Caladenia arenaria Fitzg. Recovery Plan

Threatened Species Unit, Western
The State of New South Wales, Department of Environment and Conservation, 2004
ISBN: 0731365399

8 Biology & ecology

Little is known of the specific details of the biology C. arenaria though they are believed to conform closely with other spider orchids, some of which have been well documented eg. Caladenia hastata (Carr 1988). The genus Caladenia is relatively well known taxonomically and biologically by virtue of the interest of enthusiasts and researchers. Caladenia arenaria, in common with other spider Caladenias, produces a single leaf in autumn or early winter. Flowers open in late August or September and persist for about a month, depending on seasonal conditions. Hot or dry conditions tend to result in a shorter flowering period.

Pollination in spider orchids is accomplished by male thynnid wasps in a syndrome of sexual deceit called pseudocopulation (Stoutamire 1983; Bower 1992, 1993). The wasps are attracted to the flowers by chemical analogs of the female thynnid sex pheromones. The male wasps lands on the central labellum (lip) which mimics the female wasp. He attempts to copulate with the labellum, and in the process come into contact with a structure known as the viscidium against which the pollen lies. The viscidium is sticky, and the glue produced by the viscidium allows the pollen to adhere to the insects thorax (back) when it backs out of the flower after its unsuccessful attempt at copulation. If the insect is attracted to another receptive flower, pollen is transferred to the stigma, and fertilisation is effected.

This process ensures that cross pollination (outcrossing) predominates in the population rather than self-pollination (selfing). There appear to be no molecular barriers to fertilisation, so selfing , and hybridisation with other Caladenia species is possible. Hybrids have been found in all four populations examined to date. Several other Caladenia species are involved. Presumed intermediates between pure C. arenaria and C. callitrophila, C. rileyi, C. stellata, C. sp. aff. tentaculata and C. concinna have been identified. Introgression has occurred, where plants that represent back crosses from the hybrid to one of its parents were found. At Buckingbong State Forest the morphology of some hybrids indicates at least two species other than C. arenaria in some hybrid combinations.

For germination orchid seeds require infection by a suitable fungal symbiont/partner. The fungus supplies nutrients for germination and initial seedling growth (Rasmussen 1995). Caladenia species possess a swollen stem (the collar) immediately below the leaf just under the soil surface. The mycorrhizal fungus invades collar. It is believed that before the orchid produces a leaf each year, reinfection of the mycorrhizal zone (collar) by the fungal partner must occur (D. Jones, pers. comm).

The implication of the specialised pollination (which is believed to be species specific) and dependence on a fungal symbiont (partner) for C. arenaria is that a functional ecosystem supporting these organisms is essential. Disturbance to the system that adversely affects the pollinator or fungal partner may clearly disadvantage the orchid. The identity of the fungal partner (which is normally free-living and reliant on leaf litter for its nutrition) or the pollinator of C. arenaria is not known, let alone their habitat requirements.