Rare and alien plant floras on cliffs and collapsed areas in the Ogasawara Islands surveyed with UAVs

Abstract

The conservation importance and alien plant invasion of cliffs and recently collapsed lands due to slope failures in the Ogasawara Islands was previously unknown due to difficulties in human access. In this study, a UAV was used to approach the cliffs and collapsed lands and take images to figure out the flora growing in these inaccessible areas. A total of 100 species were identified as a result of image interpretation, of which 20 were rare species and 33 were alien species. A generalized linear model was used to analyze under what conditions rare and alien species appear in large numbers. The results showed that the number of rare species was higher on cliff slopes with low annual solar radiation and at the crown top edge (the transition between the top of the cliff and collapse lands and the forest), while the number of alien species was higher on inhabited islands and in uplifted areas (collapsed sediment deposition areas). A comparison using Fisher's exact test was employed to understand how the appearance of these species identified differed between cliffs and collapsed lands and forest floor, and between cliffs and collapsed lands. The comparison between cliffs and collapsed lands and forest floor showed that the majority of rare and alien species appear more frequently in cliffs and collapsed lands. The comparison between cliffs and collapsed lands showed that both rare and alien species were more frequent in cliffs than collapsed lands. This study revealed that the flora grows on cliffs and collapsed lands in the Ogasawara Islands, and indicated that these areas are important habitats.