Flexibility in antipredator behaviour of Tasmanian macropods to altered devil abundance

Trophic cascades have been occurring at an increased rate due to the global decline of top predators. Top predators are important in maintaining the biodiversity of ecosystems through their top-down influence on prey species, both consumptive via predation which affects population vital rates and behavioural in response to the risk of predation. Predator presence and density is known to have strong influence on prey behaviour and demographics. Antipredator, or risk-sensitive, behaviours of prey individuals reflect their perceived level of threat in the environment. Behavioural responses in prey to changes in predator abundance, either declines or increases, can happen after a relatively short periods exposure (weeks or months). However, the expression of such behaviours will vary between species and individuals, depending on factors such as ecological niche, body-size and age. The flexibility of prey behaviour can therefore indicate the perceived level of risk, predator-induced or otherwise, in the environment. A rare opportunity to study the simultaneous effects of top predator decline and increase is afforded by the natural decline of the Tasmanian devil (Sarcophilus harrisii) from a novel infectious disease across its distributional range in Tasmania, Australia, and an assisted translocation for conservation of the devil to an offshore island. Tasmanian devils have drastically declined in abundance in the past two decades due to the transmissible devil facial tumour disease, with some areas of the state reduced to only 5% of the original devil population. Devil decline has already begun to have an impact on the behaviour of prey species in the environment but is likely to cause greater effects to the ecosystem balance as the disease continues to spread across Tasmania. To provide a wild-living insurance population, in the event of extinction of the devil in the wild, a disease-free population of devils was introduced onto Maria Island, a historically devil-free island and National Park 5km off the east coast of Tasmania. To determine the influence of devil abundance on Tasmanian macropod antipredator behaviours, three types of antipredator behaviour were studied (vigilance behaviour, flight initiation distance and emergence time and distance from cover) following the loss and gain in abundance of devils. The three species studied, the Tasmanian pademelon (Thylogale billardierii), Bennett's wallaby (Macropus rufogriseus) and Forester kangaroo (Macropus giganteus tasmaniensis), are all susceptible to predation from Tasmanian devils, but due to species differences will express different levels of risk-sensitive behaviours. Macropod behaviours were compared at three sites with distinct devil abundances using both historic and novel data sets collected 'before' and 'after' changes in Tasmanian devil abundance. Top predator pressure proved to have little influence on the expression of antipredator behaviours in macropods. No universal changes were seen among all species in response to devil decline or increase over time. However, changes in the structural complexity of the environment (vegetation regrowth) had a strong influence on risk-sensitive behaviour, irrespective of devil abundance. Species ecology, body size (between species and within species) and age also influenced the expression of antipredator behaviours of macropods. Smaller prey animals (both due to species body size and age) are most vulnerable to predation. This vulnerability was reflected in their anti-predator behaviour which did not reflect changes in predator pressure. Larger species are more influenced by vegetation changes than predator abundance changes. These results illustrate the complex and multifaceted relationship between predator and prey and the need for more comprehensive studies on interspecies relationships to prevent further loss of top predators in the natural environment.