Aspects of the ecology of Astelia australiana, Tall Astelia

Abstract

The conservation of threatened species requires an understanding of their ecology to identify the factors that are contributing to their decline and to assess their extinction risk. The objective of this thesis was to examine the ecology of Astelia australiana, a threatened herb endemic to Victoria, Australia to inform its conservation management. To achieve this objective, I examined how the distribution and abundance of A. australiana has changed over its entire known range in the 20- year period since its demographic monitoring program began. I also examined its habitat niche and regeneration ecology and response to potential threats including drought, wildfire, disease and low light availability. I then used this new understanding to develop a population viability model to explore the viability of A. australiana to current conditions, threats and limitations.

The key findings of my research include a better understanding of the regeneration ecology of A. australiana including that it involves both sexual and asexual reproduction which occur at the same time. This reproduction occurs once in an individual’s lifetime and results in the production of flowers and fruit and three clones. Previously A. australiana had been variously described as dioecious and gynodioecious; however, it is trioecious, having male, female and hermaphrodite flowers on different plants and male and hermaphrodite flowers on the same plants. Various native bird and mammal species are involved in the regeneration ecology of A. australiana as pollinators and dispersers of fruit. Reproduction of A. australiana requires a minimum amount of available light from canopy gaps in the rainforest understorey to occur. A. australiana has declined in abundance by 57% over the 20-year period. Herbivory, and disease have contributed to mortality of A. australiana over this period but do not appear to be the only factors contributing to the observed decline. Although, A. australiana was found to be highly sensitive to drought, multiple lines of evidence including: the pattern of decline and recruitment of A. australiana; comparison of the physiological response of A. australiana to drought in an experimental trial with physiology of individuals in the rainforest; examination of the microclimate of the sites; and assessment of the growth response using dendrochronological techniques, of the dominant tree in the cool temperate rainforest, Nothofagus cunninghamii (Myrtle Beech) to drought periods, all indicate that drought is not driving the observed decline as had been previously concluded by others. Wildfire also resulted in a loss of individuals in one site that was burnt during the study period however, decline from this site was not included in the 57% change in abundance total. Changes in forest structure due to growth in the canopy reduce the light availability in the understory which limits A. australiana recruitment and increases mortality and thus contributes to the species decline. Population viability analysis of A. australiana, predicted that the species populations are at a high risk of large decline (86%) across the species range over the next 50 years under current climate.

This research is significant for the conservation management of A. australiana as it highlights new threats to the species and removes some others that were thought to be key threats. It is also significant because the magnitude of the observed decline warrants an up listing of the conservation status of A. australiana from Vulnerable to Endangered at the State and National level. This research has already been used to inform conservation management of the species populations including: fencing, to prevent further decline in one site that had been heavily browsed and disturbed by an introduced herbivore; facilitating targeted surveys for additional sites; increasing its reproductive rate within four sites through manipulation of the light environment; establishing new populations through translocation of individuals; identifying suitable sites for further translocation; establishing the monitoring program at new sites; reviewing the ex situ storage of seed; guiding and prioritising future conservation management actions of A. australiana through modelling and expert advice; and raising the species profile to obtain funding for further research and management of the species.