21

Establishing a Strategic Management Plan for Alien Invasive Plants
in the Ogasawara Islands
Takeshi OSAWA
Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University
1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397 Japan
E-mail: arosawa@gmail.com

Abstract
In the Ogasawara Islands much effort has been invested in the management of alien invasive species.
There are still many challenges to overcome to achieve management goals such as eradication andcontainment. In all cases, after determining the target alien invasive species for management, effective, lowcost management strategies are needed. Essentially there is a need to establish such management strategies
against alien invasive species, that both 1) specify what kind of management interventions to take and 2)
provide a spatially explicit management plan. In this paper I focus on three problematic alien invasive plants,
Bischofia javanica, Leucaena leucocephala and Casuarina equisetifolia. Aggressive management efforts havebeen made against these, but in some cases no strategic management plan was established so they still cause
problems in the Ogasawaras. For two of the three species, there is a lack one or both of 1) effective
management interventions and 2) a spatially explicit management plan. First, I explain what an effective
management plan consists of, with one specific case in Japan. Then, I overview the current status and futurechallenges toward managing these three alien invasive species. Finally I discuss what further steps are needed
for realizing effective management of alien invasive species in the Ogasawaras.
Key words :	Bischofia javanica, Casuarina equisetifolia, conservation science, Leucaena leucocephala,management strategy, systematic conservation planning

1. Introduction
Biological invasions represent a major ecologicaland economic threat (Mack et al., 2000; Vilà et al., 2010).
Invading alien species sometimes transform ecosystems
by altering their fundamental properties (Blackburn et al.,
2004; Crooks, 2002; Gonzalez et al., 2008). For example,
the numbers of many endemic species have been reduced
due to biological invasion (Gurevitch & Padilla, 2004),
and some alien invasive species act as ecosystemengineers, changing how ecosystems function (Crooks,
2002). Therefore, effective control of alien invasive
species is a basic management objective (Hershner &Havens, 2008).
Among the many ecosystems in the world, oceanicislands are one of the most vulnerable to biologicalinvasion (Caujapé-Castells et al., 2010; Frankham, 1998).
Oceanic islands have essentially high conservation
priority against several drivers of ecosystem degradation,
including biological invasion, because oceanic islands
typically have unique ecosystems with several endemic
species of small individual numbers (Caujapé-Castells etal., 2010; Vitousek, 1988). Unfortunately, many oceanicislands have already suffered negative impacts on their
Global Environmental Research
23/2019: 021-028
printed in Japanecosystems from biological invasions (Vitousek, 2002;
1988).
Oceanic islands often face an unavoidable issue that
promotes biological invasion. That is tourism. Although
recreation and transportation are important drivers of
biological invasion (Kolar & Lodge, 2001), in manycases it is difficult to prevent these drivers in oceanicislands because oceanic islands often attract the generalpublic to tourist sites (Ichiki, 2003). Additionally tourism
is one of the most important industries for oceanicislands, because generally they have few valuable
agricultural products or industrial resources (Ichiki,
2003). Unique ecosystems are an important tourism
resource, but promoting tourism could also be increasing
drivers of degradation, including biological invasion. A
strict balance must thus be maintained between promotion
and restraint of tourism in oceanic islands for sustainable
use of their unique ecosystems.
The Ogasawara (Bonin) Islands are typical oceanicislands, having unique ecosystems with several endemic
species (UNESCO). In the Ogasawaras, one majorindustry is tourism and about 30,000 people visit theislands from outside each year. They are far even fromthe main islands of Japan (to visit the Ogasawaras the©2019 AIRES

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T. OSAWA

only transportation available is a ferryboat which takes
about 24 hours) (Ogasawara Village, 2019).
Unexceptionally there are already too many invasive
species naturalized in the Ogasawaras (Kachi, 2010) andmuch effort has been invested in controlling theseinvasive species (Kawakami, 2010). These efforts haveachieved some positive results. For example, feral goats(Capra hircus), a severely problematic invasive mammal,
have already been eradicated from most of the
Ogasawaras (Osawa et al., 2016a). The stakeholders
believe that management of alien invasive species will beachieved some day in the Ogasawaras. Funding andmanpower, however, are generally limited for controlling
many alien invasive species that have already naturalized(Humston & Mortensen, 2005; Osawa et al., 2016b;
Osawa & Ito, 2015; Shaw, 2005). Actually, the estimatedcost of eradicating alien invasive plants in the
Ogasawaras is high even when focusing on small areas
with a few species (Ogasawara Node, 2017a). Therefore,
establishing a pragmatic management strategy and actual
practices for controlling alien invasive species is needed.

2. What is a Pragmatic Management Plan?
In all cases of managing alien invasive species,
effective, low-cost management strategies are needed(Kumschick et al., 2012; McDonald-Madden & Chades,
2011; Moilanen et al., 2009). Although several studies
have already been done on management strategies foralien invasive species, strategic management should
integrate two essential items, 1) management
interventions, i.e., establishing specific actions for thetarget species and 2) spatially explicit management plans,i.e., prioritization of target areas within the distribution
range of the target species (Foxcroft et al., 2009). To
establish a management strategy for a target invasive
species, we need methods for deciding both “what weshould do” and “where we should do it.” Specifically, forthe former, we need methods of eradicating, depressing
and/or controlling the population of the target species.
For the latter, we need information on the distribution
range of the target species to determine which areas weshould give priority to for management efforts. Land
managers and/or practitioners could establish pragmaticmanagement plans based on combinations of these
specific actions and spatial plans (Foxcroft et al., 2009;
Osawa et al., 2019).
As an example in Japan, such a pragmaticmanagement strategy exists for Rudbekia laciniata L., insome regions. Management of Rudbekia laciniata L., analien invasive herb, was proposed, based on the following
two specific actions: mowing above ground to prevent
further expansion of the invaded area, and digging up andremoving the entire plant body to eradicate it, based on
both their regenerative ability and reproductive
characteristics (Osawa & Akasaka, 2009). Additionally

their habitat requirements, i.e., a method of predicting
their potential distribution in a natural park, had alreadybeen elucidated (Akasaka et al., 2015). Using thisknowledge and local observations, Osawa & Kawano(2019) established a spatially explicit management plan.
They showed where practitioners should be mowing,
digging or monitoring on an objective map of one part ofa national park in Japan. This type of spatial priority
would be useful for allocating limited management
resources (Osawa & Kawano, 2019). This framework
could also apply to other alien invasive species in otherregions.
In the Ogasawaras, there are some especiallyseverely problematic alien invasive species which
negatively influence the ecosystem and biodiversity. In
this paper I focus on three such problematic alien invasiveplants: Bischofia javanica, Leucaena leucocephala andCasuarina equisetifolia. Establishment of pragmaticmanagement plans is at different stages for each of themin the Ogasawaras. These three species are relatively
well-studied among alien invasive species in the
Ogasawaras, and large efforts have been made for their
management, but they are still causing severe problems
in regional ecosystems and biodiversity. Practitioners
have been making efforts to manage them according to a
management plan based on what experience and scientificknowledge they have available to use. Two of these
species, however, lack one or both of 1) effective
management interventions and 2) a spatially explicit
management plan. In this paper I give an overview of thecurrent status and future challenges toward managementof these three alien invasive species and discuss what
subsequent steps should be taken to realize more effective
management.

3. Cases of Three Alien Invasive Plants
3.1 Bischofia javanica
Bischofia javanica (Fig. 1a), belonging to the
Euphorbiaceae, is a tree species native to South andSoutheast Asia but introduced to the Ogasawaras(Fukasawa et al., 2009; Yamashita et al., 2000). It was
introduced into the Ogasawaras as a part of anafforestation effort in the early 1900s, and after that
aggressively expanded its range from the afforestationsites (Yamashita et al., 2000). B. javanica has pulpy fruits
that are eaten and dispersed by birds. This species could
rapidly invade after a large disturbance due to its highfecundity, high shade tolerance (Yamashita et al., 2003)
and rapid photosynthetic acclimation (Yamashita et al.,2000). Thus, B. javanica has high adaptability in the
Ogasawaras, especially in moist forests, necessitating
concerted management efforts for conservation of thelocal ecosystems.
For this species, a pragmatic management plan
combining management interventions and a spatiallyEstablishing a Strategic Management Plan for Alien Plants
explicit management plan has already been established(Forestry and Forest Products Research Institute, 2010)and management efforts according to the plan are
ongoing (Ogasawara Node, 2009). For management of
this species, interventions combining infusion ofherbicides for mature trees and removal of the entire plant
body for seedlings, are currently considered the mosteffective eradication method (Forestry and Forest
Products Research Institute, 2010). Regarding herbicides,
Ito (2005) tested 10 major herbicide chemicals and foundthat four of them, glyphosate-isopropylammonium,
glufosinate, a triclopyr formulation and diquat, wereeffective at killing B. javanica individuals (Ito, 2005).
Among these effective chemicals, glyphosateisopropylammonium has been used for eradication of B.

Fig. 1 Pictures of Bischofia javanica in the Ogasawaras: (a) amature tree, (b) and (c) their seedlings.

23

javanica because of its availability and persistence inenvironment (Ito et al., 2009). However, applying
herbicides to small trees of less than 1 m height involvesdifficulties. Additionally these small individuals,
including seedlings, can grow rapidly after canopy trees
die back (Yamashita et al., 2003) (Fig. 1b.c). Thus, themanagement plan required that after application ofherbicides on large trees, practitioners remove small
individuals growing around the eradicated trees (Forestry
and Forest Products Research Institute, 2010).
Concerning a spatially explicit plan, Fukasawa et al.
(2009) estimated future expansion of B. javanica basedon its ecological characteristics on Haha-Jima Island,
which is most severely invaded by the species. They
predicted future expansion of the species’ distribution
range from the current one using simultaneous models ofhabitat suitability and the dispersal process (Fukasawa etal., 2009). This study provided information on which area
management efforts should be concentrated, e.g., B.
javanica could expand to forests that retain rich endemic
flora. This allows practitioners to prioritize their efforts,
concentrating their eradication efforts in certain areas
based on scientific evidence (Fukasawa et al., 2009).
However, neither eradication nor containment efforts forthis species have succeeded yet because B. javanica had
already dominated about 14% of Haha-Jima’s area
(approximately 280 ha) in 2003 (Fukasawa et al., 2009).
Thus the next challenge will be establishing more
effective intervention methods against this species.
Actually this effort is already underway and has provided
new knowledge on more effective eradication techniques(Itou et al., 2014).
There is still one mystery regarding this species. B.
javanica has behaved as an alien invasive species only in
the Ogasawaras. The other two species, namely Leucaena
leucocephala and Casuarina equisetifolia, are known asalien invasive species in other regions as well (Lowe etal., 2000; Rodgers & Gamble, 2008), whereas B. javanica
is not. Actually the pest risk score based on the Hawaii
Pacific Weed Risk Assessment (HPWRA) of B. javanica
is not high enough to make it a concern (score 7)
compared to Leucaena leucocephala (score 10) andCasuarina equisetifolia (score 21) (Hawaii-Pacific Weed
Risk Assessment, 2019). Elucidating the reason for thatwill be another important challenge.
3.2 Leucaena leucocephala
Leucaena leucocephala L. (Fabaceae) (Fig. 2a) is anevergreen shrub naturally distributed in South America,
which was introduced to Southeast Asia and the
Ogasawaras in the late 1800s (Kimura et al., 1984). After
forming dense monotypic thickets in disturbed areas (Fig.
2b), L. leucocephala prevents the germination of woody
species and understory herbaceous species (Hata et al.,2010a). As a result, L. leucocephala promotes the invasionand establishment of alien plants that have high invasive

24

T. OSAWAability such as B. javanica (Yoshida & Oka, 2004). Givensuch impacts on native ecosystems, L. leucocephala islisted as one of the World’s 100 Worst Alien Invasive
Species (Lowe et al., 2000) and as a “caution needed
invasive species” by the Japanese Invasive Species Act(Ministry of the Environment, Japan, 2019). Additionally
Osawa et al. (2016a) showed that eradication of feral
goats enhanced the expansion of L. leucocephala becauseof the loss of grazing pressure from the goats (Osawa etal., 2016a). Currently feral goats exist on Chichi-Jima
Island only but L. leucocephala is growing and expanding
its distribution range on almost all islands of the
Ogasawaras. Thus, continuing management efforts andmonitoring of L. leucocephala is required, especially on

Fig. 2 Pictures of Leucaena leucocephala L. in the Ogasawaras:(a) mature tree, (b) monotypic forest of this species, (c)forest that has recovered after eradication activities.

islands where the goats have been eradicated (Osawa etal., 2016a).
No pragmatic management plan has been established
for this species yet but some key knowledge has alreadybeen gained. As for practical management interventions,the basic method is digging up and removing the entire
plant body (Ogasawara Node, 2016; 2017b). However,this species can quickly recover from both its root andpropagules, i.e., seeds, so continued efforts are required
to prevent its recovery after removal. To prevent recovery
from its roots, infusion of herbicides is conducted using
the same chemical as for B. javanica, glyphosateisopropylammonium (Ogasawara Node, 2017b).
Although glyphosate-isopropylammonium is not as
effective against L. leucocephala as compared with B.
javanica, it is used because of its availability (Hayama,2016).
This species can recover after eradication activities
mainly through its propagule supply (Hata et al., 2010a).
Osawa et al. (2019) provided an idea that might solvethat problem: a priority method, i.e., a spatially explicit
plan for effective management L. leucocephala in
Nakoudo-Jima Island, one of the Ogasawaras. In thismethod, its distribution range is divided into four classes:
1) adequate propagule supply and habitat suitability, 2)adequate propagule supply but limited habitat suitability,3) limited propagule supply but adequate habitat
suitability, and 4) mid- to low-level propagule supply andlimited habitat suitability, but with categorization still inprogress in some cases. This classification is not only
useful for allocating management efforts, but also helps
avoid the recovery of this species from its propagule
supply. Osawa et al. (2019) proposed setting the order of
eradication according to this classification. In specific,
Osawa et al. (2019) proposed that Class 3 should begiven priority for eradication of the focal species becausethat class is relatively less likely to be recolonized after asingle eradication effort due to the limited propagule
supply. Thus, eradicating these areas contributes tosuppression of the propagule supply in the other areas aswell. Class 2, which has limited habitat suitability but anadequate propagule supply should take lowest priority,
because reinvasion after eradication may become lesslikely after suppression of the propagule supply. Although
there are some challenges regarding applying that idea inpractice, such as reliability of estimating the propagule
supply and habitat suitability (Osawa et al., 2019), thisidea could contribute to establishment of an effective
spatially explicit plan for the management of L.
leucocephala.
Therefore, the first challenge toward the pragmaticmanagement of L. leucocephala is to establish a spatiallyprioritized management plan for the target areas andimplement a combination of both management
interventions and the spatially explicit plan. The second
challenge is the same as for B. javanica, establishing a

Establishing a Strategic Management Plan for Alien Plants

25

more effective eradication method for this species.
Effective chemicals capable of preventing recovery areespecially important.
3.3 Casuarina equisetifolia
Casuarina equisetifolia Forst. (Casuarinaceae) (Fig.
3a), native to Malaysia, southern Asia, Australia and thePacific islands, has been introduced to tropical andsubtropical islands, including those in the Caribbean, theMascarene, Galapagos and Hawaiian islands (Weber,2017) and the Ogasawaras. This species is already widelydistributed in the Ogasawaras and has a negative
influence on the native ecosystem. For example, thedensity and species richness of saplings of native woody
species in forests dominated by C. equisetifolia are much
lower than those in forests dominated by native species(Hata et al., 2009). Additionally this species provides
large amounts of litter and accumulates a deep layer oflitter under its canopy (Hata et al., 2010b) (Fig. 3b,c).
This accumulation prevents the establishment and growthof native trees (Hata et al., 2010c; Hata & Kachi, 2017)and alters water and nitrogen release rates (Hata et al.,2016; 2015; 2012). Thus, this species can alter
fundamental ecosystem properties as an autogenic
ecosystem engineer (Jones et al., 1994). Although there ismuch research on the impacts of this species on
ecosystems, pragmatic management plans for this species
have yet to be established.
Currently, the basic method for eradicating thisspecies is infusion of a herbicide using same chemical as
for B. javanica and L. leucocephala, glyphosateisopropylammonium (Ogasawara Node, 2016; 2017b).
The effectiveness of this chemical and adequate quantity
have already been tested, showing that it is more effective
against C. equisetifolia than B. javanica (Fujinuma et al.,2008). On the other hand, spatially explicit plans have yet
to be established based on the ecological characteristicsof this species. It is difficult to predict its distribution
range expansion, because this species is characterized byanemochory, namely dispersal of seeds primarily by wind(Batianoff et al., 2009), whereas B. javanica is dispersed
mainly by birds and L. leucocephala, mainly by gravity.
Oceanic islands, such as the Ogasawaras, suffer fromheavy wind, and are often affected by typhoons, so
predicting expansion of the distribution range of
anemochoric species is more difficult there than in inland
regions. Additionally anemochoric species can recover,i.e., be re-introduced after eradication efforts, even fromlong distances. This is difficult to prevent. Thus, the mostimportant challenge toward establishing a pragmaticmanagement plan for C. equisetifolia is establishing amethod to predict its dispersal for an effective spatially
explicit plan. A second challenge will be establishing amethod of removing the accumulated litter and aiding
recovery of the ground environment, such as by providing
nutrients and adjusting soil moisture. In recent years,

Fig. 3 Pictures of Casuarina equisetifolia in the Ogasawaras: (a)mature tree, (b) accumulated litter under a forest dominatedby this species, (c) current situation of the forest floor in aforest dominated by this species.

another possible way to overcome the accumulated litterhas been shown—planting a native species. Hata andKachi (2017) planted native trees Schima mertensiana
under a C. equisetifolia-dominated forest and found thenative trees could establish themselves (Hata & Kachi,2017). Of course, to apply that planting method
effectively, a spatially explicit plan is also needed.

4. Conclusion and Perspectives
In this paper I have taken three cases of alien
invasive species, reviewed their current status andproposed subsequent challenges for establishingpragmatic management plans in the Ogasawara Islands.
Management strategies for invasive species should be26

T. OSAWA

based on the given circumstances and total costs required(Grice et al., 2011). In the case of the Ogasawaras, it isdifficult to restrain tourism, which exacerbates biologicalinvasion, because tourism is an important industry.
Although there are many ways to prevent biologicalinvasion such as washing shoe soles before boarding theferryboat, many alien invasive species are alreadynaturalized. Thus we have to manage alien invasive
species for sustainable tourism to the degree that alien
invasive species can negatively influence the uniqueecosystems of the Ogasawaras.
The Ogasawaras have a scientific committee that can
discuss the direction of sustainable management of naturein the Ogasawaras, including management of alien
invasive species. Thus, current management strategies forthese three species already have enough background tohave scientific knowledge available for use to somedegree. At the same time, however, scientific researchoften requires a lot of time and resources. Thus,
sometimes science-based knowledge is not available foruse in the practical fields (Osawa & Ueno, 2017).
Nevertheless, scientific knowledge will be important in
the establishment of pragmatic management plans. The
management plans for the three alien invasive species
discussed here have been improved through bothscientific research and feedback from implementation.
This procedure could contribute to the establishment andrefinement of pragmatic management plans for otheralien invasive species. Glen et al. (2013) indicated that
eradication of invasive species is more feasible on islands
than on continents based on a global meta-analysis (Glenet al., 2013). They emphasized the importance of benefits
for the local community to gain their support, engaging
them in management efforts (Glen et al., 2013).
Accumulating research while refining pragmaticmanagement plans and continuing management efforts
with public engagement could provide us the tools toeliminate the threats to the unique ecosystems of the
Ogasawara Islands.

Acknowledgments
I would like to thank the Ministry of Environment
and the Department of National Forests in the Ogasawara
Islands for allowing us to conduct our field survey. Drs.
N. Kachi and K. Hata provided me much useful
information about alien invasive species in the
Ogasawaras. This study was supported by a grant-in-aid
for scientific research B (No. 24710038) from the Japan
Society for the Promotion of Science.
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Takeshi OSAWA
Takeshi Osawa has been an associate professor atTokyo Metropolitan University since 2018.
Before acquiring his current position, he worked
with the National Institute for AgroEnvironmental Sciences, Japan. He received his
Ph.D. in Science from Kobe University in 2010.
His research interests are biodiversity informatics,
applied ecology and conservation science. He is studying management ofalien invasive species in the Ogasawara Islands, and recently has beeninterested in cultural ecosystem services for tourist. He is on the steeringcommittee of the Japan node of the Global Biodiversity Information
Facility (JBIF).
(Received 13 September 2019, Accepted 1 December 2019)