Outstanding performance of an invasive alien tree Bischofia javanica relative to native tree species and implications for management of insular primary forests.

Invasive alien tree species can exert severe impacts, especially in insular biodiversity hotspots, but have been inadequately studied. Knowledge of the life history and population trends of an invasive alien tree species is essential for appropriate ecosystem management. The invasive tree Bischofia javanica has overwhelmed native trees on Haha-jima Island in the Ogasawara Islands, Japan. We explored forest community dynamics 2 years after a typhoon damaged the Sekimon primary forests on Haha-jima Island, and predicted the rate of population increase of B. javanica using a logistic model from forest dynamics data for 19 years. During the 2 years after the typhoon, only B. javanica increased in population size, whereas populations of native tree species decreased. Stem diameter growth of B. javanica was more rapid than that of other tree species, including native pioneer trees. Among the understory stems below canopy trees of other species, B. javanica grew most rapidly and B. javanica canopy trees decreased growth of the dominant native Ardisia sieboldii. These competitive advantages were indicated to be the main mechanism by which B. javanica replaces native trees. The logistic model predicted that B. javanica would reach 30% of the total basal area between 2017 (in the eastern plot adjacent to a former B. javanica plantation) and 2057 (in the western plot distant from the plantation site), which is a maximum percentage allowing to eradicate under the present guideline of the National Forest. The results suggest immediate removal of B. javanica is required to preserve native biodiversity in these forests.

Associations among species traits, distribution, and demographic performance after typhoon disturbance for 22 co-occurring woody species in a mesic forest on a subtropical oceanic island.

To support conservation and restoration, it is important to understand how differences in species functional traits relate to the distribution and demographic performance (i.e., changes in rates of growth, survival and recruitment) of co-occurring endemic, indigenous, and alien species on oceanic islands, where species are especially vulnerable to invasion. We examined interspecific differences in leaf and wood traits, and their associations with species origin (endemic, indigenous, and alien), distribution patterns, and demographic performance after typhoon disturbance for 22 co-occurring woody species at Sekimon on Hahajima Island in the Ogasawara Islands. Principal components analysis showed that the first and second principal components were associated with trait variations along spectra of leaf and wood economics (LES and WES). Species origin was not significantly associated with these components. Conservative species with low resource acquisition along the LES were abundant before the typhoon but acquisitive species had higher recruitment after the typhoon. After the typhoon, acquisitive species along the WES showed higher recruitment, relative growth rates but low survival rates on soil substrates. Endemic and indigenous species had lower recruitment and relative growth rates and endemic species had lower survival rates than alien species. Alien, endemic and indigenous species have similar functional space along the LES and WES, but these functional differences do not simply explain high demographic performance of alien species after the typhoon under the conditions of the species composition shifting from endemic species to alien species after repeated typhoon disturbances.

Plant species diversity, community structure and invasion status in insular primary forests on the Sekimon uplifted limestone (Ogasawara Islands).

Native forests on oceanic islands are among the most threatened ecosystems. The forests formed on Sekimon uplifted limestone in Haha-jima Island (Ogasawara Islands) have not yet been destroyed by human activities and remain as primary forests harboring several narrow endemic endangered plants. In this paper, we described the plant species diversity, community structure, and status of invasion by alien plants in the mesic forests of Sekimon. The Sekimon forest was characterized by low tree diversity (37 species), high stem density (1731 ha-1), and high basal area (63.9 m2 ha-1), comparing with natural forests in world islands. The forests were dominated in the number of stems by the sub-tree Ardisia sieboldii followed by the trees Pisonia umbellifera and Elaeocarpus photiniifolius. The invasive tree Bischofia javanica ranked fourth for basal area and third for the number of stems (DBH ≥ 10 cm), and its distribution expanded, especially near a past plantation site. Surveys of forest floor vegetation revealed that species richness of vascular plants was 109 species and that many alien plants had already invaded the forests. Despite the low species richness of alien (16% for vascular flora and 8% for trees), the high frequency of aliens on the forest floor suggests that they have colonized successfully in the Sekimon forest. Extrapolation analysis based on the rarefaction curves predicted that the vascular plants in the Sekimon (25 ha) accounted for 135 species (29.9% of the vascular flora of the Ogasawara Islands) and endemic plants were 85 species (62.0%). The fact that the 39 vascular species recorded in our plots were listed in Japanese Red List suggests that the Sekimon forest should be conserved as a sanctuary of biodiversity. Because alien plants are invading the forests without apparent anthropogenic disturbance, immediate action to eradicate these invaders is highly needed.

Complete genotyping in conservation genetics, a case study of a critically endangered shrub, Stachyurus macrocarpus var. prunifolius (Stachyuraceae) in the Ogasawara Islands, Japan.

More than 3,000 species are listed as critically endangered worldwide, and various conservation measures such as habitat restoration, assisted reproduction and establishment of ex situ populations would be required to prevent their extinction. We determined the genotype of all 15 known wild clumps using nuclear microsatellite markers for Stachyurus macrocarpus var. prunifolius, a critically endangered shrub endemic to the Ogasawara (Bonin) Islands of Japan. In addition, the seedlings propagated from seeds taken from one wild clumps were genotyped. The results of complete genotyping showed that both wild and nursery populations had population-specific alleles. Two alleles were detected only in the nursery population, indicating the existence of undiscovered mature individuals in the wild. Four alleles were found only in the wild and were detected in two geographically isolated clumps, and this finding may propose that re-introduction and transplantation between different sites requires sensitive handling in terms of the conservation of evolutionary significant units. These results show that complete genotyping can provide essential genetic and ecological information for effective management of endangered species.

Flower-visiting behavior of male bees is triggered by nectar-feeding insects.

Bees are important pollinators for many flowering plants. Female bees are thought to be more effective pollinators than male bees because they carry much more pollen than males. Males of some solitary bee species are known to patrol near flowers that females visit. Because patrolling males visit flowers to mate or defend their territories, they may function as pollinators. However, the significance of patrolling males to pollination has not been studied. We studied males of a solitary bee, Heriades fulvohispidus (Megachilidae), patrolling near flowers and visiting flowers that attracted nectar-feeding insects, including conspecifics, on the Ogasawara (Bonin) Islands. To test the hypothesis that patrolling male bees may function as pollen vectors, we compared the frequency of visits by H. fulvohispidus to flowers of an endemic plant, Schima mertensiana (Theaceae); comparisons were made among flowers with a dead H. fulvohispidus, a dead beetle, a piece of plastic, and nothing (control flowers). Patrolling H. fulvohispidus more frequently visited flowers with a dead conspecific, a dead beetle, or a piece of plastic than the control flowers. Our experiment demonstrates that nectar-feeding insects (including conspecifics and other insects) enhance the flower-visiting frequency of patrolling H. fulvohispidus males on S. mertensiana flowers. Furthermore, we observed S. mertensiana pollen on patrolling males as well as females, suggesting that male bees may also function as pollen vectors.

Threatened pollination systems in native flora of the Ogasawara (Bonin) Islands.

BACKGROUND AND AIMS: Various alien species have been introduced to the Ogasawara Islands (Japan). A survey was made investigating whether the native pollination systems fit an 'island syndrome' (biasing the flora to dioecy, with subdued, inconspicuous flowers) and whether alien species have disrupted the native pollination network. METHODS: Flower visitors and floral traits were determined in the field (12 islands) and from the literature. Associations among floral traits such as sexual expression, flower colour and flower shape were tested. KEY RESULTS: Among the 269 native flowering plants, 74.7 % are hermaphroditic, 13.0 % are dioecious and 7.1 % are monoecious. Classification by flower colour revealed that 36.0 % were white, 21.6 % green and 13.8 % yellow. Woody species (trees and shrubs) comprised 36.5 % of the flora and were associated with dioecy and white flowers. Solitary, endemic small bees were the dominant flower visitors and visited 66.7 % of the observed species on satellite islands where the native pollination networks are preserved. In contrast to the situation on the satellite islands, introduced honeybees were the most dominant pollinator (visiting 60.1 % of observed species) on the two main islands, Chichi-jima and Haha-jima, and had spread to satellite islands near Chichi-jima Island. CONCLUSIONS: The island syndrome for pollination systems in the Ogasawara Islands was evident in a high percentage of dioecious species, the subdued colour of the native flora and solitary flower visitors on satellite islands. The shape and colour adaptations of several flowers suggested native pollination niches for long-proboscis moths and carpenter bees. However, the domination and expansion of introduced honeybees have the potential for disruption of the native pollination network in the two main, and several satellite, islands of the Ogasawara Islands.

Loss of extrafloral nectary on an oceanic island plant and its consequences for herbivory.

Two Hibiscus (Malvaceae) species coexist on the oceanic Bonin (Ogasawara) Islands: Hibiscus glaber (an endemic species) and H. tiliaceus (the ancestral non-endemic species). Hibiscus tiliaceus produces extrafloral nectar from the sepals, while H. glaber does not. To clarify the effects of extrafloral nectar loss on Hibiscus-insect relationships, we examined herbivory and insect communities on flower buds of H. glaber and H. tiliaceus. Larvae of the endemic moth Rehimena variegata (Lepidoptera: Pyralidae) attacked 20% of the flower buds on H. glaber, while less than 0.2% of buds on H. tiliaceus were attacked. Introduced species of ants frequently visited the flower buds of H. tiliaceus to collect extrafloral nectar from the sepal, while they rarely visited those of H. glaber. Therefore, extrafloral nectar on H. tiliaceus sepals may function as a facultative defense against flower bud herbivory. The loss of extrafloral nectaries of H. glaber sepals may be related to the original paucity of native herbivores and ants on the Bonin Islands.

Size distribution, growth and inter-year variation in sex expression of Bischofia javanica, an invasive tree.

Flowering activity and sex expression of Bischofia javanica Blume were investigated for 3 years. B. javanica is an invasive dioecious tree of subtropical forests on the Bonin Islands in the western Pacific of Japan. The sex ratio showed a significant male bias (1.25-2.33). Smaller trees were significantly male biased, whereas larger trees showed no significant difference in sex expression, suggesting that males tend to be more precocious in sexual reproduction. We found evidence for sex changes in B. javanica; these have not been reported previously. Most of the 1,653 census trees remained non-flowering (58.1 %); 3.7 % of them showed sex changes, and the percentage of trees repeatedly flowering as males and females was 10.5 and 3.4 %, respectively. Sex changes were observed in both directions but a larger percentage of male trees became female. Flowering frequency and sex expression were significantly related to tree size (i.e. diameter at breast height). Over the 3 years, trees that were consistent females were the largest; inconsistent trees (switching sex between years) were intermediate in size, whereas consistent males were the smallest. There were no significant differences in relative growth rate (RGR) among trees of different sex or flowering frequencies. These results suggest that the maintenance of female reproduction is not related to changes in RGR of diameter but to flowering frequency or the reversal to the male form, dependent upon the internal resource status of individual trees.

Flower bud abortion influences clonal growth and sexual dimorphism in the understorey dioecious shrub Aucuba japonica (Cornaceae).

Sexual differences were investigated to determine the significance of flower bud abortion in the dioecious shrub Aucuba japonica Thunb. The mean number of flowers per inflorescence and the mean number of flowering inflorescences (as opposed to aborted inflorescences) per individual were greater in males than in females in 1997 and 1998. Reproductive investment by males was 0.4-times (1997) and 1.4-times (1998) that by females. In addition, females aborted 30.9% (1997) and 42.7% (1998) of their total flower buds without blooming, whereas no male flower buds aborted. One of the architectural traits of this shrub is that in the year that a flower bud is produced at the shoot apex, the shoot will branch into two or more shoots. Thus, there was less sexual difference in the number of current shoots per individual than there was in the number of flowering inflorescences. The relationship between annual growth and reproduction, and the probability of reproduction in the following year, suggested that the higher investment in female reproduction was manifested as a cost for reproductive frequency rather than as a cost for annual growth. The spatial distribution of both males and females was clumped, which may be the result of clonal growth. In addition, overall sex ratios were not skewed and the number of sprouts did not differ significantly between sexes. These results suggested that flower bud abortion by females might reduce sexual dimorphism in terms of clonal growth.