The history of anthropogenic disturbance and invasive alien species impact on the indigenous avifauna of the Ogasawara Islands, southern Japan

Abstract

The Ogasawara Islands are subtropical oceanic islands in the Northwestern Pacific. Since their unique ecosystem is recognized to have outstanding universal value, they are registered as a UNESCO World Natural Heritage site. However, human settlement since 1830 has seriously impacted the biota. So far, the breeding of 20 land birds and 21 seabirds, including three introduced species, has been recorded on the islands. Seven endemic species/subspecies have become extinct, and breeding populations of five species have disappeared completely. Such extinctions have been caused by habitat loss, overhunting, and alien species invasion. Specifically, alien mammals are thought to have had devastating effects on avian populations. At least ten mammal species have been introduced, five of which (goats, cats, black rats, Norway rats, and house mice) still occur there today. Owing to their serious ecological impacts, eradication programs have been implemented. Feral goats had spread to 20 islands, but have been completely eradicated from all except Chichijima. Feral cats currently exist on only four islands and are being removed from natural areas on Chichijima. Non-native rodents have invaded almost all of the islands. Eradication programs using rodenticide are ongoing on some of the islands and have succeeded on four of them. After successful eradication, some land birds and seabirds have increased in numbers and expanded their distribution. However, undesired results also have been confirmed. For example, invasive plants have increased following goat eradication due to the elimination of grazing, and increasing seabirds have become dispersers of alien plants with adhesive seeds. Rat populations have likely increased after cat eradication due to their release from predation. These accumulated experiences should inform future adaptive management of the islands based on scientific evidence of specific species interactions.