Release protocols and release environment: what influences reintroduction success for the brushtail possum?

Abstract

This thesis examines the relative influence of release protocols and the release environment on short-term establishment and long-term persistence in reintroduction programs, using a brushtail possum reintroduction in semi-arid South Australia as a case study. I present a comparison of three release treatments, as release methods are known to influence post-release parameters but vary greatly by species and other factors such as source and release environment. The survival, post-release dispersal, change in body mass and reproductive condition of possums in each treatment are compared and results highlight a mismatch between release protocols that may benefit a species and being able to deliver them effectively. I investigated whether exposure to predators was likely to influence the antipredator behaviour of possums, and whether this translated to a difference in post-release survival between predator-exposed and predator-naïve possums. Antipredator behaviour was assessed using a variety of methods. Predator-exposed possums exhibited heightened antipredator behaviour compared to predator-naïve possums in all tests. However, post-release survival of both source populations was high, suggesting that successful fox control may negate the need to source predator-exposed animals for release. With predation the leading cause of reintroduction failure in Australia, the successful control of foxes at the reintroduction site provided an opportunity to investigate the impact of habitat quality. Habitat quality was assessed via the availability and recruitment of hollow bearing trees, mid storey vegetation cover and known preferred food plants in arid systems. Effects of habitat quality were measured post-release. Body condition and mass was maintained or increased over the study period following an initial post-release drop, and reproduction was constant. Most mortalities were due to predation by feral cats. Results suggest that habitat quality varies according to the factors tested but that despite a history of degradation at the site, short term reintroduction success was achieved. However, longer term persistence may be compromised unless further habitat restoration occurs. I investigated the interaction between diet and time since release to understand acclimatisation patterns and likelihood of long–term persistence. Next-generation DNA sequencing was used to identify plant genera within possum scats. Vegetation surveys were conducted to measure plant availability. Diet changed significantly over time and suggested that acclimatisation periods revealed by diet may be longer than indicated by other commonly used measures. Results have implications for reintroductions and restoration. Finally, I documented the survival, movement and growth of juvenile possums, as an indicator of the feasibility of population growth and persistence. I found sex effects for movement and growth, which interacted with maternal effects. Dispersal involved multiple movement phases and was male-biased. Most mortalities were attributed to predation by feral cats, but were not high enough to arrest population growth. Dispersal behaviour is interpreted in light of other studies of mammalian dispersal, and the influence of sex, maternal behaviour and environmental conditions are discussed. Release environment was found to be more important for short-term reintroduction success for brushtail possums than release protocols, but its influence on long-term persistence should be carefully monitored over time and, particularly, through drought. This thesis includes three chapters that have been published (chapters 2, 3 & 6) and two chapters that have been submitted to journals (chapters 4 & 5). Formatting styles may thus vary slightly between chapters. A single reference list is provided at the end of the thesis.