Yellow-bellied glider (Petaurus australis) population on the Bago Plateau - endangered population listing

The Scientific Committee, established by the Threatened Species Conservation Act, has made a Final Determination to list a population of the Yellow-bellied Glider Petaurus australis Shaw 1791 on the Bago Plateau as an ENDANGERED POPULATION in Part 2 of Schedule 1 of the Act. Listing of endangered populations is provided for by Part 2 of the Act.

This determination has been superseded by the 2014 Determination (endangered population listing revision).

NSW Scientific Committee - final determination

The Scientific Committee has found that:

1. The Yellow-bellied Glider Petaurus australis Shaw 1791 (family Petauridae) is not currently listed as an Endangered Species in Part 1 of Schedule 1 or a Critically Endangered Species in Part 1 of Schedule 1A and as a consequence populations of this species are eligible to be listed as endangered populations.

2. The Yellow-bellied Glider is a relatively large (450–700 g) arboreal marsupial distinguished by its whitish to yellow fur on the belly, long fluffy tail, large bare ears and unusual loud vocalisations (Russell 1995). Yellow-bellied Gliders feed predominately on eucalypt sap, which is acquired by incising V-shaped grooves in the trunk of eucalypt trees. An individual glider feeds from only a small number of selected trees. The diet also includes nectar, arthropods and insect exudates (Goldingay and Jackson 2004). Yellow-bellied Gliders spend a large proportion of their nocturnal active period foraging or travelling (Goldingay 1990). They are strictly arboreal, gliding 100 m or more between tall trees. During the day, Yellow-bellied Gliders shelter in tree hollows, either alone or with a mate and their young. A number of den trees are used over time (Craig 1985; Brown et al. 2007). In the Victorian central highlands, dens were located in  Eucalyptus cypellocarpa (Mountain Grey Gum) that ranged in size from 0.9–2.2 m (average: 1.5 m;  n = 8) diameter at breast height (DBH) and in  E. obliqua (Messmate) from 1.3–2.2 (1.6 m;  n = 7) DBH (Craig 1985). Yellow-bellied Gliders live in small groups (2–6 individuals) often consisting of a single family group, although polygamous groups also occur (Craig 1985; Goldingay and Jackson 2004; Brown  et al. 2007). These social groups occupy exclusive territories of 25 to 84 ha in New South Wales (Goldingay and Jackson 2004). Individuals live for up to 6 years. A single young is born to each breeding female in most years, but females forego reproduction during years of adverse environmental conditions (Craig 1985; Goldingay 1992).

3. The Yellow-bellied Glider inhabits mixed eucalypt forests along the escarpment and coastal plain of eastern mainland Australia. An isolated population occurs in northern Queensland that is separated by the Burdekin Gap from a population occurring south from around Mackay to Victoria (Brown et. al 2006). In New South Wales, there are several isolated occurrences west of the Great Divide on the Northern, Central and Southern Tablelands (Atlas of NSW Wildlife).

4. A population of the Yellow-bellied Glider occurs on the Bago Plateau – a westward extension of the Kosciuszko highlands in southern New South Wales (Kavanagh and Stanton 1998; Atlas of NSW Wildlife). The population is disjunct owing to the steep valleys and unsuitable habitat surrounding the Bago Plateau and, in addition, because of cleared agricultural land to the west and the Tumut River and Talbingo Reservoir to the east. The area of the population includes a large portion of Bago and Maragle State Forests, a small area of Kosciuszko National Park and some freehold land. For the purposes of this Determination, the Bago Plateau population is defined to occur above the 900 m ASL elevation contour and north of a line coinciding with the southern boundary of Maragle State Forest (i.e. north of -35.93° S). Survey records indicate that a small population of Yellow-bellied Gliders exists 35 km to the south of Bago Plateau, and a low density population may extend south from the plateau to this point.

5. A habitat model produced for the Southern Comprehensive Regional Assessment estimates the Bago Plateau contains approximately 27 550 ha of high quality habitat and 20 270 ha of medium quality habitat for the Yellow-bellied Glider (NPWS 2000). This habitat consists of tall wet sclerophyll forest dominated by Eucalyptus delegatensis (Alpine Ash),  E. dalrympleana (Mountain Gum),  E. radiata (Narrow-leaved Peppermint) and  E. rubida (Candlebark) (Kavanagh and Stanton 1998; NPWS 2000).  E. dalrympleana is the most important sap-feed eucalypt for gliders in the area, with  E. fastigata,  E. ovata and  E. viminalis also used where present (NPWS 2003). The Bago Plateau contains a sufficiently large area of potential habitat ( c. 48 000 ha) to sustain a viable population of Yellow-bellied Gliders (Goldingay and Possingham 1995).

6. A large proportion of this potential habitat on the Bago Plateau occurs in Bago and Maragle State Forests and is subject to timber harvesting operations, which are particularly intensive in the high elevation forests that are also high quality habitat for the Yellow-bellied Glider (Kavanagh and Stanton 1998; NPWS 2000; IFOA Southern Region 2002). The production of high-quality sawlogs from the Tumut Subregion was expected to increase by one third above 2002/3 levels to meet the maximum levels allowed under the Regional Forest Agreement for Southern New South Wales (2001; URS 2004). The area harvested per year in the Tumut Subregion is variable, but has increased from an average of 417 ha/year during 1998-2001 to 596 ha/year during 2002-2006 (Forests NSW in litt. January  2008). Timber harvesting operations in Bago and Maragle State Forests involve the removal of logs either by a process of ‘single tree selection’ or ‘Australian group selection’, which involves the clear-felling of numerous small (less than 0.5 ha) areas, silvicultural thinning, and in some cases, post-harvest burning. Australian group selection is promoted in areas dominated by mature or ‘over-mature’ trees in order to maximise subsequent regeneration (Forests NSW 2003). Silvicultural prescriptions in harvest plans stipulate the removal of ‘defective and over-mature’, ‘larger mature’ and ‘lower quality’ trees to promote the growth of merchantable trees for the next harvesting operation (in approximately 20-25 years; Forests NSW 2006). Harvesting operations are also required to implement a number of general and species-specific measures prescribed under terms of the Threatened Species Licence for the Tumut Subregion of the Southern Region (IFOA Southern Region 2002). General measures include retention of a minimum of 10 hollow-bearing trees and 10 recruit trees per ha, and harvest exclusion zones along all mapped watercourses and some ridges and headwaters. All recently used (within 2 years) sap feed trees identified during pre-logging surveys and 15 potential feed trees within 100 m of sap feed trees are excluded from logging. Further areas are excluded because of prescriptions for other threatened species and to prevent soil erosion. There are currently no studies by which to determine whether these prescriptions are sufficient to conserve populations of the Yellow-bellied Glider in areas subject to repeated and widespread timber harvesting operations, including on the Bago Plateau.

7. The family social structure, territoriality, specialist foraging behaviour and low fecundity of Yellow-bellied Gliders places viable populations at risk from relatively minor changes in environmental conditions. A population viability analysis estimated that 150 glider groups were needed to achieve a probability of extinction less than 0.05 (Goldingay and Possingham 1995). However, population viability was sensitive to environmental variability that affects breeding success and particularly to small changes in adult mortality. For example, with an increase in the rate of adult mortality from 0.20 to 0.22, the probability of extinction of a population of 150 groups increased from 0.05 to approximately 0.40 (Goldingay and Possingham 1995). The population viability analysis of Goldingay and Possingham (1995) was based on data from populations occupying largely undisturbed forests. Although the model did incorporate an estimate of natural variation in environmental conditions, it did not explicitly account for additional threats, such as from timber harvesting operations. A viable population on the Bago Plateau is therefore expected to require somewhat more than 150 glider groups.

8. The estimated area required to support a viable population of glider groups must account for their often patchy distribution even in continuous forest (Goldingay and Possingham 1995). On the south-west slopes, Kavanagh and Stanton (1998) detected Yellow-bellied Gliders at 29% of 138 sites above 900 m elevation. At this rate of occupancy, gliders are estimated to occupy approximately 13 920 ha of the 48 000 ha of mapped habitat on the Bago Plateau. Depending on variation in territory size between 25 and 84 ha, this occupied habitat could support between 557 and 165 glider groups. Extrapolating from previous studies of Yellow-bellied Gliders, the mean home range size on the Bago Plateau is probably about 60 ha (e.g. Goldingay and Kavanagh 1993; R. Goldingay pers. comm. 4/08.), which suggests approximately 232 glider groups are present. These groups may require larger home ranges if there is a reduction in habitat quality, resulting in a lower density of occupied territories.

9. Numerous studies have found a positive association between the occurrence of Yellow-bellied Gliders and unlogged mature forest or forest containing a greater abundance of large hollow-bearing trees (Milledge et al. 1991; Smith  et al. 1994, 1995; Lindenmayer  et al. 1999; Incoll  et al. 2001; Alexander  et al. 2002; McAlpine and Eyre 2002; Wormington  et al. 2002; Kavanagh and Stanton 2005; Eyre 2007). Yellow-bellied Gliders are often absent from recently logged forests (Smith  et al. 1994). Some studies and anecdotal observations show that Yellow-bellied Gliders can persist in selectively logged forests and make use of corridors of retained habitat (Goldingay and Kavanagh 1991, 1993; Kavanagh and Bamkin 1995; Kavanagh  et al. 1995). Timber harvesting operations (logging, culling and burning) reduce habitat quality and occupancy and cause some level of population decline; however, logging intensity appears important in determining the persistence of Yellow-bellied Gliders. The threat to the Bago Plateau population is exacerbated because timber harvest operations affect a large proportion of available habitat, with very little area of unlogged mature forest and no area conserved in perpetuity. The potential for logging operations to reduce habitat quality, thus increasing home range size and decreasing population density, makes the species more susceptible to stochastic events.

10. Large mature trees provide a greater abundance of den hollows and are preferentially selected as sap feed trees by Yellow-bellied gliders (Craig 1985; Milledge et al. 1991; Goldingay 2000, Eyre and Goldingay 2005). In a wide scale survey of sap trees in south east Queensland, 43% of sap trees were in the size class 41–60 cm DBH, yet these comprised just 7 % of available trees, and trees in the largest size class were most preferred (Eyre and Goldingay 2005). Sap feed trees in some areas are primarily located mid-slope and on ridges, with few located in gullies (Goldingay 2000). This has implications for the value of harvest exclusion zones along watercourses. Although sap trees are marked for retention, pre-logging survey time is limited to 4 person-hours per 200 ha (IFOA Southern Region 2002) and some sap trees are likely to be missed. Nectar is also an important foraging resource that may be reduced in logged forests. Logging reduces the canopy area of medium to large trees, which flower more frequently than small regrowth trees (Law  et al. 2000). Large trees also shed more bark and provide a more productive foraging resource for invertebrates than smaller trees (Recher 1996; Pook  et al. 1997). Mature ‘old-growth’ forests may also provide other characteristics that make this habitat more suitable for Yellow-bellied Gliders (Scotts 1991; Recher 1996).

11. The Bago Plateau is bisected by several 80-100 m wide easement corridors that are cleared of trees to allow for overhead electricity transmission lines. These corridors of cleared land may hinder the movement of gliders, reducing the connectivity of populations occurring on either side of the electricity transmission lines. Partial fragmentation of the Bago Plateau population decreases its viability.

12. The Bago Plateau population is threatened because of its highly restricted geographic distribution. This makes a large proportion of the population vulnerable to a single large-scale catastrophic event, such as wildfire or disease. Furthermore, the population faces a continuing decline in habitat quality, primarily because of timber harvesting operations.

13. The population of the Yellow-bellied Glider Petaurus australis Shaw 1791 on the Bago Plateau is eligible to be listed as an endangered population as, in the opinion of the Scientific Committee it is facing a very high risk of extinction in New South Wales in the near future as determined in accordance with the following criteria as prescribed by the  Threatened Species Conservation Regulation 2002:

Clause 19

The population is facing a very high risk of extinction in New South Wales in the near future as, in the opinion of the Scientific Committee, it satisfies the following paragraph and also meets the criteria specified in the following clause:

(a) it is disjunct or near the limit of its geographic range.

Clause 21

The geographic distribution of the population is estimated or inferred to be highly restricted and:

(a) a projected or continuing decline is observed, estimated or inferred in:

(ii) geographic distribution, habitat quality or diversity, or genetic diversity

Professor Lesley Hughes
Chairperson
Scientific Committee

Proposed Gazettal date: 21/11/08
Exhibition period: 21/11/08 – 23/01/09

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