tree martins on a sugar glider-free-island in December 2018 – January 2019. Nestling body condition was not negatively affected, signifying that if owl broadcasts stressed tree martins, this was not detectable in the body condition of their nestlings. This study indicates that owl call broadcasts do not deter sugar gliders from preying on bird nests, nor does it deter birds from nesting. Although sugar gliders responded with alarm calls to owl call broadcast, this did not affect predation rates on nests. Sugar glider denning and reproductive activity was observed to increase over the summer in nest boxes monitored in this study, and our results suggest that sugar glider mating behaviour in Tasmania is potentially connected to the seasonality of nesting birds as a food resource. Sugar glider predation on bird nests may be determined by unmeasured factors such as mating seasonality and territorial distribution, rather than fear of predation per se. My study suggests that auditory signaling of a predator may not always result in fear-induced predator avoidance by prey species. This suggests that more direct nest protection measures are necessary to reduce the impacts of sugar glider predation in Tasmania. My thesis contributes a field-based example to the landscape of fear literature and confirms the severe threat that sugar glider predation poses to cavity-nesting bird species in Tasmania. The importance of experimental, field-based studies that are hypothesis driven can provide evidence to inform best practice conservation. Show more