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National Parks and Wildlife SA
In partnership with Threatened Plant Action Group, February 2003
Note: This publication has been superseded by the Recovery Plan for Twelve Threatened Orchids in the Lofty Block Region of South Australia 2010
The larger subpopulations of A. woolcockiorum discovered recently have been in loamy soils near gullies in Sugar Gum ± Long-leaf Box ± SA Blue Gum (Eucalyptus cladocalyx ±E. goniocalyx ±E. leucoxylon) open forest or woodland (see the locations west and north-west of The Linklands in Figure 1), with Golden Wattle / Curry Bush (Acacia pycnantha / Cassinia laevis) overstorey and Guinea Flower, Mat-rush and Native Flax-lily (Hibbertia exutiacies, Lomandra densiflora and Dianella revoluta respectively) (see Appendix). In this habitat the Arachnorchis woolcockiorum are on gentle or moderate slopes, in or near clearings with herbaceous plants such as Acaena spp., Bulbine bulbosa, Caesia vittata, Plantago spp. and Wurmbea spp. It is thought that spider orchids benefit from more open habitat because of the extra light, reduced competition and the suitability of conditions for pollinators (Bates, pers. comm.).
Historically Arachnorchis woolcockiorum was found “in sparse forests on rock ledges of difficult access and rocky slopes in shallow soil” (Jones, 1991). The type location for the species is the mossy ledges of Upper Mambray Creek. These ledges are typically in Eucalyptus microcarpa grassy woodland with Acacia pycnantha, Bursaria spinosa and Cassinia laevis. The soils are shallow dark loams with sandstone gravel (Threatened Plant Population Database [TPPD]). Many of these ledges have become overgrown with weeds in recent years, however the sites remain potential habitat.
Adelaide‘s Plant Diversity Centre also has collections of Caladenia woolcockiorum from near Mt Remarkable summit, 3kms west of Melrose (see Figure 1). The plant communities in these areas are similar to those associated with the larger recently discovered subpopulations, although the slopes are steeper and the soils are shallower and stonier. The slopes of Mount Remarkable summit and Upper Mambray Creek were searched in 1999 but no A. woolcockiorum were discovered.
The scattered individuals found near Blue Gum Flat and the Eaglehawk Dam track (2 – 3km west of the main subpopulations) are in clay loam on the edge of clearings, in open Blue Gum / Drooping Sheoak ( Eucalyptus leucoxylon / Allocasuarina verticillata) woodland. The dominant overstorey vegetation includes Acacia pycnantha, A. gracilifolia, Pultenaea graveolans and Cassinia laevis (Bickerton & Bond, 2000).
Rainfall data indicate that extant and historical locations of Arachnorchis woolcockiorum populations receive an average annual rainfall of 500 – 700 mm. This rainfall information, when coupled with vegetation mapping of the Southern Flinders Ranges (see Figure 1), indicates that the most suitable areas of habitat extend from the southern slopes of Mt Brown CP to Telowie Gorge CP, Napperby and the adjacent property managed by Forestry SA.
N.B. The southern slopes of Mt Brown CP have been included in the range of potential habitat even though the rainfall is presumed to be less than 500mm annually, because it is likely that isolated pockets within Mt Brown CP receive higher rainfall.
The main threats to Arachnorchis woolcockiorum include competition from weeds, inappropriate fire regimes, herbivory and inappropriate track maintenance activities. It is important to note that the species has not been seen recently near Upper Mambray Creek, on rocky ledges that have been invaded by weeds such as Milk Thistles and Quaking Grass. This suggests that A. woolcockiorum does not tolerate competition from weeds. Arachnorchis woolcockiorum is found predominantly in areas of MRNP that are currently weed-free. However, the species usually grows in open habitat. Such habitat is susceptible to weed invasion particularly if disturbed by such activities as track maintenance or fire fighting. A large portion of the Park called The Linklands (see Figure 1) was grazed by cattle until recently annexed to the Park, and because of its close proximity to the larger A. woolcockiorum subpopulations a diverse range of weed species may disperse into the orchid‘s habitat.
It is unknown what type of fire regimes were used by indigenous Australians prior to European settlement, but it is likely that the change in fire use with European habitation and then Park management has had an impact on the orchid. Since spider orchids typically live for approximately 15 years, a low intensity burn every 10 to 15 years would have reduced competition. However, since European settlement, fires may have become less frequent, but more intense and damaging to orchids. Conversely, if fires occur too frequently, the orchids could fail to reach maturity (typically two to five years for spider orchids), and recruitment would be diminished. For example, the weeds currently found on the rocky ledges of Upper Mambray Creek appear to have become established after fires in 1982 and 1988. Therefore inappropriate fire regimes within MRNP also pose a threat to the species.
Although there are no data on herbivory for A. woolcockiorum, it is known that other species of the Arachnorchis introduced vertebrates and invertebrates (Bickerton, 1999; Robertson & Bickerton, 2000). Of particular concern are kangaroos, euros, rabbits, hares and caterpillars. The vertebrates are of concern because of the proximity of most A. woolcockiorum populations to The Linklands (see Figure 1), an area of land which was until recently cattle pasture, but is now annexed to the Park. Such land tends to attract vertebrates that feed on the pasture at twilight and take shelter in the remnant vegetation during the day. These herbivores have the potential to severely reduce the number of flowers, and consequently the recruitment rate, in a population of spider orchids. Also caterpillars are known to have an impact on spider orchids found in the Adelaide Hills (Bickerton, 1999), and it is possible that they also graze on A. woolcockiorum in MRNP, since the rainfall is similar to some areas in the Adelaide Hills.
Most known A. woolcockiorum subpopulations are found adjacent to vehicle tracks, possibly because of the reduced competition and because of the tendency for pollinators to follow tracks when foraging or seeking mates (Although the identity of the pollinator is uncertain, spider orchids are usually pollinated by male thynnid wasps). The tracks are intended for use by National Parks' staff and fire fighting crews, and it is important that Park management and maintenance staff are aware that over-ambitious earthmoving activities could be detrimental to the species. An earthmoving vehicle is potentially a vector for the spread of weeds, and also frequent clearing could favour more aggressive species and alter the structure of the habitat unfavourably.
The pruning of vegetation for track maintenance may have an impact on A. woolcockiorum subpopulations through (a) physical damage to individuals and / or habitat by machinery or (b) smothering of the orchids by discarded prunings. Careful planning and implementation of track maintenance will limit damage to plants.
The pre-European extent of occurrence of Arachnorchis woolcockiorum is unknown, but it is likely that much of its suitable habitat has disappeared because of land clearing for grazing. Sugar gums are dominant or co-dominant in A. woolcockiorum habitat, and they have long been favored for firewood for kilns and ovens. It is probable that land clearance and the resultant fragmentation of habitat has led to the decline of the species. However, since the majority of habitat now critical or suitable for A. woolcockiorum is found within National Park reserves, there is no current threat of further land clearance.