Alteration of secondary-successional pathways on northwestern pacific islands by the invasion of leucaena leucocephala

It is commonly believed that plant communities on continental islands are less susceptible to invasion by exotic plant species than are those on oceanic islands, due to their greater native diversity. To test this hypothesis, we compared the successional pathways in abandoned fields invaded by Leucaena leucocephala on Haha-jima, an oceanic island, and Miyako-jima, a continental island. Although L. leucocephala scrub established on both subtropical islands immediately after agricultural abandonment, there was a significant difference in the secondary-successional pathways. On Haha-jima, the invasion of this alien species altered the secondary-successional pathway irreversibly by restricting the establishment of indigenous species. In contrast, it did not affect the secondary-successional pathway as seriously on Miyako-jima, where indigenous species of fast-growing secondary trees such as Macaranga tanarius and Melanolepis multiglandulosa replaced dense thickets of L. leucocephala. These results provide evidence for the hypothesis that simple communities on oceanic islands are more susceptible to invasion by L. leucocephala than diverse communities on continental islands. It is possible that the apparently high ability of L. leucocephala to invade Haha-jima is due to the scarcity of indigenous, fast-growing secondary forest species