Evolution of mutualism in plant-pollinator interactions on islands.

ABSTRACT

The evolution and ecology of interactions between plants and pollinators are discussed based on the studies on the Izu Islands and mainland Honshu, Japan. The species assemblage is depauperate, and long-tongued pollinators are absent or rare on the islands. Bumblebees, one of the most important pollinators in Japan, are generally absent. Plants depending strongly on bumblebee pollination are absent on Izu Islands, but those depending on varied pollinators including bumblebees display smaller flower sizes and accommodate smaller pollinators than their mainland counterparts. Breeding systems of these species also shift to partial inbreeding, possibly an evolutionary result of the decrease in pollinator availability. Changes in flowering phenology between mainland and island populations also occur. Plants in the islands tend to reproduce vegetatively less frequently and produce greater numbers of smaller seeds than those in the mainland. The possibility of evolution on the side of island pollinator species is also discussed, although there are few data on this topic.