University Of Tasmania
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Investigating the conservation requirements of the endangered Tasmanian wedge-tailed eagle (Aquila audax fleayi)

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posted on 2023-05-28, 00:30 authored by James PayJames Pay
Predators play a key role in maintaining ecosystem integrity in terms of ecosystem functions, long term ecological stability and even species composition. Raptors are important predators in many ecosystems, yet they are disproportionately at risk of extinction. Threats to these species include habitat loss and degradation, illegal persecution, unintentional poisoning and collisions with anthropogenic structures such as powerlines and wind turbines. The impacts of each threat will vary between populations due to inter- and intra-specific differences in behaviour as well as regional differences in landscapes and anthropogenic modification. An understanding of both the behavioural ecology and the specific combination and/or extent of threats faced by a raptor species is therefore crucial to the planning of effective in situ conservation management. The overarching goal of the research described in this thesis was to inform the conservation management of the Tasmanian wedge-tailed eagle (Aquila audax fleayi), an endangered subspecies endemic to the island of Tasmania. Prior to this study, conservation efforts had focussed on the threats of breeding habitat loss and disturbance to nesting eagles, but little is known about the habitat requirements of other life stages, or the extent of other threats that may impact survival. Addressing these knowledge gaps will be important for future successful conservation management. Within this overall goal, the first aim, addressed in Chapter 2, was to identify the extent of lead exposure in Tasmanian wedge-tailed eagles and explore patterns in exposure detected. Avian predators and scavengers are known to be susceptible to lead poisoning through the ingestion of lead-based bullet fragments in shot animals. Herbivores are shot widely in Australia, including over one million macropods annually just in Tasmania, yet lead exposure is not presently recognised as a major threat to any Australian predator. I detected elevated lead levels in 10.4% of 106 liver samples and 3.7% of 108 femur samples opportunistically collected from dead wedge-tailed eagles across the island. I also detected lead in 95.9% of blood samples taken from 24 eagle chicks captured at the nest, 8.3% of which had levels indicative of potential clinical poisoning. Femur sample lead levels were higher in adults than in immature birds, suggesting chronic exposure. Isotopic similarities found between lead from these samples and lead from ammunition supported my interpretation that lead ammunition may be a significant source of exposure. My results indicate that lead exposure is an overlooked threat to the Tasmanian wedge-tailed eagle and highlight the need for regulation of lead-based ammunitions and use of lead-free alternatives. In Chapter 3 I tested the extent to which Tasmanian wedge-tailed eagles are exposed to anticoagulant rodenticides, and investigated which factors influence exposure within the population. Anticoagulant rodenticides are used worldwide to control rodent pests, but non-target exposure to raptors is known to occur through predation of poisoned animals. In Australia, many anticoagulant rodenticides are unrestrictedly available for private and commercial use, but there has been little work evaluating the impacts of anticoagulant rodenticides on native predatory species. Anticoagulant rodenticide residues were detected in 74% of 50 dead eagles, with 16% having liver concentrations that were lethal. Anticoagulant rodenticide levels and probability of exposure increased with proportion of agricultural habitat and human population density in the estimated home range. I identify anticoagulant rodenticide exposure as a previously unrecognised threat to the Tasmanian wedge-tailed eagle population, and indicate a need to review the current registration of these compounds for residential and agricultural use. The third aim, addressed in Chapter 4, was to provide a detailed understanding of juvenile Tasmanian wedge-tailed eagle behaviour and habitat use through GPS-tracking of 25 birds. The pre-dispersal period (147-575 days) was much longer than previously predicted. The birds exhibited three distinct behavioural states: perching, short flights and long flights. During all behaviours, the eagles selected for areas less than 75 m from forest edges and with topographic slopes over 15vÄv¿. Long flights occurred more frequently over forests, whilst open habitats were generally avoided regardless of behaviour type. Cross-validated habitat selection models performed well in predicting the areas where short flights and long flights occur in Tasmania. These models can be developed into management tools, to estimate impacts of land-use changes and predict high risk areas for collisions with powerlines and wind turbines on a life stage about which very little was previously known. In Chapter 5, I describe the development of a morphological method to sex Tasmanian wedge-tailed eagles. An individual's behaviour, and thus the impacts of different threats to it, are likely to differ between sexes; the demographic consequences of these threats are therefore likely to depend on the population sex ratio. Yet molecular analyses are costly, and there is currently no validated method to sex Tasmanian wedge-tailed eagles through morphology. Using my method 100 free-flying birds could be sexed with 97.6% accuracy using forearm, tarsus width and hallux length, whilst 25 late-stage nestlings could be sexed with 95.4% accuracy using hallux width, hallux breadth and tarsus breadth. I present the corresponding equations as tools for use in future research and management of the subspecies. In this thesis, I took a multi-dimensional approach to the conservation ecology of the Tasmanian wedge-tailed eagle. Combined, this research provides new guidance and tools for the conservation of the population. I have identified previously unknown threats which may also apply to other species both in Tasmania and more widely in Australia. In addition, I have developed novel habitat selection models for different flight behaviours, and a low-cost morphological tool for identifying sex. These tools will assist managers to better predict, understand and mitigate the effects of unnatural mortalities on this endangered raptor population. Future research priorities to build on this work include the incorporation of my findings into a population model and the development of new techniques to fill life-history knowledge gaps still present. Such a model will guide, and support conservation managers needing to plan, assess and regulate the impacts of proposed new human activities on the population.


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