Bull. Natl. Mus. Nat. Sci., Ser. B, 38(3), pp. 131–137, August 22, 2012

Discovery of Geodorum densiﬂorum (Orchidaceae) on the Ogasawara(Bonin) Islands: A Case of Ongoing Colonisation Subsequent toLong-distance Dispersal
Tomohisa Yukawa1,* Dairo Kawaguchi2, Akitsugu Mukai2 and Yoshiteru Komaki3
1

Department of Botany, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan
2
Ogasawara Branch Ofﬁce, Bureau of General Affairs, Tokyo Metropolitan Government,
Hahajima, Ogasawara-mura, Tokyo 100–2211, Japan
3
Botanical Gardens, University of Tokyo,
3–7–1 Hakusan, Bunkyo-ku, Tokyo 112–0001, Japan* E-mail: yukawa@kahaku.go.jp(Received 31 May 2012; accepted 26 June 2012)

Abstract Geodorum densiﬂorum (Lam.) Schltr. (Orchidaceae) is newly recorded for the Ogasawara
Islands, Japan. The species was found on Mukoujima Island, Hahajima Group, where only a single
population of 108 individuals occurs. This case probably represents recent long-distance dispersal.
Regular monitoring in the future may allow the process of colonisation of an oceanic island to be
documented.
Key words : colonisation, Geodorum densiﬂorum, Japan, long-distance dispersal, new record,
Ogasawara Islands, Orchidaceae.

Identiﬁcation of a Geodorum species from theOgasawara Islands
The Ogasawara (Bonin) Islands are an archipelago of about 30 subtropical islands, situated1,000 km south of Tokyo (Fig. 1). They are oceanic islands formed around 48 million years ago.
Since they have never been connected with acontinent, the biota has undergone unique evolutionary processes and about 43% of the plant
species are endemic to the islands (Ono et al.,1986).
In 7 December 2009, park rangers of Tokyo
Metropolitan Government (the second author
together with Kyoko Ohno and Yuko Honma)
conducted a vegetation survey on Mukoujima
Island, an uninhabited, small island of 1.38 km2
belonging to the Hahajima Group (Fig. 1). They
found about ten orchid plants in a 5×5 m squarethat did not match any known orchid species

recorded from the Ogasawara Islands. Following
regular surveys, ﬂowering plants were found on
20 August 2011 (Fig. 2).
The plants are identiﬁable as Geodorum densiﬂorum (Lam.) Schltr. (Fig. 3) but the taxonomic
status of this entity is still not stabilized (e.g.,
Garay and Sweet, 1974; Seidenfaden, 1983;
Clements, 1989; Kores, 1989). The basionym
Limodorum densiﬂorum Lam. is based on a preLinnean taxon ʻBela Polaʼ in Hortus Malabaricus(Rheede tot Draakestein, 1692). The Ogasawara
plant agrees with the lectotype [tab. 35 associated with the description of ʻBela Polaʼ; selected
in Seidenfaden (1983)] collected from south
India. Descriptions and illustrations of G. densiﬂorum from the same area (e.g., Santapau andKapadia, 1966; Rao, 1998) also coincide with
our material. We therefore identiﬁed the Ogasawara plant as G. densiﬂorum.
Geodorum densiﬂorum is widely distributed

Tomohisa Yukawa et al.

132

Fig. 1.

Location of the Ogasawara Islands. Mukoujima Island belongs to the Hahajima Group.

throughout tropical Asia and Australasia. The
range encompasses Sri Lanka, India, Himalayas,
Indochina, south China, Taiwan, and the Ryukyu
Islands to Southeast Asia, north Australia, Melanesia, and Micronesia (Barretto et al., 2011). We
observed material collected from the Ryukyu
Islands and Indonesia and found it to be identicalto the Ogasawara plants in both vegetative andreproductive morphological characters. The central callus of the lip is variable in shape, but variation exists even within a single individual (Fig.
4). We further consulted illustrations of thisentity in ﬂoras of other regions [e.g., Pearce andCribb, 2002 (Bhutan); Barretto et al., 2011
(China); Su, 2000 (Taiwan); Smith, 1908 (Java,
as G. purpureum R. Br.); OʼByrne, 1994 (NewGuinea); Hallé, 1977 (New Caledonia, as G.
pictum (R. Br.) Lindl.); Dockrill, 1992 (Australia); Raulerson and Rinehart, 1992 (Mariana
Islands)] and conﬁrmed the conspeciﬁcity acrossthe distribution range.

Characteristics of the Ogasawara population
The Ogasawara population of Geodorum
densiﬂorum was investigated on 20 August and
28 November 2011. The plants constitute a single
population in the northern part of Mukoujima
Island. In total, 108 individuals were foundgrowing along a stream 150 m in length andranging in elevation from 70 to 95 m a.s.l. Seven
plants ﬂowered in August. Both vegetative andﬂoral dimensions vary greatly (number of leaves:
1–4, length of the uppermost leaf: 3–31 cm,
width of the uppermost leaf: 5–70 mm, length of
inﬂorescence: 12.5–35 cm, number of ﬂowers:
10–17, length of bract: 8.5–14 mm, length ofpedicellate ovary: 7.5–11 mm, length of dorsal
sepal: 11–12.5 mm, length of lateral sepal:
11–12.5 mm, length of petal: 11.5–12.5 mm,
length of lip: 10.5–13 mm).
The plants inhabit rather open, dry to mesic
sites along the stream. We recorded vegetation inassociation with Geodorum densiﬂorum at threepoints (Table 1). Methods for recording vegeta-

Discovery of Geodorum densiﬂorum on the Ogasawara Islands

133

Fig. 2. Geodorum densiﬂorum (Lam.) Schltr. from the habitat of Mukoujima Island, Ogasawara Islands, Japan.
A. Flowering plant; B. Inﬂorescence; C. Habitat; D. Fruiting plant. Photographs taken on 9 September 2011(A), 20 August 2011 (B, C) or 17 February 2011 (D).

tion composition followed Shimizu (1994). Point
A was a dry, open hill dominated by Casuarina
equisetifolia L. in the tree layer and by Gahnia
tristis Nees in the herbaceous layer. Point B was
scrub in which Planchonella obovata (R. Br.)
Pierre, Casuarina equisetifolia, and Rhaphiolepis
indica (L.) Lindl. ex Ker var. umbellata (Thunb.)
H. Ohashi were predominant. Point C was scrub
bordered by the stream and at this site mostGeodorum plants had been damaged by a ﬂood.
This site was dominated by Leucaena leucocephala(Lam.) de Wit, Livistona boninensis (Becc.)
Nakai, and Pandanus boninensis Warb. in thetree layer and by Oplismenus compositus (L.) P.
Beauv. in the herbaceous layer. These sites represent typical secondary vegetation of the Oga-sawara Islands (Shimizu, 1994). Casuarina equisetifolia and L. leucocephala are invasive alien
species that alter the original vegetation.
Mizutani et al. (2003) surveyed twelve habitats of Geodorum densiﬂorum on the Ryukyu
Islands. In that study, the species was found tofavour open sites, such as roadsides, edges ofsecondary forests, and grasslands. These secondary habitat types had developed artiﬁcally usually as a result of deforestation or road construction. The ecological characteristcs of the Ryukyu
populations are in line with those of the Ogasawara
population. It is clear that G. densiﬂorum is apioneer species that colonises disturbed places.
Fig. 5 shows the size distribution of the population as determined by the length of the upper-

134

Tomohisa Yukawa et al.

Fig. 3. Geodorum densiﬂorum (Lam.) Schltr. A. Flower, side view; B. Dorsal sepal; C. Petal; D. Lateral sepal; E.
Lip; F. Lip and column, side view; G. Column, from below; H. Column, side view. Operculum and pollinarium
detached; I. Column, after auto-pollination. Pollinia deposited on the stigma; J. Pollinarium; K. Operculum.
Drawn from Dairo Kawaguchi 1, Japan, Ogasawara Islands, Mukoujima Island, 20 August 2011 (TNS) by
Mutsuko Nakajima. Scale bar＝5 mm (A–F), 3 mm (G–I) or 1 mm (J, K).

most leaf. Since this species does not form clonalcolonies (T. Yukawa, unpublished observation),most individuals of smaller size classes represent
seedlings. Mizutani et al. (2003) estimated thatfour years or more are needed to reach the reproductive stage in this species. Our data indicatesthat Geodorum densiﬂorum can shift to thereproductive stage when the uppermost leaf
attains 10 cm or more in length. It is thus reasonable to assume that seedling recruitment occurred

relatively recently in this population.
Dispersal and colonisation of
the Ogasawara population
The Flora of the Ogasawara Islands has been
well investigated and documented (e.g., Tuyama,
1970; Kobayashi and Ono, 1987; Toyoda, 2003).
It is unlikely that previous surveys overlooked
such a conspicuous plant in this small island

Discovery of Geodorum densiﬂorum on the Ogasawara Islands

135

Fig. 4. Variation in lip morphology of Geodorum densiﬂorum (Lam.) Schltr. from plants of different localities.
Two ﬂowers in a single inﬂorescence were illustrated. A and B. Daisuke Ishikawa 1010, Japan, Ryukyu
Islands, Ishigakijima Island, ﬂowering in cultivation 10 November 2011, Tsukuba Botanical Garden (TNS); C
and D. Dairo Kawaguchi 2, Japan, Ogasawara Islands, Mukoujima Island, 20 August 2011 (TNS); E and F.
Dairo Kawaguchi 1, Japan, Ogasawara Islands, Mukoujima Island, 20 August 2011 (TNS); G and H. Ayub
Parnata s. n., Indonesia, ﬂowering in cultivation 12 October 2009, Tsukuba Botanical Garden (TNS). Drawn
by Mutsuko Nakajima. Scale bar＝5 mm.

group. It is also improbable that the plants wereintroduced artiﬁcially because Geodorum densiﬂorum does not have any economical or ornamental merits and Mukoujima Island has
remained uninhabited. Besides, only a single
population of this species has been found on
Mukoujima Island. Consequently, the Ogasawara
population of G. densiﬂorum is probably derived
from recent migration by long-distance dispersal.
The colonising nature of Geodorum densiﬂorum
has been well documented by its distribution on
oceanic islands such as Micronesia (Yap andGuam) and Melanesia (Solomon Islands, Vanuatu, Fiji, Samoa, Tonga, and Niue). Furthermore,this species was recorded at early successional
stages of the vegetation on the island of Krakatau

in the middle of Sunda Strait, about 40 km from
Sumatra and Java, after the volcanic eruption in
1883. The eruption completely sterilised three of
the islands in the Krakatau group and ﬂoristic
surveys of these islands have been conductedsince. G. densiﬂorum was recorded as one of 21
pioneer orchid species in 1929, 46 years after the
eruption (Partomihardjo, 2003).
A high rate of fruit set also characterises
Geodorum densiﬂorum. Mizutani et al. (2003)
observed that 58.3% of ﬂowering individuals set
fruits. We recorded that two of the three ﬂowering plants of the Ogasawara population bore
capsules in 2011 (Fig. 2D). The operculum of
this species detaches easily and auto-pollination
occurs frequently due to limited development of136

Tomohisa Yukawa et al.

Table 1. Vegetation associated with the Geodorum densiﬂorum population on Mukoujima Island, Ogasawara
Islands.
Point
Tree coverage (%)
Herbaceous coverage (%)
Relative abundance of tree species (%)
Leucaena leucocephala (Lam.) de Wit
Trema orientalis (L.) Blume
Livistona boninensis (Becc.) Nakai
Neisosperma nakaianum (Koidz.) Fosberg et Sachet
Hibiscus glaber (Matsum. ex Hatt.) Matsum. ex Nakai
Melicope grisea (Planch.) T. G. Hartley
Drypetes integerrima (Koidz.) Hosok.
Cinnamomum pseudopedunculatum Hayata
Planchonella obovata (R. Br.) Pierre
Pandanus boninensis Warb.
Syzygium cleyerifolium (Yatabe) Makino
Melia azedarach L.
Rhaphiolepis indica (L.) Lindl. ex Ker var. umbellata (Thunb.)
H. Ohashi
Celtis boninensis Koidz.
Casuarina equisetifolia L.
Wikstroemia pseudoretusa Koidz.
Ligustrum micranthum Zucc.
Myrsine maximowiczii (Koidz.) E.Walker
Composition of herbaceous species
Oplismenus compositus (L.) P. Beauv.
Euphorbia cyathophora Murray
Rivina humilis L.
Pteris boninensis H. Ohba
Asplenium setoi N. Murak. et Seriz.
Trachelospermum asiaticum (Siebold et Zucc.) Nakai
Carex sp.
Gahnia tristis Nees
Sphenomeris biﬂora (Kaulf.) Tagawa
Miscanthus boninensis Nakai ex Honda

A
20
40

B
70
15

C
70
20

0
0
0
0
0
0
0
0
7
11
13
0
5

1
1
1
0
3
1
0
2
35
8
8
1
14

26
3
12
6
7
9
2
6
10
12
4
1
1

0
58
5
1
0

0
19
3
2
1

1
0
0
0
0

○ (dominant)○○○○ (dominant)○○ (dominant)
○
○
○
○
○
○

Fig. 5. Size distribution of the population of Geodorum densiﬂorum (Lam.) Schltr. on Mukoujima Island, as
determined by the length of the uppermost leaf. Values on the x-axis are the upper size limit for each class.
Dark grey bars show the number of the non-ﬂowering plants and light grey bars show the number of the ﬂowering plants.

Discovery of Geodorum densiﬂorum on the Ogasawara Islands

the rostellum (Fig. 3I). Facultative autogamy
facilitates expansion of this species where potential pollinators are absent or few.
The closest extant populations of Geodorum
densiﬂorum from the Ogasawara Islands are either
those of the Ryukyu Islands or those of Micronesia, both of which are about 1500 km distant fromthe Ogasawara Islands. Use of molecular markers
could elucidate the history of long-distance dispersal of the Ogasawara population.
Our ﬁnding probably represents recent longdistance dispersal. Regular monitoring of thispopulation and of the archipelago as a whole in
the future may allow the process of colonisationof an oceanic island to be documented.
Acknowledgments
We would like to thank Kyoko Ohno and Yuko
Honma for the assistance of ﬁeld surveys,
Stephan Gale for critical reading of the manuscript, and Mutsuko Nakajima for preparing
illustrations. This study is partly supported by aGrant-in-Aid to Scientiﬁc Research from theJapan Society for Promotion of Science (No.
21370038) and a research grant from the
National Museum of Nature and Science for T.
Yukawa.
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