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2. Spider-orchids (Genus: Caladenia, Section: Calonema)

2.1 Description

All taxa covered by the current recovery plan are members of the Genus Caladenia, Section Calonema, commonly referred to as spider-orchids. Taxa within this section are characterised by their large, attractive flowers with long filamentous segments often ending in fine, elongated tips, which may be conspicuously clubbed or densely covered, with dark-coloured glandular hairs. These organs are the source of sexual attractants for the pollinators, usually male thynnid wasps (Jones 1988).

Caladenia species are deciduous terrestrial orchids that die back annually to small, spherical subterranean tubers that are protected by a tough, fibrous tunic. They have a single green basal leaf, which is generally long, narrow and conspicuously hairy. Plants flower for only a very limited time each year.

Many spider-orchids have specific pollinators that are attracted to the flowers under the false pretence of copulating with what they believe to be a female of the species. Once pollination has occurred, the flower closes and a capsule is formed containing the microscopic seed (see 2.4 - Life History).

Descriptions of all species addressed in this plan can be found in Backhouse and Jeanes (1995) and Walsh and Entwisle (1994).

2.2 Taxonomy

The taxonomy of Caladenia has been in a state of flux and confusion for some time despite recent efforts by various workers to resolve it (Carr 1991, Clements 1989, 1993, Jones 1991, 1994). In particular the Caladenia carnea, Caladenia dilatata, Caladenia patersonii and Caladenia reticulata complexes have been studied and split up into numerous new species, some of which have not yet been adequately described (Backhouse and Jeanes 1995). Revision of the genus is still currently being undertaken and many new species have been recently described. Further descriptions and name changes are likely in the future (Jones 1999).

Many of the species addressed in this recovery plan have been described in the last ten years. All species fall into one of three former complexes, namely the Caladenia dilatata group (C. amoena, C. tensa), the Caladenia reticulata group (C. hastata, C. lowanensis, C. robinsonii, C. thysanochila, C. xanthochila) and the Caladenia patersonii group (C. audasii, C. formosa, C. fragrantissima ssp. orientalis, C. rosella, C. versicolor). A number of other threatened species also occur within these groups including some newly described species (Jones 1999) and some as yet undescribed taxa (G. Carr, Ecology Australia P/L, pers. comm.).

There is believed to be some taxonomic confusion in South Australia relating to C. tensa and C. clavula (Peter Lang, EH, pers. comm.). At present these two taxa are listed as one entity on the South Australian flora database although herbarium records adequately differentiate between the two species on taxonomic and distributional grounds (R. Bates, SHSA, pers. comm.). Similarly, there has been some past taxonomic confusion between C. formosa and C. colorata in South Australia (Peter Lang, pers. comm.). Records of C. formosa in South Australia may need to be treated with some caution until accurate identification of populations can be confirmed.

2.3 Distribution and Habitat

The threatened Caladenia taxa occupy a wide distribution and a range of different habitats. Five of the taxa (C. formosa, C. lowanensis, C. tensa, C. versicolor and C. xanthochila occupy plains areas of western Victoria and southeast South Australia. The large proportion of known populations for these species occur within the Murray-Darling Depression and Naracoorte Coastal Plain interim bioregions of Victoria and South Australia (Commonwealth of Australia 1999). C. hastata also occurs in the Naracoorte Coastal Plain interim bioregion being restricted to coastal heathland environments in southwest Victoria. Three taxa (C. amoena, C. audasii and C. rosella) occupy box and ironbark forests and woodlands of inland Victoria within the Victorian Midlands interim bioregion. The three remaining taxa, C. fragrantissima ssp. orientalis, C. robinsonii and C. thysanochila are endemic to Victoria and occupy coastal areas to the southeast of Melbourne within the Southeast Coastal Plain interim bioregion.

Table 2 summarises the distribution and habitat of threatened Caladenia taxa of southeast Australia while Figures 1-12 illustrate known populations of these taxa in Victoria. Some populations have not been confirmed in the field in the last 10 years.

A defining feature of most of the species is that they occupy uncommon habitats. This may be due to these habitats having been naturally uncommon prior to European settlement or more typically because of clearing for agriculture and urbanisation since European settlement. Most species occur within severely fragmented ecosystems that are subject to a range of potentially threatening processes typical of such environments.

Table 2: Distribution, abundance and habitat of twelve threatened Caladenia taxa in Victoria and South Australia.

Species name	Distribution		Abundance	Habitat/Ecological Vegetation Class*
	Past	Present
Caladenia amoena	Restricted area to the north east of Melbourne in the Greensborough-Plenty-Hurstbridge area within the Victorian Midlands interim bioregion.	Two populations at Plenty (public land) and Wattle Glen (private land) - see Figure 1.	Approximately 45 plants remaining in two populations.	Grassy Dry Forest; Box Ironbark Forest Sandy loams derived from sandstone and mudstone.
Caladenia audasii	Disjunct distribution from central Victoria, near Bendigo to southeast South Australia near Bordertown.	Restricted to three populations in Victoria at Bendigo, Kingower and Deep Lead - see Figure 2.	Five plants remain in the wild. Extinct in South Australia.	Grassy Dry Forest; Box Ironbark Forest. Typically auriferous soils with 'buckshot' gravel derived from sandstone and mudstone.
Caladenia formosa	Presumed to have occupied aeolian sand deposits in western Victoria (border to southern Grampians) & southeast South Australia in areas with generally 400-500 mm annual rainfall within the Naracoorte Plain bioregion.	Restricted to isolated public land forest blocks south of Edenhope and north of Cavendish and some adjoining private properties. May occur in the Grampians (Victoria) and is known from Mt Scott and Mt Monster Conservation Parks in South Australia (Briggs & Leigh 1995; Adrian Stokes EH pers. comm.) and from several private properties in the Naracoorte, Coonawarra and Kingston areas (Kath Alcock, Naracoorte, pers. comm.) - see Figure 3.	Highly restricted but locally abundant in Victoria (1000's of plants). Total population in South Australia is unknown.	Damp-sands Herb-rich Woodland; Plains Sedgy Woodland; Shallow Sands Woodland; Seasonally Inundated Shrubby Woodland. Typically occurs on sandy soils that are moist in winter and dry in summer, which often promotes annual and geophytic (seasonal) plants.
Caladenia fragrantissima ssp. orientalis	Near coastal habitats extending from the eastern shores of Port Phillip Bay to Wilsons Promontory in South Gippsland within the Southeast Coastal Plain bioregion.	Disjunct populations at Rosebud (Mornington Peninsula), Wonthaggi, Cape Paterson and Walkerville - see Figure 4.	Total number of plants <100.	Coastal Heathland; Heathy Woodland generally occurring on deep siliceous sands.
Caladenia hastata	Confined to a small area of southwest Victoria from south of Portland to Gorae, approximately 15km northwest of Portland and far southeast South Australia.	One naturally occurring population at Point Danger, south of Portland. One additional translocated population occurs at Nelson Bay. Extinct in South Australia - see Figure 5.	Total number of plants ca. 20.	Damp Heathland; Damp Heathy Woodland on aeolian sand deposits.
Caladenia lowanensis	Presumed to have occupied low sandy rises in the Murray-Darling Depression bioregion, north of the Little Desert, in areas with generally 300-400 mm annual rainfall. Also occurred in South Australia north of Bangham (R. Bates, pers. comm.).	Known with certainty from Kiata Flora Reserve and a nearby private land site. Unconfirmed population recorded at Glenlee Flora and Fauna Reserve, 10km north of Kiata and at West Wail Flora Reserve. May occur in southeast South Australia (Adrian Stokes, EH, pers. comm.) - see Figure 6.	Total number of plants ca. 240. Numbers unknown in South Australia	Cypress-pine/Yellow Gum Woodland on sandy loams derived from Tertiary and Quaternary aeolian deposits.

Species name	Distribution		Abundance	Habitat/Ecological Vegetation Class*
	Past	Present
Caladenia robinsonii	Restricted to sandy areas on the eastern shores of Port Phillip Bay in the Southeast Coastal Plain bioregion.	Known with certainty from Rosebud. May also occur near Frankston and at Langwarrin - see Figure 7.	Approximately 20 plants within a single population.	Heathy Woodland; Damp Sands Herb-rich Woodland; Heathy Herb-rich Woodland on Tertiary siliceous sand deposits.
Caladenia rosella	Former distribution is uncertain but may have been scattered through the Box Ironbark woodlands and forests of the Victorian Midlands bioregion.	Disjunct populations north east of Melbourne at Cottlesbridge, Research and Christmas Hills and an unconfirmed record from near Stawell - see Figure 8.	Approximately 120 plants in four populations.	Grassy Dry Forest; Heathy Dry Forest on sandy clay loams derived from sandstone and mudstone.
Caladenia tensa	Widespread on aeolian sand deposits surrounding and including the Little Desert in western Victoria and southeast South Australia within areas of the Murray-Darling Depression bioregion with generally 300-400 mm annual rainfall.	Widespread in and surrounding the Little Desert in western Victoria. Also known from southeast South Australia where considered widespread but uncommon (Bob Bates, SHSA, pers. comm.). Examples include Telowie Gorge, Murray Bridge and Mt Boothby CP - see Figure 9.	Locally abundant in suitable habitat in western Victoria and eastern South Australia.	Cypress-pine/Yellow Gum Woodland, Heathy Woodland and Mallee on sands and sandy loams derived from aeolian sand deposits.
Caladenia thysanochila	May have occurred at several locations on the Mornington Peninsula including Mount Eliza, Mount Martha and Arthurs Seat within the Gippsland Plain bioregion.	Only known from a single site at Mount Eliza on the Mornington Peninsula - see Figure 10.	Only known from two specimens, both of which have not flowered since 1992.	Damp-sands Herb-rich Woodland Soils are generally fine-coarse granitic sands.
Caladenia versicolor	May have been scattered in low lying areas between the Grampians and St Arnaud and other low-lying areas on the plains to the west of the Grampians and into southeast South Australia in the Penola flats area (Briggs and Leigh 1995, R. Bates. pers. comm.).	Restricted to Lake Fyans (south west of Stawell, Victoria). Record from Deep Lead Flora and Fauna Reserve is unconfirmed. Known from three collections in South Australia, including one site southeast of Penola - present status of these populations is unknown - see Figure 11.	Locally abundant at Lake Fyans where several hundred plants occur. Presumed extinct in South Australia	Plains Sedgy Woodland; Shallow Sands Woodland. Soils are generally sandy/silty clay loams derived from Quaternary alluvial and swamp deposits.
Caladenia xanthochila	Restricted to low sandy rises and outwash areas in the Wimmera and Riverina bioregions in areas with generally 400-600 mm annual rainfall. Also occurred in South Australia and maybe NSW.	Known with certainty from two locations, Murtoa and Inglewood. A report of the species from NSW is awaiting confirmation. Presumed extinct in South Australia (Briggs and Leigh 1995; R. Bates pers. comm.)) - see Figure 12.	Total number of plants ca. 120. Only one plant from Inglewood has flowered in the recent past. Presumed extinct in South Australia	Shallow Sands Woodland; Alluvial Terraces Herb-rich Woodland. Occupies Quaternary alluvial or aeolian deposits derived from a variety of different geologies.

Refer to VicRFASC (1999) and ECC (in prep.) for descriptions of Ecological Vegetation Classes.

Past and present distribution of twelve threatened Caladenia taxa in Victoria and South Australia (sources: Victorian Flora Information System and South Australian Threatened Plant Population Database)

Figure 1 - Caladenia amoena	Figure 2 - Caladenia audasii Extinct in South Australia
Figure 3 - Caladenia formosa	Caladenia formosa
Figure 4 - Caladenia fragrantissima ssp. orientalis	Figure 5 - Caladenia hastata
Figure 6 - Caladenia lowanensis May also occur in southeast South Australia	Figure 7 - Caladenia robinsonii
Figure 8 - Caladenia rosella
Figure 9 - Caladenia tensa	Caladenia tensa
Figure 10 - Caladenia thysanochila	Figure 11 - Caladenia versicolor Presumed extinct in South Australia
Figure 12 - Caladenia xanthochila Presumed extinct in South Australia

2.4 Life History

All Spider-Caladenia species are terrestrial, deciduous herbs, emerging annually from spherical, subterranean tubers that are protected by a tough, fibrous tunic. Most plants shoot in response to soaking rains in early autumn, first producing only a leaf that remains almost dormant through the winter. All of the taxa covered by the current plan flower in late-winter to spring ranging from C. amoena that usually flowers in August to C. versicolor that may flower as late as early November. Table 3 details approximate flowering and fruiting times for threatened Caladenia taxa of Victoria and South Australia. Flowers may remain open for a few days to a few weeks depending on pollination and climatic factors.

Table 3: Approximate flowering and fruiting times for twelve threatened Caladenia taxa of Victoria and South Australia.

Species name	Flower numbers	Flowering time	Fruiting time
Caladenia amoena	One (rarely two)	August - early September	October
Caladenia audasii	One	September	November
Caladenia formosa	One or two	Late September - October	October - November
Caladenia fragrantissima ssp. orientalis	One or two	October - early November	November - December
Caladenia hastata	One to three	October	Late November
Caladenia lowanensis	One	September - October	October - November
Caladenia robinsonii	One (rarely two)	October	November - December
Caladenia rosella	One	August - September	October - November
Caladenia tensa	One (rarely two)	September - October	October - November
Caladenia thysanochila	One or two	October	November
Caladenia versicolor	One or two	Late September - November	October - November
Caladenia xanthochila	One (rarely two)	September	October

Fruits usually take 5-8 weeks to mature following pollination. Each mature capsule may contain tens of thousands of microscopic seeds that are dispersed by the wind when the capsule dries out.

Most spider-orchids are believed to grow in a complex relationship with mycorrhizal fungi (Warcup 1981). The fungus assimilates some nutrients for the orchid, but the degree of dependence upon the fungus is not known. Longevity of most spider-Caladenia species is not known, but there are examples of individuals of Mellblom's Spider-orchid Caladenia hastata having survived for at least 17 years in the wild (Carr 1999.).

Most members of the Spider-Caladenia group are pollinated by sexual deception through a process called pseudocopulation (Jones 1988). Spider-orchids are characterised by the often large, attractive flowers with long tapering sepals and petals ending in clubs or covered with dark glandular hairs that are the source of the sexual attractants for the pollinators, usually male thynnid wasps, attracted to the flowers by scent mimicking female thynnid wasp pheromone. Once it reaches the flower, the male attempts to copulate with the labellum of the flower, mistaking it for the female wasp, and effects pollination. The identities of the pollinator(s) for most species are not known. While thynnid wasps are the most likely pollinators, a small native calictid bee (Neoproctus species) has been reported as a pollinator of the Rosella Spider-orchid Caladenia rosella (C. Beardsell pers. comm.).

Observations suggest that the period available for effective pollination may be quite short, maybe only a few days. Successful pollination may be influenced by the receptivity of the stigma to pollen, the number of pollinators in an area, insect behaviour and climatic conditions. Higher rates of pollination usually occur immediately following wasp-emergence as inexperienced juvenile wasps attempt copulation with many flowers (G. Carr, pers. comm.). It has been suggested that pollinators may become habituated to the presence of flowers in their territory after a period of time and rates of pollination may decline as a result (C. Bower, pers. comm.). Attempted copulation rates by wasps, a critical step for pollination, has been shown to be as low as 7.5% for some Caladenia species (Peakall and Beattie 1996) while anecdotal evidence suggests that non-synchronous flowering may prevent pollination in some taxa (G. Walker, FOBCCR, pers. comm.). These may be important factors when considering species with critically low population numbers in the wild.

The role of fire in the ecology of the various Spider-orchid species is not known, but is likely to be an important factor, at least for some species which have exhibited strong flowering responses in the years following fire (eg. Caladenia fragrantissima sens. lat.). Fire is an integral part of the physical environment of most vegetation types in southern Australia (Gill in press), and is required to maintain plant diversity (Wark 1996). Vegetation response to fire is not directly known, but may be a combination of one or more factors initiated by fire. Fire removes surrounding vegetation, increasing light levels and temperature at ground level, and possibly increasing moisture levels with reduced plant competition for moisture (Purdie 1977). Soil ecology changes and mycorrhizal fungal symbionts become more prevalent. There is also a reduction or removal of the allelopathic inhibition by the surrounding vegetation (Gill et al. 1981). Seedling establishment may be critically dependent on fires.

The response of some Caladenia species to hot summer fires can be quite spectacular, with profuse flowering of C. australis at Anglesea, C. fragrantissima ssp. fragrantissima near Portland and C. insularis on French Island observed after hot summer fires. Timing of fire is important, with the best time for orchids during late summer or early autumn, after seed dispersal but prior to new shoot growth. Fuel reduction burning of state forests in spring and autumn is considered to be a threatening process for many orchid species. The variation in seasonal climatic conditions, most notably rainfall and temperature also influences flowering. Flowering is often aborted when periods of sustained hot, dry weather follow flower opening (J. Todd, unpubl. data).