ORIGINAL ARTICLE

Potential role of frugivorous birds in the recovery process of forestvegetation after feral goat eradication in Mukojima Island,the Bonin Islands

Naoko Emura • Kazuto Kawakami •

Tomohiro Deguchi • Koichi Sone

Received: 12 December 2010 / Accepted: 7 May 2011 / Published online: 23 September 2011

� The Japanese Forest Society and Springer 2011

Abstract Some introduced frugivorous birds disperse

introduced plants and are thus a substitute for extinct native

birds. Introduced birds have negative and/or infrequently

positive effects on local ecosystems. It is important for

management of the ecosystems to understand the rela-

tionships between native and introduced frugivorous and

plant species. In this study, we elucidated these relation-

ships in Mukojima Island, the Bonin Islands, where was

anthropologically deforested and Japanese White-eye

Zosterops japonicus and some plants were already introduced.

We examined the habitat selection of frugivorous birds,

actual dispersed seeds in bird feces, and the distribution of

the potentially dispersed plant species. The Japanese

White-eye and the native, Blue Rockthrush Monticola

solitarius, were dominant on this island. The former mainly

used the forest area and dispersed only small seeds and

frequently introduced plant seeds. The latter mainly used

the open area and dispersed both small and large seeds.

Some small-seed plants occurred not only in the forest but

also in the open area. Their seedlings were distributed

farther from their adult trees than the large-seed species.

These indicate that small-seed plants would be more spread

than the large-seed plants because the two bird species

disperse their seeds in different environments. This intro-

duced bird species may be important in vegetation recov-

ery, although it may contribute to the distribution of

introduced plants on this island.

Keywords Frugivorous birds � Introduced species �Japanese White-eye � Mukojima Island � Seed dispersal

Introduction

The biota of oceanic islands includes many endemic and

rare species because of limited immigration across the sea

(Carlquist 1974). However, the ecosystems of the oceanic

islands around the world have been altered by habitat

destruction and the introduction of invasive species with

human activities (Loop et al. 1988; Vitousek et al. 1997,

Shimizu 2003; Kawakami 2010). Changes in species

composition of frugivorous birds and fleshy-fruited plants

might induce changes in the original vegetation of an

island. For example, the extinction of large native frugiv-

orous species may alter forest structure because of a lack of

seed dispersal (Meehan et al. 2002; Poulsen et al. 2002).

Introduced frugivorous birds frequently disperse the seeds

of introduced fleshy-fruited plants (Richardson et al. 2000;

Mandon-Dalger et al. 2004) and those of native plants in

areas where native frugivorous birds are few or extinct

N. Emura

Graduate School of Agriculture, Kagoshima University,

1-21-24 Koorimoto, Kagoshima 890-0065, Japan

Present Address:N. Emura (&)

Rikkyo University, 3-34-1 Nishi-ikebukuro,

Toshima-ku, Tokyo 171-8501, Japan

e-mail: emurako@yahoo.co.jp

K. Kawakami

Forestry and Forest Products Research Institute,

1 Matsunosato, Tsukuba, Ibaraki 305-8687, Japan

T. Deguchi

Yamashina Institute for Ornithology, 115 Konoyama,

Abiko, Chiba 270-1145, Japan

K. Sone

Faculty of Agriculture, Kagoshima University,

1-21-24 Koorimoto, Kagoshima 890-0065, Japan

123

J For Res (2012) 17:352–359

DOI 10.1007/s10310-011-0300-7

(Kawakami 2006; Foster and Robinson 2007; Kawakami

et al. 2009; Bellingham et al. 2010). Therefore, to control

and conserve the local ecosystem, it is important to

understand the interaction among organisms, including

both native and introduced species.

In Mukojima, the Bonin (Ogasawara) Islands, the dis-

appearance of forest vegetation was mainly caused by an

introduced goat species, Capra aegagrus (Shimizu 1993),

similar to the Hawaiian Islands and the Galapagos Islands

(Loop et al. 1988; Duncan and Holdaway 1989). At the

same time, the population of some native seed dispersers,

for example the Japanese Wood Pigeon Columba janthina

nitens, Bonin White-eye Apalopteron familiar familiar, and

Brown-eared bulbuls Hypsipetes amaurotis had already

dwindled or disappeared completely (Shibazaki and Hoshi

2006; Emura and Deguchi 2009; Kawakami et al. 2009).

The Japanese White-eye was introduced to this island

around 1945 (Hasuo 1969). Introduced fleshy-fruited

plants, for example Lantana camara, have also become

established (Shimizu 1993; Yamamoto et al. 2003). All

goats on Mukojima were eliminated by 2003 (Japanese

Wildlife Research Center 2009); thus, the vegetation is

expected to recover given the release from grazing pres-

sure. Moreover, an initiative to eradicate the introduced

black rat Rattus rattus which is seed predator has been

conducted since 2008 (Watanabe et al. 2003; Abe 2007;

Hashimoto 2010). If this is successful, dispersed of seeds

by frugivorous birds may effectively re-establish vegeta-

tion enabling the recovery of native vegetation on the

island.

In this study, to understand the relationships between

native and introduced frugivorous birds and plant species

on the island, we examined the habitat selection of fru-

givorous birds, actual dispersed seeds in bird feces, and the

distribution of the potentially dispersed plant species.

Furthermore, on the basis of these results, we discuss the

conservation of the island’s ecosystems including both

native and introduced species.

Materials and methods

Study site

The Bonin Islands comprise the Mukojima, Chichijima,

and Hahajima Islands (Fig. 1a). The Mukojima Islands

(27�400N, 142�080E) are all uninhabited and include

Mukojima, Nakodojima, Yomejima and some small islands

(Fig. 1b). Mukojima (area 257 ha, maximum altitude

88.4 m) is the largest of the Mukojima Islands.

Feral goats were introduced with the human settlement

on the islands in the 1880s. Then, forced evacuation during

World War II left the Mukojima Islands uninhabited since

1944. Thus, the number of feral goats increased, eventually

reaching a population density of 300 km-2 in 1991 (Tokida

1992). Eradication of goats on the Mukojima Islands was

conducted between 1997 and 2003 (Japanese Wildlife

Research Center 2009).

We conducted surveys in the western region of

Mukojima Island (Fig. 1b). Two forest patches remain on

the island, one in the west and one in the east (Figs. 2, 3).

The forests consist of low shrubby trees (approximately

6 m high), mainly Ardisia sieboldii and Neisosperma na-

kaianaum. Calophyllum inophyllum is also dominant in the

eastern forest area. In the open area, herbaceous species

such as Chrysopogon aciculatus is dominant.

Habitat selection of birds

To clarify the habitat preference of each land bird, we set

up four to six census points with 30-m radii in each of the

four habitat types; forest (FO), forest edge (ED), and two

types of open area, namely, OP50 and OP100 (50 and

100 m from the edge, respectively) (Fig. 2). FO is mostly

covered by trees more than 3 m tall. ED is the boundary

area between FO and open habitat. OP50 and OP100 were

set approximately 50 and 100 m, respectively, from the

center of each ED plot. Between February and May 2009,

we recorded the number and species of birds observed at

the census points. Recordings were conducted in 20-min

Study site

Mukojima IslsChichijima Isls

(b) Mukojima Islands

Yomejima

Mukojima

Nakoudojima

(a) The Bonin Islands

Japan

Bonin Isls

Hahajima Isls

0 4 km

0 100 km

Fig. 1 Study site. The Bonin Islands (a) and Mukojima Islands (b)

J For Res (2012) 17:352–359 353

123

trials at each point in each habitat type between 1300 and

1600 hours throughout the study period (total recordings;

FO, 23; ED, 19; OP50, 14; OP100, 11). The numbers of

each bird species observed were compared among habitats

by use of a Kruskal–Wallis test.

Analysis of feces

We examined the contents of feces of each land bird spe-

cies to assess its role as a seed disperser. Sampling of feces

was conducted from February to May in 2008 and 2009.

We obtained samples of feces from birds trapped in mist

nets set in the forest or at the forest edge (Fig. 2). Each mist

net was checked every hour from 1300 to 1700 hours.

Captured birds were placed in individual cloth bags and

allowed to evacuate for approximately 15 min, then

released. Feces in the bags were collected. In addition, we

obtained feces samples from Blue Rockthrush Monticola

solitarius at 15 song points in the open area because few

individuals were captured in mist nets. Some of the points

were located up to approximately 100 m from the forest

edge. We collected\10% of the total number of available

samples of feces from each song point.

All seeds found within the feces were counted and

identified at the species level. The minor diameter of each

identified seed (n = 1–13) was measured after removal of

the fleshy part. Two native Neolitsea sp., N. boninensis and

N. aurata, were indistinguishable (Yamamoto et al. 2003),

and were thus treated as Neolitsea sp. Identified seeds were

classified into five groups on the basis of a previous study

(Yamamoto et al. 2003), including seed origin (native,

introduced, and unknown) and sizes (smallest diameter

\0.55 or C0.55 cm). Seeds\0.55 cm are considered to be

effectively dispersed by introduced Japanese White-eyes

(Kominami et al. 2003). To assess the contribution of each

bird species to seed dispersion, their feces content was

examined. Furthermore, to assess their contribution as seed

dispersers for introduced plants, the occurrence of feces

containing introduced plant seeds and the frequencies of

such seeds compared with all seeds detected were com-

pared among bird species. These analyses were conducted

using a chi-squared test for independence.

Distribution of plant species detected by analysis

of bird feces

A total of 47 plots (10 9 10 m) were established in five

habitat types: 10 plots in each of FO, ED, and 30 m (OP30)

and 60 m from the forest edge (OP60), and 7 plots in

OP100 (Fig. 3). The plots in FO, which was dominantly

covered by trees [3 m tall, were placed more than 10 m

away from the forest edge. The number of individuals of

each plant species whose seeds were detected in more then

two samples of bird feces was recorded. The height was

measured for each tree within each plot. Seedlings of 1.2 m

or less in height and 2.0 cm or less in diameter at the root

collar were defined as seedlings whereas larger trees were

regarded as adults. We were unable to count the number of

Myoporum boninense and Solanum nigrum individuals

because these species grow in very thick clusters. Thus, we

recorded the presence or absence of these species in each

plot. The numbers of seedlings and adult trees per plot

were compared among habitats by use of a Kruskal–Wallis

test.

To compare the distribution pattern of seedlings from

adult trees among tree species whose seeds were detected

in feces, the ratio of the number of seedlings distributed

more than 10 m from an adult tree to those distributed

100m0

Fig. 2 Site of captured birds and census point plots, western region

of Mukojima Island. The shaded area is forest and the rest is open.

The dashed line shows the area in which birds were captured by use

of mist nets. Blue Rockthrush’s feces samples were obtained at 15

song points in the open area. Each symbol indicates location of census

point plots: open circles, forest (FO); filled circles, forest edge (ED);

open diamonds, 50 m from the edge (OP50); open triangles, 100 m

from the edge (OP100)

0 100m

Fig. 3 Vegetation surveyed plots in the study site, western region of

Mukojima Island: open circles, forest (FO); filled circles, forest edge

(ED); open diamonds, 50 m from the edge (OP50); open triangles, 100 m

from the edge (OP100). The shaded area is forest and the rest is open

354 J For Res (2012) 17:352–359

123

within 10 m of an adult tree were compared between intro-

duced and native species and between large- and small-seed

species by use of a Fisher’s exact probability test.

Results

Habitat selection of birds

In total, five bird species, including three residents (Japa-

nese White-eye, Blue Rockthrush, and Brown-eared Bul-

bul) and two migratory (Short-tailed Bush Warbler

Urosphena squameiceps and Brambling Fringilla monti-

fringilla), were recorded in the point census (Table 1).

Among these, Japanese White-eyes and Blue Rockthrushes

were constantly observed during the surveyed periods.

Japanese White-eyes were mainly observed at FO and ED.

Blue Rockthrushes were observed in open area plots and

ED. The occurrences of each bird species were signifi-

cantly different among the four habitats (Japanese White-

eye: H = 47.7, P \ 0.001; Blue Rockthrush: H = 17.7,

P \ 0.001).

Analysis of feces

We collected samples of feces from nine bird species

captured by use of mist nets. Residents included the

Japanese White-eye (number of feces samples, n = 344),

Blue Rockthrush (n = 7), White’s Thrush Zoothera dauma

(n = 3), and Japanese Bush-Warbler Cettia diphone

(n = 2). Migratory species included the Brambling

(n = 5), Short-tailed Bush Warbler (n = 3), Pale Thrush

Turdus pallidus (n = 2), Japanese Waxwing Bombycilla

japonica (n = 1), and Dusky Thrush Turdus naumanni

(n = 1). In addition, 415 feces samples of Blue Rockthrush

were collected at song points in the open area. Among the

nine species, the feces of Japanese White-eyes, Blue

Rockthrushes, and Japanese Waxwing contained seeds.

Seeds of 14 plants were detected, which included two

introduced small-seed species (Ficus microcarpa and L.

camara), five native small-seed species (S. nigrum, Trema

orientalis, M. boninense, Cassytha filiformis, and Celtis

boninensis), three native large-seed species (Scaevola

taccada, Elaeagnus rotundata, and Neolitsea sp.), and four

unidentified small-seed species (Table 2). One sample of

feces from a Japanese Waxwing contained two seeds of

M. boninense. Of the samples of feces from Japanese

White-eye and Blue Rockthrush, 44.9 and 40.8%, respec-

tively, contained seeds, with no significant differences

between them (v2 = 1.86, df = 1, P = 0.17). The Japanese

White-eye defecated small seeds only, whereas the Blue

Rockthrush defecated both small and large seeds. The

composition of seed species in feces differed significantly

between the two bird species (v2 = 607.67, df = 5,

P \ 0.001). The feces containing introduced plant seeds to

natives and the frequency of number of those to all seeds

detected were significantly higher for the Japanese White-

eye (25.3 and 44.9%) than the Blue Rockthrush (6.4 and

15.7%) (v2 = 157.30, df = 1, P \ 0.001, v2 = 23.29,

df = 1, P \ 0.001).

Distribution of plant species detected in bird feces

Of the plant species detected in fecal analysis, six occurred

in the plots examined (F. microcarpa, L. camara,

S. nigrum, M. boninense, T. orientalis, and E. rotundata)

(Fig. 4). For the introduced small-seed group, the numbers

of both adult trees and seedlings were not significantly

different among habitat types (F. microcarpa; H = 2.76,

P = 0.60 for adult tree, H = 8.39, P = 0.08 for seedling,

L. camara; H = 1.78, P = 0.78, H = 5.71, P = 0.22).

Adult trees of T. orientalis, which belong to the native

small-seed group, occurred more frequently in ED

(H = 25.88, P \ 0.001) than the other habitat types,

whereas seedlings occurred in all habitat types and the

number of seedlings did not differ significantly among the

habitat types (H = 7.15, P = 0.13). E. rotundata, of the

native large-seed group, occurred only in FO and ED plots.

The number of both adult trees and seedlings differed

significantly among habitat types (H = 12.69, P \ 0.05 for

adult trees, H = 23.29, P \ 0.001 for seedlings). S. nigrum

occurred in two FO and one ED, and one OP60 plot.

M. boninense occurred in two ED plots.

The number of seedlings of introduced species \10 m

from conspecific adult trees was significantly lower than

for native species (Fisher’s exact probability: P \ 0.001)

(Table 3). The number of seedlings \10 m from a con-

specific adult tree of the large-seed group was significantly

higher than for small-seed group (Fisher’s exact probabil-

ity: P \ 0.001) (Table 3).

Discussion

The role of birds as seed dispersers

Only the introduced Japanese White-eye and native Blue

Rockthrush were observed continually in this study. This

result is consistent with the most recent records from this

island (Emura and Deguchi 2009). In addition, only a few

individuals of another potential seed disperser, the native

Bonin flying fox Pteropus pselaphon, were observed (Inaba

et al. 2004; Harada 2009). Thus, the Japanese White-eye

and Blue Rockthrush seem to be the main seed dispersers

on this island. The former was observed mainly in forested

areas and dispersed only small seeds, consistent with

J For Res (2012) 17:352–359 355

123

previous studies (Kominami et al. 2003; Foster and Rob-

inson 2007; Kawakami et al. 2009). In addition, because

this bird species was the most frequently observed, it is

expected to make the greatest contribution to seed dispersal

on this island. Conversely, the Blue Rockthrush was

observed mainly in open areas and dispersed both small

and large seeds, which is also consistent with previously

reported data (Hayashi 1988; Kawakami 2009). Therefore,

these two species are expected to play different roles as

seed dispersers on this island.

Many populations of native seed dispersers are already

extinct or seriously diminished on the island (Emura and

Deguchi 2009). Indeed, the introduced Japanese White-eye

is suggested to replace the lost function of the extinct

Bonin Island White-eye as the seed disperser, because their

distributions and food items are widely overlapped (Ka-

wakami and Higuchi 2003; Kawakami et al. 2009). The

populations of other native large-seed dispersers, for

example the Brown-eared Bulbul, have very few individ-

uals remaining on the island. Therefore, we expect that

Blue Rockthrushes are the major seed dispersers on the

island, even though they are sparse and may mainly dis-

perse seeds in open area. In the future, Brown-eared Bul-

buls are expected to increase their populations and become

Table 1 Result from analysis

of feces of Japanese White-eye

and Blue Rockthrush

Introduced Japanese White-eye Native Blue Rockthrush Seed size

(cm)

n = 1–13

Seed

typeNumber

of feces

samples

containing

seeds

Total

number of

detected

seeds

Number

of feces

samples

containing

seeds

Total

number of

detected

seeds

Introduced

Ficus microcarpa 35 433 11 144 0.09 ± 0.01 Small

Lantana camara 5 7 0.30 ± 0.04 Small

Native

Solanum nigrum 4 31 28 389 0.03 ± 0.01 Small

Trema orientalis 92 472 35 262 0.15 ± 0.01 Small

Myoporum boninense 17 26 9 35 0.35 ± 0.05 Small

Cassytha filiformis 1 1 0.29 Small

Celtis boninensis 1 1 0.46 Small

Scaevola taccada 5 5 0.57 ± 0.04 Large

Elaeagnus rotundata 81 81 0.59 ± 0.07 Large

Neolitsea sp. 1 1 0.80 Large

Unknown

Sp-1 1 7 0.02 ± 0.01 Small

Sp-2 1 1 0.04 Small

Sp-3 1 1 0.04 Small

Paspalum sp. 2 2 0.10 ± 0.00 Small

Number of seed

contained feces

158 172

Number of seeds 980 919

Number of

analyzed feces

344 422

Table 2 Observed bird species

among the four habitat types

Fo forest; Ed forest edgea Values in parentheses are the

number surveyed

Bird species Scientific name Habitat types (mean ± SD)

Fo (23)a Ed (19) 50 m (14) 100 m (11)

Japanese White-eye Zosterops japonicus 7.1 ± 2.2 6.2 ± 2.4 0.9 ± 0.9 0.2 ± 0.8

Blue Rockthrush Monticola solitarius 0.3 ± 0.7 0.5 ± 0.7 0.8 ± 0.4

Brown-eared bulbul Hypsipetes amaurotis 0.0 ± 0.2 0.1 ± 0.5

Asian Stubtail Warbler Urosphena squameiceps 0.0 ± 0.2

Brambling Fringilla montifringilla 0.2 ± 0.7

356 J For Res (2012) 17:352–359

123

an effective disperser after the recovery of the forests. They

are known to be a major seed dispersal agent in the Bonin

Islands and mainland Japan (Hirata et al. 2006; Kawakami

et al. 2009).

Japanese White-eye and Blue Rockthrush dispersed the

seeds of introduced plants. The former is known to disperse

introduced plants on other islands where it has been

introduced (Foster and Robinson 2007; Kawakami et al.

2009). One out of every eight seeds detected in the feces of

Blue Rockthrush was from an introduced plant; in a pre-

vious study on the Bonin Islands, eight of ten were from

introduced plants (Kawakami 2009). The open habitats of

the inhabited islands mainly contain human settlements, in

which more introduced plant species are grown than on the

uninhabited islands (Toyoda 2003).

The revegetation process and seed dispersal by birds

F. microcarpa, an introduced plant with a small seed, was

dispersed by two bird species, and the seedlings occurred in

FO and ED. Japanese White-eyes frequently dispersed the

seeds and mainly used FO and ED habitats. The fruit of this

plant has soft flesh that contains many small seeds, sug-

gesting that its seeds should be dispersed by many bird

species. A previous survey of this island found that many

seedlings of this species were also observed in open areas

(Japanese Wildlife Research Center 2009). Blue Rockth-

rushes may contribute to this distribution. It is possible that

we did not detect these seedlings in open areas because an

insufficient number of plots was examined. This plant is

also invasive in the other regions where it has been intro-

duced, for example the Hawaiian Islands (Starr et al. 2003).

It is necessary to control this plant by extermination. L.

camara, another introduced plant species, occurred from

FO to open areas. Blue Rockthrushes were not found to

disperse the seeds of this plant in this study, but have been

reported to do so on inhabited islands in the Bonin Islands

(Kawakami 2009). Therefore, we expect that such dispersal

would occur on Mukojima during the peak of the fruiting

period of this plant. Given that L. camara has been des-

ignated one of the world’s worst 100 invasive alien species

(Lowe et al. 2000), there is an immediate need for further

investigations into the distribution of this plant on the

island.

T. orientalis, a native plant with a small seed, was fre-

quently dispersed by two bird species and occurred from

FO to open areas. In addition, some of the seedlings

occurred far from adult trees. Adult trees in open areas are

expected to facilitate seed dispersal by birds because the

birds will use them for perching (Wenny and Levey 1998).

Artificial dispersal of the seeds may help to effectively

revegetate the open areas of the island. The dispersal of S.

nigrum and M. boninense seeds may occur via two bird

species. S. nigrum occurred from FO to open areas.

Because M. boninense mainly grow in coastal areas, and its

seeds are also dispersed by sea currents (Satake et al.

1989), the occurrence of this plant in ED may be a result of

seed dispersal by birds. E. rotundata, a native plant with a

large seed, was frequently dispersed by Blue Rockthrushes,

0

1

2F. microcarpa

0

1

2L. camara

0

2

4T. orientalis

0

10

20

30

40

50E. rotundata

0

2

4 S. nigrum

0

2

4 M. boninense

Pop

ulat

ion

dens

ity

Freq

uenc

y

Fig. 4 Population densities of adult trees and seedlings of the four

plant species and frequency of occurrence of the two among five

habitat types. White bar, adult tree; black bar, seedling

Table 3 The number of seedlings of plant species more than 10 and

\10 m from conspecific adult tree

Species Under 10 m Over 10 m Seed sizea

Introduced

F. microcarpa 3 3 Small

L. camara 4 2

Native

T. orientalis 20 3 Small

C. boninensis 7 0

E. rotundata 114 1 Large

Neolitsea sp. 15 0

a Small is\0.55 cm in minor diameter, large is more than 0.55 cm in

minor diameter. The detailed size of each species is shown in Table 1

J For Res (2012) 17:352–359 357

123

although the adult trees and seedlings occurred only in FO

and ED and the seedlings were mostly found under adult

trees. The seeds of this plant may be impossible to establish

in the open area because of habitat preference.

Although the survey season was limited in this study,

the frugivorous birds should disperse various species of

seeds in the other seasons. Fruiting seasons of endozo-

ochoric plants have not been fully elucidated in previous

phenological studies in the Bonin Islands. To evaluate the

function of the seed dispersers in more detail, it is neces-

sary to survey throughout the year. The recovery of forest

vegetation will take a long time on Mukojima, and it is

necessary to continuously monitor the process of revege-

tation, particularly given the eradication of introduced

goats and rats.

Acknowledgments This study was conducted under a project

examining Short-tailed Albatross immigration to Mukojima, con-

ducted by the Yamashina Institute of Ornithology and the United

States Fish and Wildlife Service. We thank Megumi Sasaki, Hitoshi

Saito, Haruko Ando, Yuka Kondo, and Yukiko Aoyama for field

assistance, captain Koji Yoshida and Hiroshi Fujii for transportation,

Reiko Hirata for teaching us how to capture birds, Hideo Maruoka for

acquiring background literature, and Kunihiko Hata and Keisuke

Ueda for assistance and suggestion. We also thank the referees and

editor for their helpful comments.

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