Comparison of Per-Pixel and Object-Oriented Approaches for Mapping Plant Communities in Hawaii Using IKONOS Imagery

Background Alien or non-indigenous species are now considered a major element of global environmental change (Vitousek et al. 1996; Mack et al. 2000). Invasive species pose a significant threat to many ecosystems worldwide by their direct and indirect impacts to native species and by their effects on broader scale ecological patterns and processes (Cole and Landres 1996). Biological invasions have been recognized for their effects in the degradation of global biological diversity (Davis 2003). Natural environments in tropical oceanic islands have been impacted by nonindigenous species more than continental areas (Elton 1958; Denslow 2003). The Hawaiian Islands, in particular, have experienced rapid wide-ranging changes in land cover and loss of native biodiversity as a result of the spread of aggressive non-native invasive species. More native species have been extirpated in Hawaii than anywhere else in the United States (Ewel et al 1999; Loope 1998). Alien species pose the single greatest threat to Hawaii’s unique flora and fauna (Ewel et al. 1999). Specifically, plant invasions have replaced diverse tropical rainforest ecosystems by displacement of native species. Invasive plant characteristics such as alleopathy, high fecundity and vegetative spread have exhibited dominance over native plant populations resulting in the formation of monotypic stands (Smith 1985). Three of the most destructive invading plants affecting intact native ecosystems on O`ahu are Strawberry guava (Psidium cattleianum), manuka (Leptospermum scoparium) and rose apple (Syzygium jambos). These disruptive weeds gradually convert native forest to single-species stands and alter ecosystem-