Elsevier

Biological Conservation

Volume 142, Issue 11, November 2009, Pages 2438-2448
Biological Conservation

Modelling human impacts on the Tasmanian wedge-tailed eagle (Aquila audax fleayi)

https://doi.org/10.1016/j.biocon.2009.05.010Get rights and content

Abstract

The wedge-tailed eagle is Australia’s largest bird of prey and one of the largest eagles in the world. Aquila audax fleayi is an endemic Tasmanian subspecies isolated for 10,000 years from the nominate subspecies on the Australian mainland. The Tasmanian wedge-tailed eagle is classified nationally and at a State level as endangered due to its small number of breeding pairs, low breeding success and high rate of mortality from unnatural causes. The subspecies experiences mortality throughout its range from shooting, poisoning, trapping, road accidents, electrocutions and collisions with wind turbines, aircraft, fences and overhead wires, which we term ‘un-natural mortality’. A portion of the subspecies’ range is managed for timber production, which can lead to disturbance of nest sites and the loss of nest trees. We use a model of the eagle population from the Bass District in northeast Tasmania to explore the relative importance of different sources of mortality and nesting habitat loss, and the potential for mitigating impacts associated with unnatural mortality, disturbance, nesting habitat loss and human access to forests. We create a habitat map including suitable nest sites and link it to a dynamic landscape population model based on life history traits and disturbance responses. Using the program RAMAS-Landscape, we model alternative forest management scenarios, ranging from no timber harvesting and a natural wildfire regime, to scenarios prescribing native forest harvesting and regeneration and different levels of conversion of native forest to plantation under the same natural wildfire regime. The results indicate that the Tasmanian wedge-tailed eagle is sensitive to unnatural mortality, plantation establishment and native forest harvesting. The predicted decline over the next 160 years (∼65%) will most likely be driven largely by loss of current and potential future nest sites associated with harvesting activities, exacerbated by unnatural mortality in the wider landscape. Interventions that minimise unnatural mortality, reduce nest disturbance, and retain breeding habitat and nest sites may improve the prospects for the subspecies in the Bass District. If nest disturbance and unnatural mortality continue at the rates modelled here, the species appears to face a high risk of declining substantially in the region.

Introduction

The Tasmanian regional forest agreement (RFA) attempted to balance the interests of commercial production forestry with environmental concerns, including biodiversity conservation (Commonwealth of Australia and State of Tasmania, 1997, Slee, 2001). The objective was to establish a comprehensive, adequate and representative system of forest reserves and to promote ecologically sustainable forest management, while providing resource security to the timber industry. The RFA allocated land to management zones with different priorities that included reservation, forest protection, logging and re-establishment of native forest, and conversion to plantation.

Approximately 65,000 ha of Tasmania’s native forest were cleared for plantation establishment (“plantation conversion”) between 1999 and 2006 (Green, 2004, Resource Planning and Development Commission, 2002, Forest Practices Authority, 2006), representing just over half of the total loss of native forest since 1996 (Forest Practices Authority, 2006). The implementation of the RFA since 1997 led to debate about the interpretation of criteria for conservation (JANIS, 1997, Kirkpatrick, 1998) and the effectiveness of conservation measures (Bonham et al., 2002, Taylor et al., 2003). In signing the Tasmanian RFA agreement, the Federal Government recommended that the consequences for sensitive species of conversion of native forest to plantation be quantified (Commonwealth of Australia and State of Tasmania, 1997). This study is part of a larger project that explored the relationships between predicted population size, extinction risk, and the amount and spatial configuration of habitat for 11 forest-dependent, rare, threatened or sensitive species from north-eastern Tasmania. The study used population viability analysis (PVA) to quantify the risks faced by species from fire, native forest harvesting, plantation conversion and other disturbances (Fox et al., 2004a).

This paper presents the PVA for the Tasmanian subspecies (Aquila audax fleayi) of the wedge-tailed eagle (A. audax), which is classified nationally and at a State level as endangered (Environment Protection and Biodiversity Conservation Act, 1999; Tasmanian Threatened Species Protection Act, 1995). Beyond natural fatalities, the subspecies experiences mortality from shooting and illegal poisoning, accidents involving electrocution and collision with vehicles, aircraft, wind turbines, overhead wires and fences and it is sensitive to various types of human disturbance including some forestry activities. Habitat changes due to intensification of forest management can have important detrimental impacts on breeding (e.g., Mooney, 1997, Mooney, 2005). The model was therefore constructed to test the consequences of human impacts including different forestry scenarios on the persistence and predicted population size of the subspecies in the Bass District of northeast Tasmania.

Section snippets

Study area

The study area in northeast Tasmania, within Forestry Tasmania’s Bass District (Fig. 1), comprises approximately 1 million hectares (ha), 61% of which in 2001 was native forest or existing plantations on public and private lands. The Bass District ranges in elevation from the coast to the northeast highlands, dominated by Ben Lomond (1572 m above sea level) in the southwest corner of the district. The average annual rainfall ranges from 500 mm to 1800 mm with temperatures ranging from an average

Changes in carrying capacity

The availability of nesting habitat for the Tasmanian wedge-tailed eagle is expressed in changes in carrying capacity K in the study region over 160 years. The predicted carrying capacity was highest in the model that excludes harvesting, remaining close to the initial value of 142 breeding adults (which it was not allowed to exceed), and reduced under all other scenarios to approximately half this value (Fig. 4).

The initial gradual decline in K in Scenario 1 is a result of the bimodal age-class

Discussion

The results and sensitivity analyses indicate that the Tasmanian wedge-tailed eagle is sensitive to plantation establishment and native forest harvesting, resulting in an expectation of a roughly 70% probability of a 75% decline at least once during a 160 year period when all factors are included in the model (Fig. 6). Even in the absence of 1080 poisoning, nest disturbance and mortality associated with improved access (that is, even if we assume these sources of mortality are eliminated

Acknowledgements

This project was supported by an ARC Linkage Grant, funded by the Australian Commonwealth Government, with Forestry Tasmania as the industry partner. Wintle was supported by ARC Project LP0347473 to Burgman and DP0774288. Bekessy was also supported by ARC Project LP0454979. H.R. Akçakaya and M. McCarthy provided technical advice on landscape and population modelling methods. S.J. Grove and M.J. Brown provided useful input throughout this project.

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