Eradication of the Invasive Tree Species Bischofia javanica and Restoration of Native Forests on the Ogasawara Islands

Abstract

Bischofia javanica is the most invasive alien tree species on the Ogasawara Islands; it is seen as a significant problem for biodiversity conservation. B. javanica has colonized four Ogasawara islands, namely, Ototojima, Chichijima, Hahajima and Hirashima. On Ototojima, B. javanica was never abundant and has been almost eliminated through actions of an eradication project operating since 2005. However, larger numbers of the alien trees are distributed over wider areas on Chichijima and Hahajima, and eradication will be problematic. Aerial images captured in 2003 showed that the invaded areas on Chichijima and Hahajima covered 50.9 ha (2.1%) and 296.5 ha (14.7%) of the islands, respectively. The potentially invasible habitat area on Hahajima was estimated to be 59.2% of terrain (judged from the vegetation type distribution on the island). A model predicting the alien tree's distribution on Hahajima was developed by mapping environmental factors (elevation, slope, drainage basin, curvature, and topographic openness) and distance from seed source. We developed an occurrence probability map (potential habitat map) estimated solely from measured environmental factors. The expected risk of invasion was estimated by comparing present distributions with potential habitat areas. A survey of forest development after clear-cutting B. javanica stands demonstrated natural regeneration potential in a diversity of native tree species. Based on available ecological knowledge and the results of eradication projects, we propose a management system aiming to eradicate B. javanica from whole island systems.