Insular evolution in plants

Oceanic islands provide unique opportunities to study evolution. On these islands, the morphological and ecological evolution of land plants can involve habitat changes (niche expansion), life form alterations (woodiness), sex expression variations (dioecism), and modifications in seed dispersal modes (loss of dispersal abilities). However, a lesser-known phenomenon is the loss of extrafloral nectaries in plants, observed on the Hawaiian Islands, which are isolated oceanic islands. This loss is believed to be related to the absence of native ants on the Hawaiian Islands.

Like other oceanic islands, the Ogasawara Islands support many endemic plants, with an endemism rate of approximately 70%. We observed a similar loss of extrafloral nectaries in a hibiscus species endemic to the Ogasawara Islands. The ancestral non-endemic species, Hibiscus tiliaceus, produces extrafloral nectar from the sepals, whereas the endemic species, H. glaber, does not.

Impacts of invasive plants on endemic insects

The Ogasawara Islands support many endemic plants (approximately 70%) and insects (approximately 40%). The insect fauna of these islands is characterized by forest-dwelling species such as wood-feeding beetles. However, natural forests have been replaced by invasive non-native trees, such as Bischofia javanica and Casuarina equisetifolia. Since insects and plants are reciprocally dependent, invasive non-native trees may impact endemic insects. We investigated the current status of native insect diversity and their associations with non-native trees on the Ogasawara Islands.

Bionomics of endemic bees

Like other oceanic islands, the Ogasawara Islands originally had no social bees. There are nine (or ten) solitary bee species on the Ogasawara Islands, all of which are endemic. These endemic bees are considered important pollinators. However, since the 1980s, endemic bees other than the large carpenter bee (Xylocopa ogasawaraensis) have rarely been found on inhabited islands due to the negative impacts of non-native organisms. As a result, the pollination behavior of these endemic bees is poorly understood. To address this, we studied the flower-visiting behavior of endemic bees to examine their roles in pollinating native plants.

Invasive predators on islands

The Ogasawara Islands support many endemic land snails, with an endemism rate of approximately 90% (Dr. S. Chiba's web site). A predatory flatworm, Platydemus manokwari (Global Invasive Species Database), feeds on live snails of any species. The introduction of P. manokwari has been considered a cause of the decline of endemic land snails on tropical oceanic islands. On the Ogasawara Islands, the non-native predatory flatworm has spread widely across the largest inhabited island, Chichijima, eradicating the land snail fauna. We demonstrated the rapid decrease in land snail survival caused by P. manokwari predation in the field.

Since P. manokwari  can be readily transported in soil on various materials, such as potted plants, we proposed a hot water treatment to quarantine these materials.

Publication

Sugiura, S. & Hayashi, M. (2018) Functional compensation by insular scavengers: the relative contributions of vertebrates and invertebrates vary among islands. Ecography, 41: 1173–1183. 

Sugiura, S. (2016) Impacts of introduced species on the biota of an oceanic archipelago: the relative importance of competitive and trophic interactions. Ecological Research, 31: 155–164. (invited review)

Sugiura, S., Tsuru, T. & Yamaura, Y. (2013) Effects of an invasive alien tree on the diversity and temporal dynamics of an insect assemblage in an oceanic island. Biological Invasions, 15(1): 157–169.

Sugiura, S. (2010) Prey preference and gregarious attacks by the invasive flatwormPlatydemus manokwari. Biological Invasions, 12(6): 1499-1507.

Sugiura, S. (2010) Species interactions-area relationships: biological invasions and network structure in relation to island area. Proceedings of the Royal Society B: Biological Sciences, 277: 1807-1815. 

Sugiura, S. (2010) Associations of leaf miners and leaf gallers with island plants of different residency histories. Journal of Biogeography, 37(2):237-244.

Sugiura, S. (2009) Seasonal fluctuation of invasive flatworm predation pressure on land snails: Implications for the range expansion and impacts of invasive species.Biological Conservation, 142(12):3013-3019. 

Sugiura, S. & Yamaura, Y. (2009) Potential impacts of the invasive flatwormPlatydemus manokwari on arboreal snails. Biological Invasions, 11: 737-742.

Sugiura, S., Tsuru, T., Yamaura, Y. & Makihara, H. (2009) Small off-shore islands can serve as important refuges for endemic beetle conservation. Journal of Insect Conservation, 13(4): 377-385.

Sugiura, S., Yamaura, Y., Tsuru, T., Goto, H., Hasegawa, M., Makihara, H. & Makino, S. (2009) Beetle responses to artificial gaps in an oceanic island forest: implications for invasive tree management to conserve endemic species diversity. Biodiversity and Conservation, 18(8): 2101-2118.

Kawazoe, K., Kawakita, A., Sugiura, S. & Kato, M. (2008) Phylogenetic position of the endemic large carpenter bee of the Ogasawara Islands, Xylocopa ogasawarensis(Matsumura, 1912) (Hymenoptera: Apidae), inferred from four genes. Zoological Science, 25(8): 838-842. (cover photo)

Sugiura, S., Yamaura, Y. & Makihara, H. (2008) Biological invasion into the nested assemblage of tree-beetle associations on the oceanic Ogasawara Islands. Biological Invasions, 10(7): 1061-1071. 

Sugiura, S. (2008) Male territorial behaviour of the endemic large carpenter bee,Xylocopa (Koptortosoma) ogasawarensis (Hymenoptera: Apidae), on the oceanic Ogasawara Islands. European Journal of Entomology, 105(1): 153-157. (PDF 252KB)

Sugiura, S., Abe, T., Yamaura, Y. & Makino, S. (2007) Flower-visiting behavior of male bees is triggered by nectar-feeding insects. Naturwissenschaften, 94(8): 703-707.

杉浦真治 (2007)  島の植物に被食防御は必要か？―海洋島での花外蜜腺の消失と移入アリによる影響. 生物科学, 58 (2)：111–114.

Sugiura, S., Abe, T. & Makino, S. (2006) Loss of extrafloral nectary on an oceanic island plant and its consequences for herbivory. American Journal of Botany, 93 (3): 491–495.