9Recent Control of Invasive Alien Animals in the Bonin Islands
Nozomu SATO
Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University
1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan
E-mail: sato-nozomu@tmu.ac.jp

Abstract
The Bonin Islands are oceanic islands of Japan that have never been connected to the continent, andinclude abundant endemic species and ecosystems. These islands were registered as a UNESCO Natural
World Heritage Site in 2011, along with other islands of the Ogasawaras. The environment of the Bonin
Islands has been disturbed by development and livestock since human settlement began, and many native
organisms remain at risk of extinction due to alien species introduced by humans. To conserve and restore thenative ecosystems, alien animals (cats, goats, pigs, bullfrogs, and rodents) have been eradicated on several
islands through alien species control projects conducted with the cooperation of administrative organs, theprivate sector, and residents. The outcomes of these projects highlight the importance of understanding theinteractions among multiple alien species and considering the indirect effects of alien species control.
However, in recent years, further alien species countermeasures have been required for the big-headed ant andland nemerteans, which have been identified as posing an ecological risk, and for a predatory snail that has
recently invaded. Control strategies for alien animals are associated with many issues, including legal
restrictions, environmental and human impacts, and technical aspects. In particular, it is difficult to eradicate
small-sized species that have high fecundity and invade easily. Therefore, for future protection of the Bonin
Islands as a Natural World Heritage Site, it is essential to strengthen the quarantine system to prevent alien
species from dispersing on other islands, and to prevent new alien species from invading.
Key words :	conservation, management, Natural World Heritage site, oceanic islands, Ogasawara Islands

1. Introduction
The Ogasawara Islands comprise more than 30
oceanic islands south-east of mainland Japan, including
the Bonin Islands (Muko-Jima Islands, Chichi-Jima
Islands, and Haha-Jima Islands), volcanic islands, andseveral small islands. The Ogasawara Islands arecharacterised by an absence of native mammals, except
for the Bonin flying fox (Pteropus pselaphon). Therefore,
endemic species on the islands (e.g. native birds such as
the ground-nesting Japanese wood pigeon, Columba
janthina nitens) have evolved in the absence ofcarnivorous mammals. Furthermore, speciation by
adaptive radiation is remarkable. Land snails show thehighest species diversity, with 98 endemic species amongthe 104 species recorded in the Ogasawara Islands (Chiba,2009). These islands were registered as a UNESCO
Natural World Heritage Site in 2011 because of theuniqueness of the ecosystems and the high academic
value of the biological evolution thereof. However, theecosystems of the Ogasawara have been subjected toextensive human disturbance since people first settled
there in the 1830s. Even today, many native organisms
Global Environmental Research
23/2019: 009-019
printed in Japanand ecosystems are disturbed directly or indirectly byinvasive alien species introduced by humans (Kawakami,
2010a; Sugiura, 2016) (Fig. 1). In general, oceanic island
natives have no defence against natural enemies, which
do not exist on the island, and are significantly vulnerable
to alien species with higher competitive, defensive, andreproductive performance (Carlqust, 1974; Vermeij,
1991). Natural World Heritage site designationnecessitates effective management. It is important tomaintain or improve the environment to ensure thesurvival of native species for future generations, and tocontrol or eradicate invasive alien species that pose athreat. Accordingly, invasive alien species control has
been implemented in the Ogasawara Islands incooperation with various entities, including government
ministries and agencies such as the Ministry of theEnvironment (MOE), Tokyo Metropolitan Government(TMG), National Research and Development Agency, andOgasawara Village, as well as non-profit organisations,consultants, private enterprises, university researchers
and residents. Many residents have a deep understandingof ecosystems and conservation, partly due to information
disclosure and the provision of feedback to residents by©2019 AIRES

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Fig. 1 Distribution of invasive alien animals in the Bonin Islands.
The Bonin Islands consist of three island groups, excluding the Volcanic Islands and other tiny islands: the Muko-Jima group (a),Chichi-Jima group (b), Haha-Jima group (c). The “E” on the legend icon indicates that eradication has already been completed. This map isbased on an electronic map provided by the Geospatial Information Authority of Japan (https://maps.gsi.go.jp/).
Distributions of invasive alien animal are according to the following references: feral cat (Nakayama, 2009; Kawakami, 2010b; 2019),
feral goat (Japan Wildlife Research Center, 1992; Tokyo Metropolitan Ogasawara Island Branch Office, 2018; Suzuki et al., 2019), feral
pig and bull frog (Karube, 2010; Kawakami, 2019), black rat, Norway rat and house mouse (Yabe, 2017; Hashimoto, 2010; Kawakami,2019), green anole (Hasegawa, 1988; Okochi et al., 2006; Toda et al., 2010; Shimizu, 2013), cane toad (Matsumoto et al., 1984; Kishimoto,2009), New Guinea flatworm (Kawakatsu, 1999; Okochi et al., 2010 ; Ohbayashi et al., 2010), honeybee (Funakoshi, 1990; Abe, 2006;
Abe et al., 2010), land nemertean (Kawakatsu, 1999; Shinobe et al., 2017), African big-headed ant (Takahashi et al., 2000; Uchida et al.,2016), red-rimmed melania (Sasaki et al., 2009).

Control of Alien Animals in Bonin Islands
the ministries and research institutions.
The invasive alien species control that occurred up
to the Natural World Heritage Site registration has alreadybeen described (Kawakami, 2008; Kawakami, 2010a;
2010b). Additionally, Sugiura (2016) reviewed domestic
studies on the effects of alien species on native ecosystems
and interspecific interactions. This work outlines trends
in alien animal control projects since registration as a
Natural World Heritage site and the problems associatedwith current alien species control efforts in the Bonin
Islands (Ogasawara Islands, excluding the volcanicislands and small remote islands). Moreover, this article
introduces some newly invading alien species, and someinvasive alien species which have recently been identified
as posing an ecological risk, and also provides an
overview of future issues. Alien plants are discussed inother articles of this journal (see Hata’s and Osawa’s
articles). In this article, the focus is on alien animals
having a severe impact on native ecosystems. The article
does not discuss termites, oriental fruit flies or the giant
African snail, which are mainly of concern in terms of
their effects on human activities, such as housing andcropping.

2. Invasive Alien Species in the Bonin Islands
2.1 Mammalia
Feral cat
The house cat (Felis catus) is a common pet
worldwide, including on islands. In recent years, cats
have posed a severe threat to wildlife due to predatory or
non-predatory killing, which has resulted in the extinctionof some birds and other organisms (Medway, 2004; Losset al., 2013; Woinarski et al., 2017). There are 80 million
or more feral cats (as many as domestic cats) in the
United States, which are estimated to kill 1.4 to 3.7
billion birds, 6.9 to 20.7 billion mammals, 228 to 871
million reptiles, and 86 to 320 million amphibians a year
(Loss et al., 2013). Cats are the greatest threat to thenative ecosystems of islands because they prey on
endemic species (Nogales et al., 2013; Loss et al., 2013;
Shinosaki et al., 2015; Maeda et al., 2019).
In the Ogasawara Islands, cats were introduced as
pets, or for rodent control, to Otōto-Jima, Ani-Jima,
Chichi-Jima, Haha-Jima, Mukō-Jima, Iwo-To, Minamitori-Shima, and Nishi-no-Shima (Kawakami, 2019) (Fig.
1). They continue to threaten the survival of many native
birds. An endemic subspecies of Japanese wood pigeon(C. j. nitens) forages and nests on the ground and shows
no alertness to alien predators. Therefore, many of these
pigeons have been killed by cats, and their populations
have decreased (Kawakami, 2008). In the spring of 2005,
a large number of carcasses of brown boobies (Sulaleucogaster) and wedge-tailed shearwaters (Puffinus
pacificus) were found at the breeding colony in

11

Minamizaki, Haha-Jima. Predation of the seabirds byferal cats was confirmed using trail cameras. By 2006, allbreeding seabirds had been lost, even though the local
government and residents of Haha-Jima began to catchthe cats (Nakayama, 2009). Elimination of feral cats from
Otōto-Jima was completed between 2007 and 2010
(Ministry of the Environment, 2011), and no cats havebeen reported on Ani-Jima since 2008. Cats currently live
on four inhabited islands, as the cat population on MukōJima disappeared naturally (Kawakami, 2010b, 2019)(Fig. 2). Subsequent to the removal of feral cats, wedgetailed shearwaters have been breeding successfully in
Minamizaki since 2007. At Chichi-Jima, feral cats havebeen captured since 2006 (Nakayama, 2009), and thiseffort is still ongoing.
Captured cats are accepted by the Tokyo Veterinary
Medical Association (TVMA), and Ogasawara Kaiun
Co., Ltd. is responsible for transporting cats off theisland. The capture initiative (capturing and surveying
cats in the mountains, transporting them, promoting
public awareness, and detaining the cats temporarily) wasestablished with the cooperation of many administrative
organs and the private sector (i.e. MOE, Forestry Agency,
TMG, TVMA, Institute of Boninology (IBO), Ogasawara
Village, Ogasawara Kaiun and local volunteers)(Nakayama, 2009). Ogasawara Village establishedregulations pertaining to the proper care and ownershipof domestic cats, which requires microchip registration,
castration and keeping cats indoors.
Feral goat
The feral goat (Capra hircus) is an herbivorous
mammal that eats grasses and trees. In addition to leaves,the feral goat also damages shoots and bark. These
animals have destroyed vegetation in places such as the
Hawaiian and Galápagos Islands, causing severe losses toecosystems (Loope et al., 1988; Campbell & Donlan,
2005). The goats’ behaviours exert both direct andindirect effects. Direct effects include the diminishmentof vegetation due to foraging for grasses and seedlings,
bare land resulting from the trampling of vegetation(Coblentz, 1978; Hamann, 1993), and disturbance ofseabird breeding grounds (Suzuki et al., 2019). Indirect
effects include soil erosion due to the destruction ofvegetation (Manzano & Návar, 2000; Hata et al., 2014),
the loss of bird nesting environments (Chiba et al., 2007),
ecosystem degradation, and reduced biodiversity(Campbell & Donlan, 2005).
Goats were introduced to each island of the Bonin
Islands for food in the early 19th century. After thereversion of Ogasawara to Japan in 1968, goats were
eliminated on at least six islands, Mago-Jima, HigashiJima, Hyōtan-Jima, Minami-Jima, Futago-Jima, andHira-Shima, by 1980 (Japan Wildlife Research Center,
1992) (Fig. 2). The habitat of feral goats at the beginningof the 1990s was limited to seven islands of the Muko-

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Fig. 2 Chronology of alien species control projects.
Icons for legends of species and references follow Fig. 1. The starting point of each line indicates the start of an alien capture project, andthe endpoint “Eradicated” indicates the year when eradication was completed. The period indicated by the black line represents the year
from the start of the capture project to the year it was completed (not including preparatiory stages).

Jima and Chichi-Jima groups. Since the 1970s, the TMG,
Ogasawara Village, and Hunting Association of themainland and Chichi-Jima began a goat eradication
project to protect and restore Bonin Island ecosystems by
capturing large numbers of animals using fences, andshooting the few remaining individuals. The goats were
almost totally eradicated from Muko-Jima, NakoudoJima, and Yome-Jima by 2003 (Suzuki et al., 2019) (Fig.
2). Soon after, rapid recovery of herbaceous plants and alarge number of native plants of Trema orientalis andCallicarpa subpubescens were confirmed on NakoudoJima (Hata & Kachi, 2012). The number of seabirds
nesting on Muko-Jima has increased dramatically since
eradication of the goats (Suzuki et al., 2019).
Projects were implemented on the majority of theislands of the Chichi-Jima group, and goat eradication
has been achieved on almost all of the islands: Nishi-Jimafrom 2002 to 2003, Ani-Jima from 2004 to 2009, andOtōto-Jima from 2008 to 2011 (Tokyo Metropolitan
Ogasawara Island Branch Office, 2018). On Chichi-Jima,
extermination work has continued since 1973 (Japan
Wildlife Research Center, 1992) (Fig. 2).
Feral pig
The feral pig (Sus scrofa domesticus) is a domestic
pig raised around the world. Pigs are omnivorous and eat
various organisms (e.g. plant leaves and roots, bird chicks
and eggs, reptiles). They are a threat to endemic species
in many island environments, including oceanic islands(Coblentz & Baber, 1987; Cruz et al., 2005; NogueiraFilho et al., 2010; Murphy et al., 2014). The impacts offeral pigs on native ecosystems include disturbance ofhabitats by excavation and trampling, direct effects of
predation and indirect effects such as dispersal of alien

Control of Alien Animals in Bonin Islands
plant seeds and alteration of vegetation (Nogueira-Filho
et al., 2010).
In the Ogasawaras, the feral pig arrived with theearly human settlers and was subsequently introduced to
Muko-Jima, Nakoudo-Jima, Otōto-Jima, Ani-Jima, ChichiJima, Haha-Jima and Ane-Jima (Kawakami, 2019). At
Otōto-Jima, the feral pig fed on various native plants,
insects, and land snails, and was present until recently(Ministry of the Environment, 2011). When controlling
an alien species, removal needs to be performed withconsideration of the interspecific interactions with otheralien species. On Otōto-Jima, feral pigs ate the alienbullfrog (Lithobates catesbeiana) and reduced its
population; therefore, exterminating the pigs could havecaused an explosive increase in bullfrogs. For this reason,
eradication was achieved by first capturing and reducing
bullfrogs, and then capturing feral pigs. Neither species
has been reported since 2008 and they have likely been
eradicated completely from Otōto-Jima (Karube, 2010).
Rodents (black rat, Norway rat, house mouse)
Rodents are pests that enter the cargo of ships, andmay carry the plague and other zoonotic diseases (Shiels
et al., 2014). Rodents have invaded islands around theworld, where they prey on native organisms, including
birds. It has been reported that 75 species of seabirds
from 10 families have become extinct or had their
population reduced severely by rodents (Jones et al.,2008). Rats have a particularly significant impact onburrowing and below-ground nesting seabirds.
The black rat (Rattus rattus), as the most problematic
species, preys on a variety of organisms, from plants toanimals larger than itself. In New Zealand, two endemicbats (Mystacina robusta and M. tuberculata) have gone
extinct due to black rats (Daniel, 1990). Furthermore, rats
predations is highly related to the decline or extinction of
the green sea turtle (Caut et al., 2008), various lizards andamphibians (Clark, 1981), and terrestrial crustaceans(Page & Meier, 2006; Nigro et al., 2017).
The distribution of rodents in the Bonin Islands(Yabe, 2017; Hashimoto, 2010; Kawakami, 2019) is asfollows. House mice (Mus musculus) are found on OtōtoJima, Chichi-Jima, and Haha-Jima. Norway rats (Rattus
norvegicus) inhabit nine islands: Haha-Jima, Mukō-Jima,
Mei-Jima, Imōto-Jima, Ane-Jima, Hira-Jima, Chichi-Jima(unknown in recent years), Kita-Iwo-To, and Iwo-To.
Black rats have invaded almost all the islands (Fig. 1).
On these islands, rodents prey on various native
organisms, including plants, land snails, and seabirds(Tomiyama, 2002; Kawakami, 2008; Hashimoto, 2010).
The Forestry and Forest Products Research Institute(FFPRI) initially undertook various experimental rodent
eradication projects with support from the MOE.
Subsequently, the MOE and TMG supervised theseprojects. Many of the islands have complex geographic
features, so it was difficult to capture rodents with traps.

13

Therefore, the extermination was mainly carried out by
helicopter using the slow-acting rodenticide diphacinone(Hashimoto, 2010). The extermination project carried out
on Nishi-Jima since 2006 was considered successful,
because there were no confirmed rat sightings over a
period of several years. However, rats have since been
seen again in Nishi-Jima. Rats have also been sighted onother islands where extermination projects were attempted,
including Otōto-Jima, Ani-Jima, Hitomaru-Jima, HyotanJima, Minami-Jima, and Tatsumi-Jima. Eradication was
successful only at Muko-Jima, Higashi-Jima and MagoJima (Fig. 2). However, in recent years, Nakoudo-Jimaand Yome-Jima have been added to the list, and theseislands are currently being monitored for rats.
Regarding whether rodent eradication has been
completely successful, Keitt et al. (2015) stated that “itmay be wise to wait two years to confirm successful
eradication.” It remains unclear whether the rediscovered
rats survived the extermination and increased again, orwere reintroduced from outside. A population can recover
within a few years if a small number of rats survive due
to uneven spraying of rodenticide on large islands withcomplex terrain. In Japan, restrictions imposed by the
Agricultural Chemicals Regulation Act preclude the useof more effective second-generation rodenticides (e.g.
brodifacoum) and limit the amount of toxins that can beapplied in a given area. Therefore, it can be difficult toeradicate rats completely once they have invaded anisland, except in the case of a tiny island. However, even
if rats cannot be eradicated, suppression of the population
allows recovery of native vegetation and renewed
breeding of birds (Kawakami & Aoyama, 2011). Thus, it
is essential to continue monitoring rat populations, and to
remove them regularly to mitigate their impact on theecosystem.
The MOE and TMG continue monitoring rats using
sensor cameras and bait stations (bait traps designed forrats only) (Hashimoto, 2010), including on islands whererats have already been eradicated. On Ani-Jima, rats havebeen controlled using bait stations with rodenticides, orby spraying using drones (because spraying by helicopter
along cliffs risks spills of the chemical into the ocean).
However, this current strategy is insufficient for thereasons mentioned above, and a stronger rodenticide or
improved spraying method is required. However, there
are several barriers to this, such as the restrictions
imposed by the Civil Aeronautics Act on the types of
spreaders that can be mounted on helicopters, restrictions
on the types of rodenticides that can be used under the
Agricultural Chemicals Regulation Act, and therequirement for consensus building with residents where
it is necessary to mitigate any impact on industry andallay concerns regarding the effects of toxins on humanhealth.

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2.2 Reptilia
Green anole
The green anole (Anolis carolinensis) is an arboreal
lizard native to the south-eastern United States that has
invaded Taiwan, Hawaii, Guam, and some islands ofJapan. Anoles prey on various organisms, including
insects, spiders, skinks, and land snails (Karube & Suda,
2004; Makihara et al., 2004; Abe et al., 2010; Takahashi
et al., 2014). The green anole invaded Chichi-Jima
sometime between 1965 and 1966, and Haha-Jima
between 1968 and 1972; its distribution was found tohave expanded throughout these islands by 1990 and
1997, respectively (Hasegawa, 1988; Toda et al., 2010)(Fig. 1). The population is estimated to be as large as 6.38
million on Chichi-Jima, and the species has had a severe
impact on the native ecosystem (Okochi et al., 2006).
Invasive anoles are predators of a wider variety of preythan are the native snake-eyed skink (Cryptoblepharus
nigropunctatus). Therefore, the skinks may have declined
due to interspecies competition or direct predation (Toda
et al., 2010). Additionally, the populations of many
diurnal insects have been reduced drastically, and someendemic species have gone extinct on Chichi-Jima(Makihara et al., 2004; Karube, 2010). As an indirect
effect, the decrease in native bee populations has resulted
in reduced pollination and lower reproduction rates ofnative plants (Abe et al., 2011).
An anole extermination project was implemented in2005 (Toda et al., 2010). The MOE began a control
project in harbour areas to prevent the spread to otherislands, and began exterminating anoles in an area ofnative insect habitats on Haha-Jima. Because anoles
move between trees and the ground, they can be captured
at low cost by installing adhesive traps on tree trunks.
Unfortunately, anoles have a “spatula” on their finger andcan climb even smooth surfaces. Therefore, a Teflon
sheet formed into a roof was attached to the top end of afence for control of anoles (Pacara et al., 1983).
Furthermore, by cutting 2 m of vegetation from aroundthe fence, the invasion of individuals from trees was also
prevented (Toda et al., 2010). Dispersion to other islands
is prevented by removing anoles from around the pier andchecking the ship’s cargo for anoles. However, invasionof anoles into Ani-Jima was confirmed in 2013 (Shimizu,2013) (Fig. 2), and there is concern about the impact on
endemic insect species, such as a grasshopper
Ogasawaraoxya anijimensis. Based on the lessons
learned from the rapid spread of anoles on Chichi-Jimaand Haha-Jima, several defensive fences and adhesive
traps were constructed immediately on Ani-Jima to
prevent expansion of the anole’s distribution.
2.3 Amphibia
Cane toad
The cane toad (Rhinella marina) is a giant terrestrial
toad native to the region from northern Mexico to central

Brazil. This species was introduced around the world for
agricultural pest control but had a severe impact on native
ecosystems due to its high fertility and toxicity (Shine,2010). In Australia, many reptiles have been killed by the
poison of the cane toad (Lever, 2001; Letnic et al., 2008;
Shine, 2010). Cane toads prey on ants, termites andbeetles, and can significantly reduce insect populations(Greenlees et al., 2006).
Cane toads were introduced to Chichi-Jima by the US
military in 1949, to control centipedes and cockroaches(Matsumoto et al., 1984). They subsequently invaded
Haha-Jima, in 1975 (Fig. 1). The cane toad preys on
various arthropods (mainly soil animals, such as ants andterrestrial crustaceans; Matsumoto et al., 1984), and is aparticular threat to nocturnal and ground-dwelling insects(Kishimoto, 2009).
Through fence installation, the MOE is working withresidents to prevent invasion of the toads into breeding
ponds of Hasu-ike in Haha-Jima, and to capture themusing traps. However, even today, large numbers of toads
inhabit broad areas of Chichi-Jima and Haha-Jima, andregional eradication and prevention of spread to otherislands are ongoing issues. Although one individual was
found on the coast of Ani-Jima, no other individuals havesince been found there, so it is considered that this species
has not settled on Ani-Jima.
2.4 Turbellaria
New Guinea flatworm
The New Guinea flatworm (Platydemus manokwari)
is a carnivorous terrestrial planarian native to New Guinea.
These flatworms were introduced for extermination of the
giant African snail Achatina fulica, but have caused theextinction of many endemic land snails in the Pacificislands (Civeyrel & Simberloff, 1996; Cowie, 2001). This
species has high regeneration and proliferation capacities,
is resistant to starvation, and grows rapidly to maturity(in about 3 weeks; Kaneda, 1990; Sugiura, 2009). P.
manokwari has a wide range of feeding habits, not only
preying on land snails but also on other species offlatworms and earthworms (Ohbayashi et al., 2010).
In the early 1990s, invasion of P. manokwari into
Chichi-Jima was confirmed (Kawakatsu, 1999). In little
more than a decade, most of the native land snail
communities on Chichi-Jima were driven to extinction by
the flatworms (Ohbayashi et al., 2010; Okochi et al.,2010; Chiba & Cowie, 2016). To prevent the spread of
the flatworms to other islands, several measures have beentaken at Chichi-Jima. Before entering the conservation
area or boarding a ship from Chichi-Jima to anotherisland, passengers must wash their soles and kill insects
or pests with concentrated acetic acid (for removal of
flatworms and their eggs, as well as the seeds of alienplants). In the land snail conservation area, the MOE is
installing intrusion-prevention fences, evacuating snails
and implementing ex situ conservation for the remaining

Control of Alien Animals in Bonin Islands
land snail populations in the invaded regions. P.
manokwari has not yet been confirmed in Haha-Jima by
to the efforts described above (Fig. 1). However, there areseveral other alien species of flatworm that could reduce
endemic land snail populations (Okochi et al., 2004). It
will be necessary to strengthen the quarantine of materials
such as seedling soil, which represent the main invasionroute for these flatworms.
2.5 Insecta
Honeybee
The honeybee (Apis mellifera) is a domestic animal
introduced all over the world for honey production. Bees
were introduced to the Ogasawara Islands for beekeeping
in the 1880s, and have since gone feral on Chichi-Jimaand Haha-Jima. Moreover, they have invaded otherislands of the Chichi-Jima group: Ani-Jima, Otōto-Jima,
Minami-Jima, and Nishi-Jima (Funakoshi, 1990; Abe,
2006; Abe, 2010) (Fig. 1).
This eusocial bee engages in more foraging activity
than solitary bees and has a negative impact on native
species due to competition (Kato et al., 1999; Traveset &Richerdson, 2006). Some recent studies have indicatedthat honeybees disrupt the pollination network of nativeplants (Abe, 2010; Abe et al., 2011; Tsujimura et al.,2015). On the islands invaded by green anoles, honeybees
are dominant and act as pollinators of native and alienplants, instead of native bees, because anoles prey much
more on native bees than on honeybees (Karube & Suda,
2004; Abe, 2006; Abe et al., 2010, 2011). This regime
shift is significant for diurnal pollinators that are prey ofdiurnal anoles (Tsujimura et al., 2015). Honeybees
function as pollinators in environments where native bees
have disappeared, but it has been pointed out thatpollination efficiency is reduced due to their excessive
consumption of pollen (Butz Huuryn, 1997). Hirota et al.,(2000) reported that honeybees often act as nectarrobbers, without completing the pollination of nativeplants such as Hibiscus glaber. In addition, some nativeplants are not visited by alien bees. The elimination ofhoneybees is technically difficult, and removal may cause
further damage to the native plant population (becausehoneybees have partially assumed the role of lost native
pollinators). For these reasons, honeybees have not yet
been eliminated.

3. Outcomes of Efforts to Control Alien Species
The alien species control projects undertaken in the
Ogasawara Islands have achieved several notablesuccesses (i.e. in the control of cats, goats, pigs, rodents,
and bullfrogs) through collaborations between manyorganisations and local residents. Vertebrates that haverelatively slow breeding cycles and populations that areeasy to capture (e.g. feral cats, goats, and pigs), have beenquickly eradicated. Small animals, such as rodents and15

anoles, that increase explosively in a short period of time,
have been eradicated only on small islands (Figs. 1 and
2). However, on the inhabited islands, which are large
and have complex terrains, such as Chichi-Jima andHaha-Jima, only local eradication or temporary density
control has been achieved. Furthermore, these islands arealways at risk of invasion by new alien species due to the
movement of people and goods.
When attempting to eradicate aliens, it is essential totake account of the fact that multiple alien species canbecome established on the same island, because alienorganisms engage in new interactions in the invaded
ecosystems (Simberloff & Von Holle, 1999), which can
indirectly produce unexpected results. The “release” ofmesopredators from the control of a top predator (Souléet al., 1988; Crooks & Soulé, 1999) is often a problem;
populations of medium-size predators released in thismanner can increase rapidly and negatively affectorganisms in lower cascades. For example, theextermination of cats on islands in northern New Zealand
resulted in an increase in population of the invasive
Polynesian rat Rattus exulans, which preyed on seabird
chicks and eggs and reduced the breeding success rate of
the providence petrel Pterodroma solandri (Rayner et al.,2007). Eradication of feral pigs and bullfrogs in OtōtoJima was achieved using a control method that consideredmesopredator release (see Feral pig).
Eradication of alien herbivore animals can result in a
rapid increase in suppressed alien plants and othercompetitors (Zavaleta et al., 2001). At Muko-Jima, aninvasive alien lead tree (Leucaena leucocephala), which
had been controlled by feral goats, flourished on bareland and expanded its distribution rapidly after eradicationof the goats (Hata & Kachi, 2009; Osawa et al., 2016).
Thus, restoration of the ecosystem could not be achieved
only by eliminating the goats, as the native vegetation
could not recover from the establishment of the lead tree
community. Also, after the goat eradication, the numberof competing black rats increased, which in fact increased
the negative impact on native organisms (Tokita, 2006).
Thus, when exterminating goats, it will be necessary tosimultaneously exterminate alien trees and rodents.
For the reasons mentioned above, when attempting
to restore an ecosystem, it is important to understand theinteractions of all alien and native organisms after
exterminations, and to control certain alien organisms andanticipate all effects resulting from species extermination(Zavaleta et al., 2001). These findings, which have been
corroborated by the alien species control projects
conducted in the Bonin Islands provide a basis for future
efforts to control newly introduced alien species.

4.	Issues Pertaining to Future Alien Species
Control
The risk to ecosystems posed by some alien species,

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whose aggressiveness has become apparent in recent
years, has also become a concern. For example, theAfrican big-headed ant (Pheidole megacephala) has asevere impact on ecosystems, killing many invertebrates(e.g. ants, thrips and spiders) and thereby reducing their
populations (Reimer, 1988; Gillespie & Reimer, 1993;
Hoffman et al., 1999; Holway et al., 2002). This species
was confirmed in Chichi-Jima in 1999 (Takahashi et al.,2000), and later expanded to Haha-Jima. P. megacephalacaused catastrophic damage to micro land snails(Achatinellidae) on Haha-Jima, and snail habitats in theinvaded areas were almost lost between 2005 and 2015
(Uchida et al., 2016). The TMG has been using bait traps
and monitoring the invaded areas since 2015, as acountermeasure against the ants. Extermination projects
began on Haha-Jima in 2015, and on Chichi-Jima in2017. Although reductions in ant populations anddistribution have been achieved in some areas, control isdifficult in areas where bait trap placement is
problematic, such as on steep slopes (Ministry of theEnvironment, 2019). Therefore, continuous control
activities are required.
Geonemertes pelaensis is a land nemertean identified
on Chichi-Jima in 1981, and on Haha-Jima in 1995
(Kawakatsu, 1999). However, its impact on the ecosystem
was not immediately obvious. In recent years, it has beenreported that this rapidly increasing nemertean species
preys on soil invertebrates, such as isopods andamphipods, causing devastating damage (Shinobe et al.,2017). Predation on land snails has also been reported,and there are concerns about the impact on native landsnails (Gerlach, 1998).
As a new alien species, Macrochlamys sp. was foundon Haha-Jima in 2016 (Ministry of the Environment,2019). This carnivorous land snail preys on native landsnails and thus has a direct negative impact on the
populations thereof (Kimura, 2015). Macrochlamys sp. iscurrently present only around some farmlands on HahaJima, and its distribution is controlled by spraying a slug
pesticide containing metaldehyde (Ministry of theEnvironment, 2019).
There is a tendency to exert less effort towards theconservation of freshwater organisms than terrestrialorganisms. Stenomelania boninensis is an endemic thiarid
snail that inhabits rivers in the Bonin Islands. On ChichiJima, the number of individuals of this species has
decreased significantly due to competition with an alien
thiarid snail, the red-rimmed melania (Melanoides
tuberculata), which has high fertility and an extensive
habitat (Sasaki et al., 2009). Several invasive alien fishes,
including the Mozambique tilapia (Oreochromis
mossambicus), guppy (Poecilia reticulata), mosquitofish(Gambusia affinis), and common carp (Cyprinus carpio),
have been introduced to Chichi-Jima (the guppy was also
introduced to Haha-Jima). The Mozambique tilapia canprey on the endemic freshwater fish Rhinogobius

ogasawaraensis (Yokoi, 2009). However, information on
the distribution of aquatic alien species, and their impacts
on native aquatic organisms is still lacking. More detailed
surveys of aquatic areas and discussions regarding alien
species control are required.
Although alien species control projects in the Bonin
Islands have many issues and complexities, thecooperation of various organisations and residents has
enabled early detection of invasion of some alien species,
such as the Macrochlamys snail. The African big-headed
ant, land nemerteans, and green anoles in Ani-Jima arerecent alien species detected by control projects. The
results indicate that the alien species control system of
the Bonin Islands is functioning well. However, as
demonstrated by previous invasive alien species control
efforts, it is almost impossible to eradicate smallvertebrates and invertebrates with high fertility once they
become established on an oceanic island. Some of thesesmall animals are often mixed in seedling soil, and can be
killed in a warm bath (Sugiura, 2008). To prevent
unexpected alien species invasions at the border, and tomaintain the integrity of the Ogasawara Islands as a
Natural World Heritage Site, it will be necessary tostrengthen quarantine systems, and the monitoring andinspection of intrusion routes.

Acknowledgements
I would like to express my appreciation to Dr. Naoki
Kachi for helpful comments, and anonymous reviewers
for providing constructive suggestions on this manuscript.
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19

Nozomu SATO
Nozomu Sato is a project researcher at theGraduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, Japan.
He received his Ph.D. from Iwate University in2017. His research interest is in the behavior andevolution of invertebrates, such as insects andland snails. He is currently working on analyzing
biological interactions and food chains in island ecosystems.
(Received 16 November 2019, Accepted 26 December 2019)