Underground orchid (Rhizanthella gardneri) Interim Recovery Plan 2003-2008

Interim recovery plan no. 127
Andrew Brown, Andrew Batty, Mark Brundrett & Kingsley Dixon
Department of Conservation and Land Management, WA, 2003

1. Background

History

John Trott discovered the first specimen of the underground orchid on his farm near Corrigin on the 23rd of May 1928 and Richard Rogers described it in August the same year, naming it in honour of the then Premier of Western Australia Charles Gardner. Between 1928 and 1959 it was found six more times, each time by chance during plowing of recently rolled and burnt bushland. All discoveries up until this time were made between the Corrigin and Dowerin areas. There was then a gap of 20 years before it was again seen (1979) by a private landowner near the town of Munglinup, some 300 kilometers south of previous known locations. During surveys of thickets of broom honey-myrtle (Melaleuca uncinata) in the Munglinup area by Dr Kingsley Dixon and members of the WA Native Orchid Study and Conservation Group (WANOSCG) 24 plants were found in three separate populations. Following these southern sightings Dr Dixon and members of WANOSCG searched areas near the original sighting at Corrigin and in 1981 and 1982 located two more populations together containing over 114 flowering plants. In 1985 a further population was discovered west of Corrigin and some 36 flowering plants were located at the site during surveys in 1989. Since that time no new populations have been located.

Natural Heritage Trust funding has been obtained by the Botanic Gardens and Parks Authority (BGPA) who are providing the research, seed collection, propagation and translocation components of this Interim Recovery Plan.

Description

Until recently, Rhizanthella was thought to be a monotypic genus confined to the south-west of Western Australia. However, in 1984, a second underground orchid, formerly known as Cryptanthemis slateri, was placed in Rhizanthella. This second species is known only from south-eastern Queensland and central-eastern New South Wales and, like the western Underground Orchid, lives out its entire life cycle under the surface of the soil.

Both species are leafless and lack chlorophyll. The flowers are positioned in inward-facing rows surrounded by fleshy, overlapping bracts.

The name Rhizanthella was coined by Richard Rogers in 1928 and refers to the rhizome-like tubers of the two orchids.

Flowering of Rhizanthella gardneri begins in late May, early June when each plant produces up to 100 small, inward facing, cream to reddish coloured flowers, surrounded by 6 to 12 large, cream or pinkish-cream bracts. These bracts form a tulip-like head that curves over the flowers forming a small opening at the soil surface. A layer of leaf and bark litter covers this opening. The plants have a horizontal rhizome 6 to 12 cm below the ground level, which, like the rest of the plant, is succulent and produces a formalin-like odour when cut. Once pollinated each flower produces a berry-like indehiscent fleshy fruit containing 20 to 150 seeds. This type of fruit is unique amongst the Western Australian orchids as species in all other orchid genera produce a dehiscing pod from which thousands of minute seeds are dispersed by the wind.

Distribution and habitat

Rhizanthella gardneri is known from two disjunct areas some 300 km apart - between Corrigin and Babakin and northwest of Munglinup. Plants occur under leaf and bark litter in thickets of broom honey-myrtle with scattered emergent Eucalyptus and Acacia species. Soil is either sandy-clay or sandy-loam.

Critical habitat

Critical habitat is habitat identified as being critical to the survival of a listed threatened species or listed threatened ecological community. Habitat is defined as the biophysical medium or media occupied (continuously, periodically or occasionally) by an organism or group of organisms or once occupied (continuously, periodically or occasionally) by an organism, or group of organisms, and into which organisms of that kind have the potential to be reintroduced. (Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)).

The critical habitat for Rhizanthella gardneri comprises:

  • the area of occupancy of known populations;
  • areas of similar habitat within 200 metres of known populations i.e. thickets of Melaleuca uncinata (these provide potential habitat for natural range extension);
  • additional occurrences of similar habitat on nearby areas of remnant bushland that are not currently known to contain the taxon but may have done so in the past (these represent possible translocation sites).

Habitat critical to the survival of the species, and important populations

Given that this species is listed as Critically Endangered it is considered that all known habitat for wild and any translocated populations is habitat critical.

Benefits to other species/ecological communities

There are no threatened ecological communities or other threatened species in the immediate vicinity of Rhizanthella gardneri. However, recovery actions implemented to improve the quality or security of the habitat of the species, such as weed control and rehabilitation, will benefit the remnant bushland habitat in which it occurs.

International Obligations

This plan is fully consistent with the aims and recommendations of the Convention on Biological Diversity, ratified by Australia in June 1993, and will assist in implementing Australia's responsibilities under that Convention. However, as Rhizanthella gardneri is not listed under any international agreement, the implementation of other international environmental responsibilities is not affected by this plan.

Role and interests of indigenous people

There are no known indigenous communities interested or involved in the management of areas affected by this plan. Therefore no role has been identified for indigenous communities in the recovery of this species.

Social and economic impacts

The implementation of this recovery plan is unlikely to cause significant adverse social and economic impacts. One population of Rhizanthella gardneri occurs on private property. However, negotiations between relevant parties have ensured that the area directly supporting this species will be left uncleared.

Evaluation of the Plans Performance

The Department of Conservation and Land Management, in conjunction with the Recovery Team will evaluate the performance of this IRP. In addition to annual reporting on progress with listed actions and comparison against the criteria for success and failure, the plan is to be reviewed within five years of its implementation.

Biology and ecology

Although Rhizanthella is like other orchids in that it relies on an association with mycorrhizal fungi for its survival, due to its subterranean habit and lack of photosynthetic capacity, its dependence is likely to be greater than most. The relationship with broom honey-myrtle is unique in the orchid world with a symbiotic micorrhizal fungus forming a link between the orchid and the Melaleuca.

It is currently unknown why the orchid occurs under some thickets of Melaleuca uncinata and apparently not under others. It is also not known why the orchid appears in two disjunct areas some 300 km apart. Distribution of essential mycorrhizal fungi may be a determining factor for its distribution. However, research using seed baiting methods for detection of suitable mycorrhizal fungi, developed by BGPA, may provide further information.

It is known that small fungal gnats pollinate flowers of the underground orchid. These are small enough to get through the gaps in the leaf and bark litter that covers the tulip-like inflorescence of the orchid. The gnats crawl through the litter into the tiny opening at the top of the floral bracts and down to the flowers that encircle the inside of the capitulum. Other insects such as termites and mosquitoes have been seen on flowers of an exposed capitulum and may also be incidental pollinators.

It is currently unknown how seed is dispersed but it is thought that small marsupials may eat the succulent fruits produced by the plant and deposit seed in their faeces.

Threats

Rhizanthella gardneri currently meets World Conservation Union (IUCN, 2000) Red List Category 'CR' under criteria B2ab(ii,iii,v); C2a(i); D due to its extreme fragmentation, a continuing decline in area of occupancy, area, extent, quality of habitat and the number of mature individuals and a total population size of less than 50 mature individuals.

Its specialized habitat survives as disjunct remnants in the central and southern Wheatbelt. A combination of drought and senescence resulting in the death of mature Melaleuca uncinata (Broom honey-myrtle) plants threatens much of its habitat in the central wheatbelt where little recruitment of broom honey-myrtle is evident and the once large thickets are becoming smaller and more open. This has resulted in vastly increased light levels and a significant drop in the level of leaf and bark litter held at the base of plants, causing the soil to become hard baked and dry. Habitat is in better condition in the area of southern populations.

Just 23 flowering plants were found during intensive surveys of three populations in the Corrigin area in May-June 2001 and a further 4 plants found in two populations near Munglinup in July 2002.

The main threats are lack of suitable habitat, degraded habitat, drought, soil compaction, road and firebreak maintenance, rising saline water tables (Meston 2001), weeds, inappropriate fire regimes, human damage during searches for the orchid and poor recruitment.

  • Lack of suitable habitat is a barrier to more populations being found. This also limits the number of areas suitable for translocation.
  • Degraded habitat is a current and continuing threat as the thickets of Melaleuca uncinata on which the orchid depends for its fungal nutrient link are dying back from the edges due to drought and possibly rising saline water tables.
  • Drought appears to be a major threat to the habitat of Rhizanthella gardneri in the central Wheatbelt.
  • Soil compaction due to poor levels of leaf and bark litter and human disturbance has resulted in poor flowering and possible deaths of Rhizanthella gardneri plants.
  • Road and firebreak maintenance threatens several southern populations.
  • Rising saline water tables are possibly already causing deaths amongst the associated Melaleuca uncinata and are likely to become an increasing future threat.
  • Weed invasion is a minor threat to all populations. The effect of weeds is uncertain but they are likely to compete for soil moisture and nutrients needed by the orchid and associated fungi. Weeds also increase the fire hazard due to the easy ignition of high fuel loads produced annually by many grass weed species.
  • Inappropriate fire may threaten populations if it occurs during the flowering period of the orchid and overly frequent fires are likely to alter its habitat. One southern population was burnt some years ago and despite several searches since then no Rhizanthella gardneri plants have been located.
  • Human damage during searches for the orchid is a continuing threat. The method used to locate plants is quite destructive requiring the removal of leaf litter beneath Melaleuca uncinata. This litter is often not replaced and when it is, is usually mixed with soil resulting in soil compaction and the drying out of the area where the orchid occurs.
  • Poor recruitment and declining populations are a major threat to the orchid. In areas where many flowering plants were located during surveys in the 1980s few plants were found during surveys in 2001 and 2002.
  • A lack of a seed dispersal agent and severe habitat fragmentation may prevent recruitment into new habitats.
Summary of population information and threats
Pop. No. & Location Land Status Year/No. plants Condition Threats
1. Babakin Nature Reserve 1982 110
2001 15
2002 10
Poor Drought, soil compaction, future rising saline water tables, degraded habitat, habitat damage during searches
2. W of Babakin Nature Reserve 1982 4
2001 6
2002 2
Good Drought, soil compaction, future rising saline water tables, lack of habitat, habitat damage during searches
3. Oldfield River Unvested Crown Land 1982 4
2002 2
Good Drought, road and firebreak maintenance, habitat damage during searches
4. NW of Munglinup Nature Reserve 1982 4
2002 0
None seen Drought, inappropriate fire, firebreak maintenance
5. NW of Munglinup Private 1981 10
2002 2
Good Drought, weeds, clearing, firebreak maintenance
6. Kunjin Townsite Reserve 1989 38
2001 2
2002 3
Poor Drought, soil compaction, degraded habitat, future rising saline water tables, habitat damage during searches

Guide for decision-makers

Section 1 provides details of current and possible future threats. Any on-ground works (firebreaks, roadworks etc) in the immediate vicinity of Rhizanthella gardneri will require assessment. On ground works should not be approved unless the proponents can demonstrate that they will not have an impact on the species, its habitat or potential habitat, or on the local surface hydrology such that drainage in the habitat of the species would be altered.