Modelling landscape distributions of large forest owls as applied to managing forests in north-east Victoria, Australia
Introduction
Large forest owls pose special challenges to forest managers because they are top predators, with large home ranges and complex habitat requirements. Some owl species may need extensive areas of old forest within their home range, as reported for sooty owls (Tyto tenebricosa) in Australia (Milledge et al., 1991) and northern spotted owls (Strix occidentalis caurina) in North America (Bart and Forsman, 1992, Marcot and Thomas, 1997). In Australia, all large forest owls need tree hollows for nesting and some species also need them for roosting (Schodde and Mason, 1980, Hollands, 1991, Higgens, 1999). Arboreal mammals form a high percentage of their prey (Higgins, 1999), and most of those mammals depend on tree hollows for daytime shelter (Strahan, 1983, Menkhorst, 1995). Large hollows suitable for these species do not form until trees are very old (hundreds of years; Ambrose, 1982, Mawson and Long, 1994). Large trees tend to contain many more hollows than small trees (Bennett et al., 1991, Soderquist, 1999).
Commercially attractive rotations for timber production are usually shorter than the time needed to develop hollows. Three primary strategies are available to conserve hollow-dependent fauna in forests used for wood production (Loyn, 1985a), of which selected stand retention (and replacement over time) is the most popular and generally the most effective. The other primary strategies are extended rotations or tree retention on coupes, with minimum levels of tree retention specified under a Code of Forest Practices (Department of Natural Resources & Environment, 1996). The stand retention strategy is particularly effective if stands are selected on the basis of their value for hollow-dependent fauna such as large forest owls. To this end, a programme was initiated in Victoria to help select valuable habitat for large forest owls and reserve it in a system of reserves and Special Protection Zones (SPZs). This was done as part of forest planning processes and establishment of Regional Forest Agreements between state and commonwealth governments (Victorian RFA Steering Committee, 1998), following a national forest policy initiative (Commonwealth of Australia, 1992). All four large owl species in Victoria are listed as endangered (barking owl, Ninox connivens; powerful owl, Ninox strenua; and masked owl, Tyto novaehollandiae) or vulnerable (sooty owl, T. tenebricosa) in the state of Victoria (Department of Natural Resources & Environment, 1999a).
This work was conducted in north-east Victoria, south-eastern Australia (Fig. 1). The region contains 1.2 million ha of forest on the inland (northern) slopes of the Great Dividing Range, in the upper catchment of the Murray River. It includes the highest mountain in Victoria (Mt. Bogong, 1986 m above sea level) and mountainous ranges falling to plains 30 m above sea level. Rainfall varies from about 500 mm in the lowlands to over 1500 mm in the mountains. Climate is generally temperate and strongly seasonal, with warm to hot summers and cool to cold winters. The lowlands have been extensively cleared for agriculture including high-value fruit and wine production. High mountains are popular for skiing in the winter. Forests form a continuous belt between the high Alps and mainly cleared valleys below, including much remote and rugged country. Timber production is economically important, especially from tall forests with high rainfall. The forests provide essential habitat for wildlife including owls.
Wildlife studies have been undertaken on a broad scale in the 1970s and 1980s (Bennett et al., 1991, Lumsden et al., 1991) and more locally at other times, but usually did not involve targeted surveys for owls. Generally, previous wildlife information is biased towards the more accessible country near the edge of cleared land (Emison et al., 1987, Menkhorst, 1995, Traill et al., 1996). Owls pose challenges for research and survey, because they are nocturnal and sparsely distributed in the forest landscape. Recent technical advances in systems of call playback have opened up new opportunities to improve this knowledge (Kavanagh and Peake, 1993, Debus, 1995).
Section snippets
Field methods
Field teams were assembled with a charter to focus on nocturnal fieldwork. Surveys were conducted at 472 sites in north-east Victoria from April to June 1996, using call playback and standard spotlighting as the main methods (see below). Two other methods were used at subsets of these sites: dusk-watch at 72 sites and a period of quiet initial listening before playback at 459 sites. Observers worked independently (each with a 4WD vehicle) but pairs of observers worked in the same general area
Mapped variables
Geographical Information Systems (GIS) were used to assign characteristics to each site, from mapped information held in the NRE corporate database. The principal characteristics considered were EVC (see above) and forest growth-stage determined by a current Statewide Forest Resource Inventory (Department of Natural Resources & Environment, 1999b). Because forest owls have large home ranges, these characteristics were also considered within selected areas around each survey site. Radii of 500
Results
Six owl species and 21 nocturnal mammal species were recorded during the field survey. Modelling focused on two large owl species which proved to be widely though sparsely distributed in the region. Powerful owl and sooty owl were recorded at 65 and 18 sites respectively. Barking owls (N. connivens) and masked owls (T. novaehollandiae) were rare and confined to distinct parts of the region (barking owls at 12 sites in dry forest or woodland in the northern lowlands; masked owls south of the
Limitations and strengths of the study
With sets of complex data, it is often possible to develop many different models with varying degrees of realism. Hence, we should not expect predictions to be perfect representations of reality but this should not deter us from developing simple models to solve specific problems (Starfield, 1997). In this study the issue was to identify, locate and conserve high-quality habitat for large forest owls. Field testing showed that the models discriminated between sites of high and low probability
Acknowledgements
We would like to thank all who contributed to this work in various ways, and especially the field teams who spent long hours at night in difficult country. Fieldwork was conducted mainly by Ed McNabb, Maria Belvedere, Dale Gibbons, Phil Maher, Chris Tzaros and Rick Webster. Charles Silveira helped conduct fieldwork to field test the models. Kylie White made expert use of the models in the course of her work in devising forest management plans and defining Special Protection Zones. Regional
References (57)
- et al.
Dependence of northern spotted owls Strix occidentalis caurina on old growth forests in the western USA
Biological Conservation
(1992) - et al.
The integration of field survey and remote sensing for biodiversity assessmenta case study in the tropical forests and wetlands of Sango Bay, Uganda
Biological Conservation
(1998) - et al.
Distribution of nocturnal forest birds and mammals in relation to the logging mosaic in South-Eastern New South Wales, Australia
Biological Conservation
(1995) - et al.
The conservation of arboreal marsupials in the montane ash forests of the Central Highlands of Victoria, south-east Australia. 1. Factors affecting the occupancy of trees with hollows
Biological Conservation
(1990) Flagships, umbrellas and keystones: is single species management passe in the landscape era?
Biological Conservation
(1998)- et al.
Forest succession, timber production and conservation of Leadbeater's Possum (Gymnobelideus leadbeateri MarsupialiaPetauridae)
Forest Ecology & Management
(1992) - Ambrose, G.J., 1982. An ecological and behavioural study of vertebrates using hollows in eucalypt branches. PhD thesis,...
The development of even-aged stands in Eucalyptus regnans F. Muell. in central Victoria
Aust. J. Bot.
(1976)- et al.
Habitat use by arboreal mammals along an environmental gradient in North Eastern Victoria
Wildlife Research
(1991) - et al.
Studies on the arboreal marsupial fauna of eucalypt forests being harvested for woodpulp at Eden, NSW. 3. Relationships between faunal densities, eucalypt occurrence and foliage nutrients, and soil parent materials
Australian Wildlife Research
(1984)