Threatened Species - White Spider Orchid
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Common white spider-orchid
Caladenia argocalla
(Photo: Y Steed) |
- Endemic to the Mount Lofty Ranges Region of South Australia;
only 1,000 flowering plants are currently known.
- Five populations ranging from 20 to 650 flowering plants
occur within a 10 km2 area near Clare.
- Only 90 flowering plants found at five sites in the Barossa
Valley, and in the hills east of Adelaide.
- Records dating back as far as 1904 verify that this species
was once growing on the Fleurieu Peninsula, in the Adelaide
Hills, the Barossa Valley and around Clare, South Australia.
It has not been recorded south of Adelaide since 1926.
- Extent of occurrence - past: 5,200 km2, present: 1300 km2
The Common White Spider-Orchid Caladenia argocalla occurs
in healthy grassy woodland communities. In South Australia this
type of vegetation has been extensively cleared for agriculture
and grazing, leaving mostly small isolated remnants. Populations
of C. argocalla are consequently small and fragmented.
Remnants of grassy woodlands are easily invaded by introduced
plant species that the orchids cannot compete with. An increase
in weed species also causes a reduction in habitat suitable
for the orchid's specialist pollinators.
- Small total population size - Currently only 1,000 flowering plants known.
- Small size of sub-populations - seven of the ten populations
comprise only 100 flowering plants between them.
- Isolation - The five southern populations are up
to 40 km apart from each other, and 90 km away from the Clare
populations. If populations are too far apart for pollinators
to fly between them, the gene pool becomes restricted.
- Lack of pollinators - Studies indicate that, at some
sites, less than 5% of flowers are setting seed. Small populations
may lack opportunities for cross-pollination and are unlikely
to be viable. Flowering plants appear to outnumber non-flowering
plants, which suggests a low rate of seed production and / or seedling establishment
- Weeds - topped lavender, soursobs, St John's Wort,
Cape Tulip, Gorse, Hawthorn and Dog Rose are threats.
- Sites are weeded to alleviate competition from exotic species.
- Flowers in very small populations are hand-pollinated, to enhance seed set.
- In small populations, tree guards are placed around individual
plants to discourage grazing by animals.
- The option of propagating plants in a nursery and transplanting
them back into the wild will be explored.
- Research will be conducted to identify the pollinator species
and its preferred food plants. If the food plants are missing,
they may be reintroduced at Caladenia argocalla locations.
- Plant identification sheets, posters and workshops are used
to inform the public of the importance of these species. Such
techniques assist in community education, and help interested
parties to identify new populations.
Orchids of the Caladenia genus (spider-orchids) have species-specific requirements:
- They require the presence of particular soil fungi mycorrhizae for germination.
- They have a limited range of pollinator insects (sometimes only one species).
- The primary food plant of the pollinator (which is rarely the orchid) must be present nearby.
Sexual deception and the spider's tongue
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Fig 1. A wasp
visiting a Caladenia
cardiochila flower
(Photo:R Bates)
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Fig 2. The
labellum of the Caladenia
clavigeraflower has been
triggered, tipping a wasp back towards the yellow pollen.
(Photo: C Bower)
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Fig 3. Pollen
from Calendenia tentacultaflower
deposited on the wasp
(Photo:R Bates)
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Little is known about the pollinators of Caladenia argocalla,
although the primary pollinator species of spider-orchids is
usually a native wasp. Some Caladenias attract native bees to
"glands" inside the flower that have the appearance of pollen
or nectar. Other spider-orchids use sexual deception; they exude
a chemical similar to the pheromone of a female native wasp.
This scent attracts a male wasp (fig. 1), and when it lands
on the labellum (the tongue of the flower), a trigger mechanism
tips it towards the column (fig. 2), which houses the pollen
and the receptive stigma. Pollen is either removed or deposited
in this process (fig. 3). Because of this complex pollination
mechanism, the ecosystem that supports Caladenia argocalla
must be relatively intact if the orchid is to survive. There
needs to be the correct species of soil fungus for seedling
establishment, the correct pollinator species and the food plants
that the pollinators need.
- Natural Heritage Trust
- Threatened Plant Action Group
- Mount Lofty Ranges and Greater Adelaide Integrated Natural Resource Management Group
- Northern and Yorke Agricultural Districts Integrated Natural Resource Management Group
Doug Bickerton,
Department for Environment and Heritage
Joe Quarmby,
Project Officer, Threatened Orchids, Department for Environment and Heritage
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