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Minami-Iwo-To: The Last Vestige of Wilderness
in the Ogasawara Islands
Kazuto KAWAKAMI
Department of Wildlife Biology, Forestry and Forest Products Research Institute
1 Matsunosato, Tsukuba, Ibaraki 305-8687, Japan
E-mail: kazzto@ffpri.affrc.go.jp

Abstract
Minami-Iwo-To is a small, isolated member of the Volcano Islands, part of the Ogasawaras. It is a rareexample of an island that has maintained a complete original ecosystem without human disturbances orinvasive alien species owing to its severe topographical conditions. The most significant feature of MinamiIwo-To is that tens of thousands of seabirds breed there. Seabirds fulfill various ecosystem functions—they
disperse seeds, add soil nutrients via their excrement and physically alter habitats. As an ecosystem withseabirds, Minami-Iwo-To can serve as a model for nature restoration programs in the Ogasawara Islands.
Key words : alien invasive species, conservation, ecological function, seabirds, Volcano Islands

1. The Value of Minami-Iwo-To
Minami-Iwo-To is a small, isolated member of the
Volcano Islands, which are part of the Ogasawara Islandsof southern Japan. It is a subtropical oceanic island withan area of approximately 3.54 km2, a diameter ofapproximately 1 km, and a maximum elevation of 916 m(Fig. 1). The island’s topography is steep and surroundedby high cliffs and it has no freshwater rivers or ponds.
The greatest asset of Minami-Iwo-To is its original
ecosystem, which has yet to be disturbed by humanactivities. Because of its topography, it has never been
inhabited by humans, and few alien species have invaded
it. One of the barriers to anthropogenic disturbance is its
isolation: the nearest island on which people and various
alien species can be found, Iwo-To, is 60 km away.
The Ogasawara Islands are an oceanic chain
composed of the Bonin Islands (Muko-Jima, Chichi-Jima,and Haha-Jima islands), the Volcano Islands, and threesmaller, remote islands. Modern human settlement began
in 1830, and as many as 14 of the islands have been
inhabited in the past (Chiba et al., 2007). Feral goats(Capra hircus), cats (Felis catus), and rodents have been
introduced to at least 20, 8 and 29 of the islands,
respectively, and these species have had a major impact
on the native ecosystems (Kawakami, 2019). Alien
plants, such as Casuarina equisetifolia and Leucaena
leucocephala, have also spread widely on the uninhabitedislands, threatening native ecosystems (Kawakami,2008).
To restore these ecosystems, Japan’s Ministry of theEnvironment, Forestry Agency and Tokyo Metropolitan
Global Environmental Research
23/2019: 077-081
printed in Japan

Government have conducted invasive alien species
eradication programs. Feral goats, cats, pigs (Sus scrofa),
and bullfrogs (Lithobates catesbeianus) on theuninhabited islands have already been exterminated, andecosystem recovery is expected (Kawakami, 2008; 2019).
While the elimination of invasive species has beensuccessful, the target ecosystem characteristics areunclear in such restoration programs because the naturalstate of ecosystems prior to human disturbance was notwell documented.
As previously mentioned, Minami-Iwo-To featuresan original, pre-disturbance ecosystem. Various species

Fig. 1 
Appearance of Minami-Iwo-To as seen from the south.
Black arrow, coastal collapse area on the southeast; whitearrow, valley with survey route. (Photo by Kazuto
Kawakami, 23 June 2017.)©2019 AIRES

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K. KAWAKAMI

are endemic to the Ogasawara Islands, original
evolutionary processes have been preserved andconservation of their biodiversity is a worthy goal.
Minami-Iwo-To Island offers a rare example of desirable
target conditions for natural restoration programs.
The Bonin Islands are old Eocene islands, but the
Volcano Islands date only to the late Pleistocene(Kawakami, 2019). The disparate geological histories
mean that the biota of the Volcano Islands differs fromthat of the other islands. The Volcano Islands comprise
three islands: Kita-Iwo-To, Iwo-To, and Minami-Iwo-To.
Since humans settled on Kita-Iwo-To and Iwo-To, theecosystems of those islands have been disturbed andmany extinctions have been reported. For example,
Japanese bush-warblers (Cettia diphone) and oriental
greenfinches (Chloris sinica) can no longer be found on
the two islands (The Ornithological Society of Japan,2012). Minami-Iwo-To therefore has a role to play inmaintaining the biota of the Volcano Islands.

2.	Research Expedition and Measures against
Alien Species Invasions
Surveys of Minami-Iwo-To have been limited by its
steep topography. Thus far, only four surveys that
included mountaintop areas have been conducted, in
1936, 1982, 2007 and 2017 (Suzuki et al., 2018b).
Minami-Iwo-To has been designated a Wilderness Areaby the Ministry of the Environment and a naturalmonument by the Agency for Cultural Affairs. Landingon the island is strictly regulated to prevent disturbanceof the ecosystem. Academic research is conducted only
once every 10 years.
The most recent survey in 2017 examined
vegetation, vascular plants, fungi, mammals, birds,
arthropods, reptiles, large soil animals, crustaceans, landsnails, corals and intertidal shellfish (Suzuki et al.,2018b). Geological surveys were conducted in 2007
(Nakano, 2008). Professional climbers, divers and adoctor were included in previous surveys to ensure
participant safety.
In the 2007 and 2017 surveys, strict quarantines
were observed to avoid introducing alien organisms andcontaminating the ecosystem (Kaga & Sasaki, 2018).
New clothes and survey equipment were used as much as
possible. If new products could not be prepared, the gear
was cleaned as much as possible by refrigeration,
washing with ethanol and visual cleaning. Baggage was
packed at Chichi-Jima, the departure point. To prevent
small seeds and animals from joining the research
expeditions, the baggage was packed in a clean fumigatedroom during daylight hours to avoid contamination by
insects attracted to artificial light. Survey members
performed wet landings, approaching the coast in an
inflatable boat and swimming before landing. This wasdone not only to accommodate the topography but also toensure that investigator luggage was washed withseawater. The investigators transported all excrement toChichi-Jima as a further measure to minimize
disturbance. As a precaution, they refrained from eating
seed-containing fruit for a week before the survey.
Because bringing organisms from Minami-Iwo-To to
Chichi-Jima could disrupt Chichi-Jima’s biota, thereturned baggage was frozen to prevent animals fromescaping. Equipment that could not be frozen was washedwith ethanol and visually checked. Although a few plantseeds, spiders, ants, rove beetles and land snails weredetected in the equipment from the pre- and post-survey
quarantines in 2017 (Kaga & Sasaki, 2018), thequarantine procedures proved useful.

3. Biota of Minami-Iwo-To
The island is surrounded by cliffs of up to 300 m inheight, and coastal vegetation is limited to thesoutheastern and southwestern collapsed areas. Cloudforests dominated by Machilus kobu and Cyathea
tuyamae are common above 500 m above sea level(Shumiya et al., 2018). Forests composed of Elaeocarpus
zollingeri var. pachycarpus and Boninia grisea
predominate at approximately 300 m above sea level, andMelia azedarach and Planchonella obovata are dominant
in the low-attitude forests (Shumiya et al., 2018).
Landslides are common, and various transitional stages,
such as herbaceous communities and shrubs, can be seenat post-landslide sites.
A total of 135 vascular plants have been confirmed,
including 44 ferns, 64 dicotyledonous plants, and 27
monocotyledonous plants (Takayama et al., 2018). In the2017 survey, an endangered orchid, Liparis hostifolia,
which had not been reported since 1938 and was thought
to be extinct, was reported (Takayama et al., 2018). No
mammal is known to inhabit the island except for Bonin
flying foxes (Pteropus pselaphon). The species isnocturnal on other islands but is known to be active
during the day on Minami-Iwo-To (Suzuki et al., 2018a),
possibly because the lack of available food on the island
may extend food search time. Six land birds and nineseabirds breed on this island, with the number of breedingseabird species being the largest in the Ogasawara Islands(Kawakami et al., 2018). Two reptiles, Cryptoblepharus
nigropunctatus and Perochirus ateles, have been reported(Kawakami & Murakami, 2018). The former is widelydistributed in the Ogasawara Islands, while the latter
exists only in Minami-Iwo-To in Japan. To date, 184
species of insects have been identified, including endemic
species, such as Temnothorax mekirai and Satozo
minamiiwoensis (Mori et al., 2018). Six species belong tolarge terrestrial isopod and decapod crustaceans (Sasaki
et al., 2018). Geograpus grayi has been confirmed in allareas, including the summit at 916 m above sea level.
Fourteen land snail species have been detected andMinami-Iwo-To: The Last Place with real Wilderness
Lamellidea sp. is possibly a new species endemic to thisisland (Wada & Chiba, 2018).
The Volcano Islands are geographically close to theBonin Islands and they share many Ogasawara endemic
species. Examples include plants (such as Pandanus
boninensis and Ficus boninsimae), Bonin flying foxes,
Bannerman’s shearwater (Puffinus bannermani), andothers. Although related species are distributed in the
Bonin Islands, in some species, the most closely relatedgroup is found on the Izu Islands and mainland Japan to
the north. This includes plants, such as Eurya japonica
and Melastoma candidum var. alessandrense, as well asbrown-eared bulbuls (Hypsipetes amaurotis) (Sugita etal., 2016). In the case of the land snail Vertigo kodamai,
its origin is presumed to be Hokkaido, 2000 km north of
the island (Nekola et al., 2018). Because island organisms
often face reduced mobility (Carlquist, 1974), related
species that have evolved in the Bonin Islands may not beable to migrate to the Volcano Islands, and distantpopulations may populate.

4. Seabird-driven Ecosystem
Numerous seabirds breed throughout Minami-IwoTo. Nine species of seabirds breed there, with tens of
thousands of breeding pairs reported. Heavier, middleweight and lighter seabirds breed on the coast, at middle
elevations and around the summit, respectively (Fig. 2;
Kawakami et al., 2018). Because weight tends tocorrespond directly with competitiveness, larger species
should be distributed at low elevations where it is easy toapproach the sea. Conversely, because less energy isrequired flying to a highland area, the cost of breeding at
a high altitude might be lower for smaller species. Lightweight Bulwer’s petrels (Bulweria bulwerii) likely breed
at low elevations because they can avoid competing withthe other burrow-nesting petrels by nesting on the ground79

at the coast.
To estimate breeding environment preferences, thelocations of seabird nests were recorded along the coastand climbing routes in June 2017 (Kawakami et al.,2018). Figure 3 shows a heat map of where the nests ofseabirds are concentrated, overlaid on a vegetation map(Biodiversity Center, Ministry of the Environment, 2014).
On the coast, seabird nesting was concentrated in thesoutheast, the lower part of a collapsed area with a shrub
forest (Fig. 1). Nesting density was low in an area withhigh cliffs and no vegetation. In mountainous areas,
nesting density was high at middle elevations, whichwere covered by well-developed M. kobu forest. Low
nesting density was reported in collapsed areas, bushes
and shrubs. Large numbers of burrow-nesting seabirds,
such as Bonin petrels (Pterodroma hypoleuca) andMatsudaira’s storm petrels (Oceanodroma matsudairae),
breed on Minami-Iwo-To. Forests with thick soils are
considered suitable environments for seabirds that nest
underground.
Most pelagic seabirds can fly hundreds of kilometers
a day, forage on fish and plankton in the sea and breed onland. This unique combination of behaviors means thatseabirds perform various ecological functions. On
oceanic islands, seed dispersal by birds plays animportant role in flora formation (Carlquist, 1974; Price& Wagner, 2018). In the Ogasawara Islands, seabirds
have been shown to be effective adhesive seed dispersers(Aoyama et al., 2012). Seabirds supply nutrients, such as
nitrogen and phosphorus, from the sea to the land throughexcrement, although plant growth can be suppressed bytrampling and burrowing (Ellis, 2005). Recently, certain
species of Lepidoptera have been found in the nests ofpetrels in the Ogasawara Islands, which can provide
preferable habitats and food resources (Nasu et al., 2014).
Because seabirds breed in large colonies containing
numerous individuals, their role in ecosystem

Fig. 2 R elationship between seabird body weight and elevation of nesting areas. Brown, areawith relative high-density colonies; peach, area with relative low-density colonies; white,area without colonies.

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K. KAWAKAMI

Fig. 3 Heat map of seabird nesting overlaid on a vegetation map. Black, circular buffer around each nest with 100 m radius; white, highdensity nesting area estimated by kernel density estimation; thick black line, approximate surveyed area; arrow, coastal collapse areawith shrub forest; pink, M. kobu forest; green, bush and shrub forests; light green; grassland; grey, bare ground.

maintenance is thought to be critical. This appears to be
the case on Minami-Iwo-To. Forest floor vegetation ispoor, likely due to disturbance associated with petrels.
Seabirds with seeds attached to their feathers have beenfound to include boobies, tropicbirds and petrels(Kawakami et al., 2018). Terrestrial crabs can be found at
the top of the mountain, and flies occur at high densities,
likely because decomposers thrive in the presence oflarge numbers of seabird corpses and spit production(Mori et al., 2018; Sasaki et al., 2018).
However, seabirds can easily be pushed to extinction
due to the effects of alien species, such as goats, cats androdents. Islands disturbed by humans tend to have fewer
species of seabirds (Chiba et al., 2007). On the
Ogasawara Islands, the distribution of seabirds prior tohuman settlement is poorly understood, but it isreasonable to assume that most of the islands once
maintained large seabird colonies. For example, soil witha high concentration of plant-available phosphate acidwas detected in a forest on Haha-Jima (Morita et al.,2010), which is thought to be evidence of a seabird
colony. The results of the surveys on Minami-Iwo-To

imply that seabirds once played essential roles in theecosystems of the Ogasawara Islands.

5. Conservation Implications
Invasions of exotic herbs, such as Cenchrus
echinatus and Bidens alba, were recently recorded on
Minami-Iwo-To (Takayama et al., 2018). They are
epizoochory plants that were likely transported to theisland by seabirds. In 2016, a tagged booby from
Minami-Jima in the Chichi-Jima Islands was found on
the coast of Minami-Iwo-To, suggesting seabird
migration among islands (Kawakami et al., 2018). In
addition, migratory birds can carry alien species fromneighboring islands. Iwo-To, the nearest island, is home
to alien plants, such as Lantana camara and Ficus
microcarpa. To prevent floral invasions of Minami-IwoTo, it will be necessary to control alien plants on the otherislands.
Minami-Iwo-To experiences frequent landslides andassociated environmental changes due to its steep
topography (Shumiya et al., 2018). This can affect theMinami-Iwo-To: The Last Place with real Wilderness
distribution and viability of plant and animal populations(e.g., Mori et al., 2018). The populations of species on
Minami-Iwo-To are essential for maintaining thebiodiversity of the Volcano Islands, as well as that of theisland itself. This is because various species have become
extinct on the other members of the Volcano Islands due
to anthropogenic disturbance. Monitoring changes inbiota and distribution is necessary for biodiversity
conservation. Additionally, restoration of the ecosystem
in Kita-Iwo-To would contribute to the protection of
Minami-Iwo-To species, because the islands share similar
environments and various species are found on both ofthem. Because the Kita-Iwo-To ecosystem has beenstrongly damaged by an invasion of black rats (Rattus
rattus), the species should be eradicated.
Surveys of Minami-Iwo-To were conducted in June
1982, 2007 and 2017. June was chosen due to calm sea
conditions in that season. The same trail was used eachtime for the mountain surveys, because it was the easiest
route to the island’s summit (Fig. 1). No information istherefore available on island conditions during otherseasons or in other areas. In the future, it will benecessary to conduct surveys in different seasons andalong different routes to obtain a better picture of thebiota of Minami-Iwo-To.
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Kazuto KAWAKAMI
Kazuto Kawakami is a senior researcher at theForestry and Forest Products Research Institute.
His field of specialization is island biology,
focusing on the ecology, biogeography andconservation of birds, mainly in the Ogasawara
Islands. Recently he has been interested in the
primary succession and ecological functions ofseabirds on the pristine new land of Nishinoshima, where new lava groundhas been established by volcanic eruptions since 2013.
(Received 2 October 2019, Accepted 21 November 2019)