A Guide to the Biology & Conservation of Singapore's Only Endemic Orchid

The flower of Singapore Nervilia, an endemic orchid species, found only in Bukit Timah Nature Reserve (Photo: Matti Niissalo/National Parks Board).

"...the contrivances by which Orchids are fertilised, are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom." - Charles Darwin, Fertilisation of Orchids.^[1]

Introduction

Few plants have the honorable status of being found only in Singapore and named after the country. One of these plants is the Singapore Nervilia, also known as Nervilia singaporensis Niissalo. In 2019, this rare and inconspicuous orchid received local media attention a year after.^[2]Being small and well-hidden among forest vegetation, much of the orchid's biology remains unknown. I was fortunate enough to see this orchid alive and in its type locality where fewer than 30 individuals are thought to exist. However, as Singapore has lost more than 95% of its native forest and this orchid is in danger of extinction if there are no efforts to conserve it. ^[3]

Orchids hold a special place in Singapore. They are celebrated in festivals such as the 20th World Orchid Conference in 2011 ^[4]and the biannual Singapore Garden Festival. ^[5]The national flower is Vanda Miss Joaqium (Papilionanthe Miss Joaquim). ^[6] As a cross between Papilionanthe teres × Papilionanthe hookeriana, it is the only hybrid in the world made a national flower. Thus, it will be a pity to lose Nervilia singaporensis, the only orchid that is uniquely found in Singapore and nowhere else.  

The iconic Singapore Garden Festival symbol displayed on a Ferris wheel (left) and plant wall (right) decorated by orchids in 2012. 

The flower of Papilionanthe Miss Joaquim (left) and its stems, leaves and aerial roots climbing on supports at the Singapore Botanic Gardens (right).

How can you support its conservation?

Provisional IUCN conservation assessment

The orchid should be classified as Critically Endangered globally in the IUCN Red list.^[7]The most probable extent of occurrence (EOO) is less than 100 km2 and area of occupancy is significantly below 10 km2. However, as a challenging species to find, the orchid may have been gone unnoticed at other localities in Peninsular Malaysia or Singapore. Fortunately, the species does not need pristine primary forest and survives in mature secondary forests. Due to the loss of forests near Choa Chu Kang, the only known population of the orchid is confined to a few square meters in Bukit Timah Nature Reserve. The quality of its habitat has also declined due to increased edge effects in the last century. Under the local Red list criteria and global IUCN assessment, the orchid should also be classified Critically Endangered. Information to support other threatened wildlife such as the orchid can be found here.

Orchid conservation and reintroduction programme

There are at least 200 native orchids in Singapore, more than half are nationally extinct.^[8]Most native orchids alive today are threatened and found in the remaining nature reserves such as Nervilia singaporensis. These orchids are affected by development and require help. Hence, the Orchid Conservation and Reintroduction Programme started in 1995 to ensure the preservation of native orchids. This is done by gathering of seed and tissue to grow in the Singapore Botanic Gardens. The National Parks Board retrieved a few capsules from the known population of Nervilia singaporensis for germination and species recovery purposes.^[7]Unfortunately, when the seeds were given orchid growing media, it did not germinate after more than a month. In order to conduct ex-situ conservation, more research is needed.

For the conservation of this orchid, financial support is necessary. Some research topics that require funding to protect Nervilia singaporensis are:

• What is the true distribution of the orchid? Could the plant be more widespread than thought in other Singapore forests? Could those forests be protected?
• Is the orchid facing inbreeding depression? How can we assign parent plants to ensure the surviving genetic population and future generations remain healthy?
• What is the main reproductive strategy of the species? Does it depend on other organisms such as mycorrhiza fungi? Does it need pollinators?

You can support this effort through the Garden City Fund. For current information on this orchid, you can find it in this webpage below.

Bulbophyllum maxillare (Lindl.) Rchb.f. is one of orchids listed as Nationally Extinct being reintroduced in Singapore under the programme. 

Name

Common Name: Singapore Nervilia

Binomial: Nervilia singaporensis Niissalo

Two emerging leaves and a mature fruit capsule of the orchid, growing above the forest floor leaf litter.

Etymology

Nervilia describes the prominent veins on the leaf. As the orchid was first found in Bukit Timah Nature Reserve, it was named after the type locality's country, Singapore.

Description

A size comparison size between my hand and the orchid leaves during surveys of it.

Nervilia singaporensis is a deciduous herb that grows on the forest floor.^[7]It is an easy plant to miss. The orchid is under 10 cm tall but grows to more than 25 cm when in fruit. The small tuber has a spindle shape and white in color. Some will have old sheaths degrading on the tuber causing it to turn light brown. Occasionally, vestigial roots may appear. Underground stem emerges from the tuber, usually a few centimeters long, may produce lateral roots or short stolons with fine hairs.

Lateral roots appearing on the side of an exposed underground stem.

The uniformly green cordate leaf is few centimeters long and shorter in diameter. Both surfaces of the leaf are glossy. The lobes are often rounded. Leaf is parallel to the ground and flat when it opens fully. The major veins are prominent and diverge palmately. Margins are entire. Membranous cataphylls sheath the true leaf. It also has an erect pseudopetiolate.

An emerging leaf from the apical node of tuber with cataphylls protecting it (left). As the leaf slowly opens, the lobes become more obvious (right). 

When flowering, the pink inflorescence arises from the tuber's apical node, may also produce other flowers laterally from the same node. Flower bracts protect the inflorescence. Elliptic sepals are brown and covered with purple spots. Equally shaped petals can be white to light yellow-brown. The oblong lip of the orchid is pinkish with bright violet spots and has three lobes. Column is white with purple spots. For two days, the flower will open and stay horizontal for day before slowly lowering. No distinct smell was captured when I did the surveys.

When the capsule matures, it releases microscopic seeds, dispersed by wind (left). The final stage of the capsule after releasing seeds, it begins to dry and wilt (right).

The flower then withers and shrinks as the fruit develops. The fruit is a capsule and dehisces when dry. It can grow to a centimeter long and half a centimeter wide with six narrow strips. Perianth will stay on the capsule even when dried. Mature fruit capsules had very few seeds (fewer than 50 per pod) , suggesting high inbreeding within the population. Seeds are microscopic, crescent in shape and have an orange embryo in its middle.

Nervilia singaporensis with new leaves emerging in foreground, old leaves in background and a dried capsule.

Diagnosis

The developing capsule of Nervilia singaporensis after flowering recently with the bract intact.

Unlike the acute lip of Nervilia punctata (Blume) Makino inflorescence, the lip of Nervilia singaporensis is truncated.^[7]Clinandrial tissue surrounds the cleft anther cap dorsally for over half its length. The flower is also cleistogamous that pollinates itself by not opening fully. 

Biology

General biology of genus Nervilia

Nervilia orchids are terrestrial and herbaceous and are characterized by their tubers.^[9]These tubers are capable of producing inflorescences, solitary leaves or runners. The runners then create new tubers, allowing the orchid to make clones of itself over a wide area. Leaves are horizontal and vary in shape. Most Nervilia orchid leaves are glossy, they can vary from dark green to maroon, even with purple strips. The inflorescence are erect, slender and terminal. Its sepals and petals are similar, either ovate or elliptic. It has fewer than a 100 species found throughout Africa and Asia. Some orchids are endemic to the country or region it belongs. The orchid is a problematic genus due to its unpredictable flowering and sometimes remain dormant belowground without leaves or flowers, leading to incomplete herbarium collections and taxonomic error.

In horticulture, Nervilia is rare. The orchids are usually grown in shade.^[10]and need a balanced mix of organic material and coarse gravel for good drainage. Before the growing season, the dormant tubers are buried at least 2 cm below the surface of the soil. Repotting of orchid is done before each growing season. Under suitable conditions, some species can produce multiple clones via its runners in a single growing season, creating a nice flower colony.

Some Nervilia have medicinal properties and compounds. In Traditional Chinese Medicine, Nervilia fordii (Hance) Schltr. also known as Qing Tian Kui (青天葵), is used to remove toxins, relieve coughs and reduce pain.^[11]In India, another sister species, Nervilia aragoana Gaud., is used as traditional Ayurveda medicine against asthma and epileptic fits.^[12]

The reflective surface of  Nervilia singaporensis under camera flash.

Cryptic diversity of Nervilia

An integrative morphological and phylogenetic approach was used to delimit the polyphyletic Nervilia punctata group in Southeast Asia.^[13] STACEY, a Bayesian coalescence approach to phylogenetic species delimitation helped to resolve the polyphyletic clusters. For unidentified samples from Thailand, three cryptic species were circumscribed using subtle morphological differences and the phylogenetic analysis of one nuclear (ITS) and two plastid (matK andtrnL-F) markers. Other new species of orchids such as Nervilia futago S. W. Gale & T. Yukawa. A from Japan^[14] and Nervilia kasiensis S.W.Gale & Phaxaysombath from Laos^[15] were also described using similar approaches. This approach is important for conservation of the genus, given the taxonomic difficulty in the orchid, ensuring its species diversity is not lost because of poor identification. 

Phenology

In the taxonomic description of Nervilia singaporensis^[7], inflorescences among multiple individuals were seen between November and December 2019. The period could have triggered flowering after the sudden drought from July to September in the same year when the orchids were dormant. This species flower rarely opens fully. When monitored over several days in November 2019, the flowers opened only slightly. There were no obvious pollinators, insects occasionally land on the flower but could be due to the presence of decomposing figs. Pollinia are observed firmly latched on the stigma. Self-pollination is very possible as there is no clear rostellum or barrier to prevent self-fertilisation. Flowers are hence autogamous and cleistogamous. The pollination strategy is very similar to other Nervilia orchids like Nervilia nipponica Makino.^[16]However, cross-breeding with other individuals was discovered to be possible in Nervilia nipponica after thorough pollination tests were done. Therefore, such investigations should be replicated in the endemic Singapore orchid to truly understand its pollination strategy. 

After a short flowering span, the orchid starts to develop its capsule as the perianth closes, the pollinia likely lands on its own stigma and undergoes self-fertilization.

Distribution

Nervilia singaporensis is only known from Singapore as there is unlikely any conspecific specimens from Peninsular Malaysia.^[7]Bukit Timah Nature Reserve, the locality of the type specimen, is surrounded mostly by dry coastal hill dipterocarp primary forest. At other near locations where the orchid could have also be found in Singapore such as Stagmount and Choa Chu Kang, the similar habitat types are no longer intact.

The 163 ha intact forest Bukit Timah Nature Reserve, where the type specimen was found, highlighted in yellow, on the Singapore map. Other nearby forests are fragmented by urbanization and unlikely to contain the orchid. (Map: Google Maps)

Ecology

The orchid grows among the undergrowth vegetation of mature secondary forests in the type locality. Other plants found in the vegetation are: Ficus spp. and introduced herbs (Dieffenbachia seguine (Jacq.) Schott, Epipremnum aureum (Linden ex André) G.S.Bunting and Heliconia psittacorum L.f.). Fortunately, this orchid does not need pristine primary forest and stand a better chance of occupying other secondary forest habitats in Singapore if funding for tests are available. ^[7]

An example of the kind of mature secondary forest in Bukit Timah Nature Reserve which the orchid occupies.

The thick undergrowth vegetation surrounding the orchid's opened capsule at bottom right of photo under the light.

A closer shot of the orchid's opened capsule in the center of photo.

The heart shape leaves of Epipremnum aureum are very similar and difficult to discern from Nervilia singaporensis in the center of the photo.

Fungal symbiotic associations in other Nervilia orchids 

Nervilia nipponica is a well-studied Japanese orchid species. However, the orchid is also a threatened species with limited distribution. Specificity to mycorrhizal fungus was suspected as a reason for its rarity.  ^[17]When mycorrhizae samples were collected from individuals throughout Japan, the fungi were suspected to be Agaricomycetes that infected most of the population using phylogenetic analyses. However, two other fungi were also found belonging to Ceratobasidiaceae and Sebacinales clades. The orchid may exploit these groups or in combination with the Agaricomycetes. Thus, this might be the same for other Nervilia orchids, including Nervilia singaporensis. More research is needed to investigate such relationships and understand this as a possible evolutionary driver for the Nervilia rarity and diversity.^[18] More importantly, such information can contribute to the orchid's conservation. 

Classification

• Domain: Eukaryota
• Kingdom: Plantae
• Phylum: Tracheophyta
• Class: Liliopsida
• Order: Asparagales
• Family: Orchidaceae
• Genus: Nervilia Commerson ex Gaudichaud-Beaupre
• Species: Nervilia singaporensis Niissalo

Phylogeny

Phylogenetic position of Nervilia in Orchidaceae

Nervilia belongs to the "LOWER EPIDENDROID" clade that was one of the 6000 equally parsimonious, successfully weighted rbcL trees, taken from Cameron et al. ,1999. Branch lengths (ACCTRAN optimization with equal weights) are represented by numbers above the tree branches. Nodes with strong bootstrap support values above 75% are drawn with a solid circle while those with weak bootstrap support of 50 to 74% are highlighted with open circles.

According to Cameron et al. ,1999^[19],  Nervilia is classified under the paraphyletic Lower Epidendroid orchids. The Epidendroid clade is considerably large, thus divided into "lower" and "higher" clades for convenience. It is the one of the five subfamilies of Orchidaceae, one of the largest family of plants. Within the clade, the Nervileae tribe, which Nervilia belongs to, is the sister group to the remaining tribes of Epidendroid. The phylogenetic analysis was done using parsimony from 171 taxa representing most orchids. Chloroplast rbcL exon coding sequences were used, each sequence was approximately 1330 base pairs. The species of Nervilia used in this study was Nervilia bicarinata Schltr (Voucher: Chase O-580 (K), Database accession: GBANAF074199).

Phylogenetic position of endemic orchid in Nervilia

ITS sequences from the ribosomes was utilized for this RAxML tree, outgroups are not shown.^[7]. It shows the position of Nervilia singaporensis compared to other Nervilia sp.  Numbers above nodes only show bootstrap support values above 75.

GenBank sequence for the type specimen for the tree above is available online. The sequence belongs to the ribosomal DNA from 18S to 26S. From the phylogenetic analysis, Nervilia singaporensis is more related to Nervilia trangensis S.W.Gale, Suddee & Duangjai from Peninsular Thailand than Nervilia punctata from Java, Indonesia. For other Nervilia species (Nervilia lanyuensis S.S.Ying, N. marmorata S.W.Gale, Suddee & Duangjai and N. infundibulifolia) that are genetically more similar to the locally endemic orchid than similar looking Nervilia punctata, their morphologies differ greatly.

Type information

The type specimen was collected on 3 December 2019 by M.A. Nissalo at Bukit Timah Nature Reserve, Singapore. Specimen ID: SING2019-1365. The holotype is a dissected inflorescence without the anther cap. It contains the fruit and stem. Holotype ID: SING0273892.

Photographs of the Nervilia singaporensis holotype parts at the type locality. Emerging leaf (A). Flower being manually opened (B). Lateral view of flower (C). Ventral side of column (D). Dissected perianth (E). (Photos: M.A. Niissalo). ^[7]

Holotype of Nervilia singaporensis drawn by Evonne Tay Koh. The vegetative parts (A-C), flower parts (D-O), fruit capsule (P) and seeds (Q). Scales drawn on right.^[7]

Glossary

References

*All photos taken by author unless stated otherwise

Gratitude towards Matti Niissalo for his permission to produce this species page on Nervilia singaporensis

This page was authored by Png Jun Qiang, Karl [karlpng1997@gmail.com]
Last curated on 8 December 2020