Blackwell Science, LtdOxford, UKPSBPlant Species Biology0913-557X2004 The Society for the Study of Species BiologyDecember 2004193197201Original ArticlePOLLINATION IN C. JAPONICA BY WHITE EYESY. K. KUNITAKE
Et al.
Plant Species Biology (2004) 19, 197–201
Role of a seasonally specialist bird Zosterops japonica on
pollen transfer and reproductive success of Camellia
japonica in a temperate area
YOKO KAWATE KUNITAKE,* MASAMI HASEGAWA,† TADASHI MIYASHITA* and HIROYOSHI HIGUCHI*
*Laboratory of Biodiversity Science, School of Agriculture and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan and
†Laboratory of Geographical Ecology, Department of Biology, Faculty of Science, Toho University, Chiba 274-8510, Japan
Abstract
Camellia japonica is considered to be pollinated by birds, but to date no studies have
clarified the contribution of birds to the seed production and reproductive success of this
species. We conducted a pollinator exclusion experiment that showed that fruit set was
sixfold greater in flowers visited by birds than in flowers without bird visitation. Field
observations revealed that most of the visitations made by birds were of the species
Zosterops japonica. Furthermore, the level of fruit set was found to be saturated after only
five visitations by Z. japonica. These results demonstrate that Z. japonica is the most
effective pollinator of C. japonica. Such a strong relationship between a plant and an
omnivorous bird pollinator has rarely been reported in a temperate zone.
Keywords: Camellia japonica, Izu islands, ornithophily, pollinator exclusion experiment, Zosterops
japonica.
Received 12 December 2003; revision received 15 April 2004; accepted 20 April 2004
Introduction
Identification of pollen vectors is considered to be essential for elucidating the mechanisms that result in the spatial pattern of genetic structure in plant populations
because the type of pollen vector affects the degree of
outbreeding and the combination of gametes (Govindaraju 1988; Ghazoul et al. 1998; Dutech et al. 2002). Powerful vectors such as wind and pollinators with high
mobility enable long-distance pollen transfer (Chase et al.
1996; Dow & Ashley 1998; Schulke & Waser 2001) and
plants depending on such vectors are expected to be panmictic with little spatial differentiation. In contrast, plants
that depend on pollinators with limited mobility show
genetic differentiation among and within populations
(Schmitt 1980; Wolff et al. 1997).
Camellia japonica is a common broad-leaved evergreen
tree that is distributed widely throughout East Asia. This
species secretes a large quantity of nectar, its flowers are
conspicuously red in color and its petals are tough; traits
consistent with ‘bird pollination syndrome’ (Faegri & Van
der Pijl 1971). Indeed studies have shown that C. japonica
Correspondance: Yoko Kawate Kunitake
Email: [email protected]
© 2004 The Society for the Study of Species Biology
is visited by two bird species in Japan, white-eyes (Zosterops japonica) and brown-eared bulbuls (Hypsipetes amaurotis) (Tanaka 1980; Yumoto 1988), but the role of birds in
the pollination success of C. japonica has not been quantitatively evaluated.
Recent molecular studies have indicated that C. japonica
has a unique genetic structure both within and among
populations. First, genetic diversity within a population
is very high (Wendel & Parks 1985), even in isolated
populations (Yeeh et al. 1996). Second, although the
overall level of population differentiation based on allelic
frequency is greater than typical outbreeding trees, allelic
compositions of local populations are similar throughout
the species range (Wendel & Parks 1985). Third, this
species has a relatively high outcrossing rate (Wendel &
Parks 1985). Previous studies have shown that birds can
enhance between-plant pollen transfer (Paton 1997) and
outcrossing rate (England et al. 2001) more than insects.
Moreover, in C. japonica, long-distance gene flow by seed
dispersal is unlikely because seed dispersal appears to
be mediated by gravity (Ueno et al. 2000) and small
mammals (H. Abe, pers. comm., 2003). Consequently, the
unique genetic characteristics of C. japonica may be mediated by pollen transfer by birds (Wendel & Parks 1985; Oh
et al. 1995; Yeeh et al. 1996), and this is compatible with the
198 Y. K . K U N I TA K E E T A L .
hypothesis that birds are the major pollen vectors of
C. japonica.
The present study aims to test the hypothesis that birds
play a major role in the pollination and female reproductive success of C. japonica. First, we observed the frequency of visits and the foraging behavior of visitors to
C. japonica flowers to identify potential pollen vectors.
Second, we conducted a pollinator exclusion experiment
to evaluate the relative importance of birds in the pollination of C. japonica. Third, by experimentally controlling
the number of visits by birds to flowers we evaluated the
efficiency of bird pollination.
Materials and methods
Study area
The field survey was carried out on Niizima Island
located approximately 157 km south of Tokyo, Japan
(34∞N, 139∞E). The mean monthly temperature is 17.3∞C
and the island has an annual rainfall of 2356 mm. The
dominant species in the natural forests is Castanopsis sieboldii (Ohyama 2001) and C. japonica is distributed widely
throughout the island. In this area C. japonica blossoms
from the end of November to early April.
Flower visitors and visitation rates
The experiment was carried out from December 1998
to March 1999, which covered most of the November to
April flowering period. In December and January, 30
treatment sets (each including open, caged and bagged
treatments) were placed on 30 individual plants; 50 treatment sets were placed on 21 individuals in February; and
40 treatment sets were placed on 20 individuals in
March. Treatments were left in place until September
1999 when the flowers bore fruit and the presence or
absence of fruit was recorded. The effect of treatment and
seasonality on fruit set was analyzed using a log-linear
model with treatment, month and presence or absence of
fruits as the three independent variables. As the three–
way interaction (treatment ¥ month ¥ fruit set) was not
significant and one of the two–way interactions
(treatment ¥ fruit set) was significant we pooled the data
from different months and compared fruit sets between
treatments using Fisher’s exact probability test to
determine which combination of treatments differed
significantly.
To estimate the possible effect of bagging on fruit set,
40 flowers were hand-pollinated with the pollen collected
from four flowers from five different individuals and the
flowers were bagged in February 2001. In September 2001
the fruit set was measured.
Effect of visitation frequency by white-eyes on fruit set
Visitation rates of animals to 14 C. japonica individuals
were monitored for 3 days from 14 February to 16 February 1999. All days were fine and not windy. Observations were made continuously from 7 AM to 4 PM. The
total observed time per individual tree was 27 h. In addition, concurrent observations of the frequency of visitations to 26 flowers from six of the individual trees being
monitored were also recorded. Flowers were identified
with numbered tape prior to the observations being
made.
Pollinator exclusion experiment
We used two types of exclosures to estimate the pollination effectiveness of possible flower visitors (Ramsey
1988). Open control flowers could be visited by all types
of animals, caged flowers allowed access to insect visitors
only and bagged flowers prevented access to all visitors.
Cubic cages (side length: 15 cm or 20 cm) with a single
aperture 1 cm ¥ 1 cm were constructed with semirigid
metal wire mesh. Cages were placed on buds before flowering and secured at the supporting stems. Netting bags
made of satin veil were placed on buds before flowering
and the bottom of each bag was secured with cotton
strings. The openings of the bags were filled with insect
repellent to restrict access by insects.
To estimate the relationship between visitation frequency
to flowers and the resultant fruit sets, visitations to
a flower by white-eyes were controlled. Initially, 50
unopened buds, randomly selected among seven individuals, were covered with bags. When the flowers opened,
the bags were removed until the flowers were visited by
white-eyes for a predetermined number of times; bags
were then replaced over the flowers to prevent further
access. The numbers of flowers with 0, 1, 5 and 15+ visit
treatments were 15, 10, 8 and 17, respectively. Flowers that
had been visited more than 15 times were left uncovered
after observation. In the following September and October the flowers were checked again to determine whether
they had set fruit or not. The effect of visitation frequency
on the presence or absence of fruit was analyzed using
logistic regression, with visitation frequency as an independent variable.
Results
Flower visitors and visitation rates
Most visitors were birds and consisted of white-eyes and
brown-eared bulbuls. Syrphid flies only visited five
times during the study period and nitidulid beetles were
occasionally observed inside the flowers. The average
© 2004 The Society for the Study of Species Biology Plant Species Biology 19, 197–201
80
40
60
30
Fruit set (%)
Visitation frequency (/tree/day)
P O L L I N AT I O N I N C . J A P O N I C A B Y W H I T E E Y E S
40
10
0
0
0
White-eyes
Fig. 1 Frequency of visitations to Camellia japonica flowers by
brown-eared bulbuls and white-eyes. Error bars represent SE.
1
5
No. visitations by white-eyes
>15
Fig. 3 Percentage of fruit set per flower in Camellia japonica after
different numbers of visitations by white-eyes.
5.7% of the caged flowers did. None of the bagged flowers
produced fruits. A log-linear model revealed that the
three-way interaction (treatment ¥ month ¥ fruit set) was
not significant (Pearson c2 = 3.26, d.f. = 4, P = 0.516), but
the interaction between treatment and fruit set was significant (Pearson c2 = 18.17, d.f. = 6, P = 0.006; Fig. 2).
Therefore, we obtained a consistent effect of treatment
over seasons. All pairwise comparisons of fruit set
between treatments revealed significant differences
(Fisher’s exact probability test: open and caged P < 0.0001,
open and bagged P < 0.0001 and caged and bagged
P < 0.003).
40
30
Fruit set (%)
20
20
Brown-eared bulbuls
199
20
10
0
Effect of the bagged treatment on fruit set
Dec.
Jan.
Feb.
Mar.
Fig. 2 Comparison of fruit set among open (), caged () and
bagged () treatments for Camellia japonica flowers.
visitation rates by white-eyes and brown-eared bulbuls
were 54.8 and 1.9 visits per individual C. japonica per
day, respectively. This difference in visitation rates
between white-eyes and brown-eared bulbuls was significant (Wilcoxon’s signed rank test, z = 3.297, n = 14,
P = 0.001; Fig. 1). Brown-eared bulbuls were often
observed eating stamens and pollen when they visited
C. japonica flowers.
Pollinator exclusion experiment
Fruit production was highly dependent on the treatment.
Of the open flowers 36.2% produced fruits, whereas only
Of the 40 hand-pollinated and bagged flowers 12 produced fruits, representing 30% fruit set. As this percentage fruit set is similar to that for the open flower
treatment any damage to flowers as a result of the
flower being bagged could be ignored. It is quite
likely that bagged flowers that did not set fruit in the
pollination exclusion experiment result from the high
self-incompatibility of this species (Shibata & Ieyumi
1991).
Effect of visitation frequency by white-eyes on fruit set
Fruit production of flowers was highly dependent on the
number of visits by white-eyes (Fig. 3). The logistic regression analysis showed that fruit set success was significantly related to visitation frequency (likelihood ratio
c2 = 10.057, d.f. = 1, P = 0.002, n = 42).
© 2004 The Society for the Study of Species Biology Plant Species Biology 19, 197–201
200 Y. K . K U N I TA K E E T A L .
Discussion
Our study revealed that pollination of C. japonica depends
greatly on birds throughout the flowering season because
the fruit set of open-pollinated flowers (without cage) was
more than sixfold greater than that of the bird-excluded
flowers (with cage), and insects larger than the grid diameter of the cages were not observed visiting the flowers.
One can argue that large insect pollinators visiting openpollinated flowers at night may have contributed to seed
production, but this is unlikely because no nocturnal species of large moths or beetles are known to be active
during winter.
Among bird visitors white-eyes appear to be the primary pollinator of C. japonica. First, the visitation frequency of white-eyes to C. japonica trees was much higher
than that of brown-eared bulbuls. Second, brown-eared
bulbuls appeared to be more likely to cause damage to a
flower rather than pollinate it. A study reporting the petals and pollen of Camellia flowers in the stomachs of bulbuls (Yamashina 1941) supports our observation that
bulbuls ate the reproductive organs of the C. japonica
flower. Furthermore, no flowers that sustained damage to
their reproductive organs bore fruit (Y. Kunitake, unpubl.
data, 1999).
As we did not conduct any observations or experiments
in April, it remains possible that other visitors, including
bumblebees and large nocturnal insects, contributed to
the reproduction of C. japonica at this time. However, the
contribution of these pollinators, even if present, appears
to be minor because only 5% of C. japonica trees
blossomed in April compared to 90% in January (M.
Hasegawa, unpubl. data, 2001).
The finding that birds are the major pollinators for
C. japonica is consistent with the hypothesis that birds are
responsible for the unique genetic structure in this species: a high outcrossing rate in comparison to other outbreeding plant species (Wendel & Parks 1985), a high level
of genetic diversity within populations (Wendel & Parks
1985; Chung & Kang 1996) and little genetic differentiation even in isolated populations (Oh et al. 1995) are all
indicative of long-distance pollen transfer. M. Chung and
S. Kang (unpubl. data, 1996) estimated the approximate
gene flow boundary among Korean Camellia populations
to be 100 km. White-eyes are known to migrate for long
distances from the mainland to the Izu islands (Kikkawa
& Kakizawa 1981). Thus, with their high mobility whiteeyes can transfer genes via pollen widely within a population and occasionally spread between populations.
We have demonstrated experimentally that white-eyes
provide a highly efficient and plentiful pollination service
for C. japonica; fruit sets were saturated after only five
visits and 84.6% of observed flowers received more than
five visits by white-eyes within only 3 days (less than half
the longevity of a flower). Pollination efficiency and the
total amount of pollen received have previously been
shown to be dependant on two components of pollinator
behavior; visitation frequency and the amount of pollen
deposited on stigmas per visit (Herrera 1987, 1989), both
of which depend highly on the level of specialization of
the pollinator to the plant species. If bird visitors use
various diets in their foraging trail both visitation rate and
the amount of pollen deposited on stigmas per visit
should decrease (Feinsinger et al. 1988). White-eyes are
typical omnivores and their food items from spring to fall
include many types of insects, spiders, fruits and seeds
(Yamashina 1941; Kikkawa & Kakizawa 1981). However,
they switch their diets to floral resources in winter and
feed primarily on C. japonica, which can comprise 70–90%
of their foraging items (Y. Kunitake and M. Hasegawa,
unpubl. data, 1999). It appears that this strong specialization during winter enables white-eyes to be effective pollinators of C. japonica despite their omnivorous diet over
the remainder of the year.
The finding that omnivorous birds are effective pollinators has rarely been reported in a temperate zone. It is
possible that ecological interactions explain this seasonal
specialization by white-eyes. The flowering season of
C. japonica is over the coldest period in central Japan. In
this season, invertebrates, fruits and other flowering
plants are very scarce, whereas C. japonica flowers secrete
large amounts of nectar and are abundant in the broadleaved evergreen forest at this time. This phenological
concordance between a pollinator’s resource shortage and
the flowering of C. japonica may have led to the seasonal
specialization of Z. japonica (an omnivorous bird), and as
a result of this specialization Z. japonica provides an effective pollination service.
Acknowledgments
We thank K. Ueda, C. Ueda, M. Eda, S. Sakai, H. Abe, R.
Kawate and S. Kunitake for field assistance. G. Fujita and
I. Washitani provided some helpful comments on our study.
K. Fujita advised us regarding the field experiment. R.
Walters improved the English of this manuscript. We also
thank the staff at the Niizima-mura museum and S. Isobe
for valuable information in the field. The research was
partly supported by the Japan Ministry of Education, Science, Sport and Culture (grant no. 13440229 to H. Higuchi).
References
Chase M. R., Moller C., Kesseli R. & Bawa K. S. (1996) Distant
gene flow in tropical trees. Nature 383: 398–399.
Chung M. G. & Kang S. S. (1996) Genetic variation within and
among populations of Camellia japonica (Theaceae) in Korea.
Canadian Journal of Forest Research 26: 537–542.
© 2004 The Society for the Study of Species Biology Plant Species Biology 19, 197–201
P O L L I N AT I O N I N C . J A P O N I C A B Y W H I T E E Y E S
Dow B. D. & Ashley M. V. (1998) High levels of gene flow in bur
oak revealed by paternity analysis using microsatellites. Journal of Heredity 89: 62–70.
Dutech C., Seiter J., Petronelli P., Joly H. I. & Jarne P. (2002)
Evidence of low gene flow in a neotropical clustered tree
species in two rainforest stands of French Guiana. Molecular
Ecology 11: 725–738.
England P. R., Beynon F., Ayre D. J. & Whelan R. J. (2001) A
molecular genetic assessment of mating system variation in
a naturally bird-pollinated shrub: contributions from birds
and introduced honeybees. Conservation Biology 15: 1645–
1655.
Faegri K. & Van der Pijl L. (1971) The Principles of Pollination
Ecology, 2nd edn. Pergamon Press, Oxford.
Feinsinger P., Busby W. H. & Tiebout H. M. (1988) Effects of
indiscriminate foraging by tropical hummingbirds on pollination and plant reproductive success: experiments with two
tropical treelets (Rubiaceae). Oecologia 76: 471–474.
Ghazoul J., Liston K. A. & Boyle T. J. B. (1998) Disturbanceinduced density-dependent seed set in Shorea siamensis
(Dipterocarpaceae), a tropical forest tree. Journal of Ecology 86:
462–473.
Govindaraju D. R. (1988) Relationship between dispersal ability
and levels of gene flow in plants. Oikos 52: 31–35.
Herrera C. M. (1987) Components of pollinator quality: comparative analysis of a diverse insect assemblage. Oikos 50:
79–90.
Herrera C. M. (1989) Pollinator abundance, morphology, and
flower visitation rate analysis of the quantity component in a
plant pollinator system. Oecologia 80: 241–248.
Kikkawa J. & Kakizawa R. (1981) Agonistic behaviour of Japanese White-eyes Zosterops japonica on Miyake Island. Journal
of the Yamashina Institute for Ornithology 13: 60–70.
Oh G. S., Kim J. H., Kang S. S., Yeeh Y. & Chung M. G. (1995)
Spatial genetic structure among Korean populations of Camellia japonica and Eurya japonica (Theaceae). Plant Species Biology
10: 155–161.
Ohyama A. (2001) Flora and plant community diversity in the
201
Izu-islands: Niizima island, Shikinezima Island, Jinaitou
Island. Niizima Museum Annual Report 2001. (In Japanese).
Paton D. C. (1997) Honey bees Apis mellifera and the disruption
of plant-pollinator systems in Australia. Victorian Naturalist
114: 23–29.
Ramsey M. W. (1988) Differences in pollinator effectiveness of
birds and insects visiting Banksia menziesii (Proteaceae). Oecologia 76: 119–124.
Schmitt J. (1980) Pollinator foraging behavior and gene dispersal
in Senecio (Compositae). Evolution 34: 934–943.
Schulke B. & Waser N. M. (2001) Long-distance pollinator flights
and pollen dispersal between populations of Delphinium nuttallianum. Oecologia 127: 239–245.
Shibata M. & Ieyumi S. (1991) Self-incompatibility in a wild
population of Camellia. National Institute of Vegetable and Tea
Science. Kurume Branch Annual Report 4: 182–183. (In Japanese).
Tanaka H. (1980) Bird-pollination of Camellia japonica. Saisyu to
Siiku 42: 382–383. (In Japanese).
Ueno S., Tomaru N., Yoshimaru H., Manabe T. & Yamamoto S.
(2000) Genetic structure of Camellia japonica L. in an oldgrowth evergreen forest, Tsushima, Japan. Molecular Ecology
9: 647–656.
Wendel J. F. & Parks C. R. (1985) Genetic diversity and population structure in Camellia japonica L. (Theaceae). American Journal of Botany 72: 52–65.
Wolff K., El-Akkad S. & Abbott R. J. (1997) Population substructure in Alkanna orientalis (Boraginaceae) in the Sinai Desert,
in relation to its pollinator behaviour. Molecular Ecology 6:
365–372.
Yamashina Y. (1941) Research on diets of birds in Japan. Tori 11:
19–21. (In Japanese).
Yeeh Y., Kang S. S. & Chung M. G. (1996) Evaluations of the
natural monument populations of Camellia japonica
(Theaceae) in Korea based on allozyme studies. Botanical Bulletin of Academia Sinica 37: 141–146.
Yumoto T. (1988) Pollination systems in the cool temperate
mixed coniferous and broad-leaved forest zone of Yakushima
Island. Ecological Research 3: 117–129.
© 2004 The Society for the Study of Species Biology Plant Species Biology 19, 197–201