2:05 PM - 2:25 PM
[O06-02] Predators initiate primary succession on oceanic islands: The rise of seabird-driven ecosystems on Nishinoshima
★Invited Papers
Keywords:Nishinoshima, Seabird-driven ecosystem, booby, tern, ecological function, Dermaptera
Nishinoshima in the Ogasawara Islands is an isolated oceanic island, 130 km from the nearest island. The island used to be home to eight species of seabirds, six species of plants and more than 30 species of arthropods. However, the eruptions in 2013-2020 covered the entire land with lava and volcanic ash, resulting in the loss of the previous ecosystem. This is a situation that can be described as the emergence of a new island in the open sea. Understanding how terrestrial ecosystems form on newly formed islands is one of the key issues in island biology. Throughout human history, new islands have arisen where biota transitions have been observed, such as Krakatau in Indonesia and Surtsey in Iceland. However, these are continental islands, less than 20 km from neighbouring islands. On the other hand, Nishinoshima is the first island in human history to be formed in extreme isolation. Therefore, elucidating the primary succession on this island will provide insights into the primary succession on oceanic islands such as the Hawaiian, Galapagos and Ogasawara Islands.
Following a major eruption in 2019-2020 (Episode IV), the transition of the biota on Nishinoshima has been monitored by the Ministry of the Environment. Immediately after episode IV, it was confirmed that a large number of seabirds, such as boobies and terns, had resumed nesting. Primary succession is generally thought to be a bottom-up type of succession, where plants become established first, followed by animals such as insects that feed on the plants. This is because terrestrial animals cannot survive without food on land. Seabirds, on the other hand, prey on fish and other organisms in the sea and can nest and rest without living things on land. This allows them to quickly settle on land, even in barren areas where there is no other life on land. Seabirds have a wide range of ecosystem functions, including delivering nutrients from the sea to land, dispersing seeds by attaching them to their plumage, engineering ecosystems to create arthropod habitats by nesting, and providing food for decomposers through regurgitation and carcasses. They form colonies at high population densities, so that together they perform very large ecosystem functions. In Nishinoshima, it is expected that seabirds, the top predators of the marine ecosystem, will first settle to provide nutrients to the land, and their carcasses will promote the establishment of arthropods that become decomposers, followed by plants, which is the reverse order of the bottom-up type of transition described above. This type of ecosystem is referred to here as a seabird-driven ecosystem, and the study focuses on changes in the distribution of seabirds and the associated changes in the distribution of other organisms. As seabirds can only colonise islands without carnivorous mammals such as foxes and cats, this type of ecosystem is unique to oceanic islands.
The rapid re-establishment of seabird colonies in 2021 can be attributed to changes in topography. Before Episode IV, the island was largely covered by AA lava and there were few flat areas suitable for nesting. However, the thick layer of volcanic ash that covered the entire island resulted in the development of available nesting environments across the island. In addition, gullies created by the flow of water created gravelly areas favoured by terns. However, the breeding success of the seabirds was very low. This was thought to be because the volcanic ash was poorly drained, so nests were easily submerged when it rained, and breeding was inhibited by water flow and volcanic steam in the gully. In other words, Nishinoshima was an ecological trap where breeding was likely to fail, even though the topography made nesting easy. Under these circumstances, seabirds have recently changed their nesting habitats and distribution on the island, suggesting behavioural adaptations. The distribution of arthropods, which act as decomposers, has also expanded as the distribution of seabirds has expanded.
Because the island's area is limited, island species are vulnerable to extinction. The island's biota is shaped by repeated introductions and extinctions, and Nishinoshima is a rare natural experiment field in the world where this process can be directly observed. Here, geological events such as volcanic eruptions and landform changes affect the distribution of seabirds, which in turn affects the processes of ecosystem establishment. Continued monitoring of terrestrial primary succession on the island is expected to provide evidence for the existence of a seabird-driven ecosystem.
Following a major eruption in 2019-2020 (Episode IV), the transition of the biota on Nishinoshima has been monitored by the Ministry of the Environment. Immediately after episode IV, it was confirmed that a large number of seabirds, such as boobies and terns, had resumed nesting. Primary succession is generally thought to be a bottom-up type of succession, where plants become established first, followed by animals such as insects that feed on the plants. This is because terrestrial animals cannot survive without food on land. Seabirds, on the other hand, prey on fish and other organisms in the sea and can nest and rest without living things on land. This allows them to quickly settle on land, even in barren areas where there is no other life on land. Seabirds have a wide range of ecosystem functions, including delivering nutrients from the sea to land, dispersing seeds by attaching them to their plumage, engineering ecosystems to create arthropod habitats by nesting, and providing food for decomposers through regurgitation and carcasses. They form colonies at high population densities, so that together they perform very large ecosystem functions. In Nishinoshima, it is expected that seabirds, the top predators of the marine ecosystem, will first settle to provide nutrients to the land, and their carcasses will promote the establishment of arthropods that become decomposers, followed by plants, which is the reverse order of the bottom-up type of transition described above. This type of ecosystem is referred to here as a seabird-driven ecosystem, and the study focuses on changes in the distribution of seabirds and the associated changes in the distribution of other organisms. As seabirds can only colonise islands without carnivorous mammals such as foxes and cats, this type of ecosystem is unique to oceanic islands.
The rapid re-establishment of seabird colonies in 2021 can be attributed to changes in topography. Before Episode IV, the island was largely covered by AA lava and there were few flat areas suitable for nesting. However, the thick layer of volcanic ash that covered the entire island resulted in the development of available nesting environments across the island. In addition, gullies created by the flow of water created gravelly areas favoured by terns. However, the breeding success of the seabirds was very low. This was thought to be because the volcanic ash was poorly drained, so nests were easily submerged when it rained, and breeding was inhibited by water flow and volcanic steam in the gully. In other words, Nishinoshima was an ecological trap where breeding was likely to fail, even though the topography made nesting easy. Under these circumstances, seabirds have recently changed their nesting habitats and distribution on the island, suggesting behavioural adaptations. The distribution of arthropods, which act as decomposers, has also expanded as the distribution of seabirds has expanded.
Because the island's area is limited, island species are vulnerable to extinction. The island's biota is shaped by repeated introductions and extinctions, and Nishinoshima is a rare natural experiment field in the world where this process can be directly observed. Here, geological events such as volcanic eruptions and landform changes affect the distribution of seabirds, which in turn affects the processes of ecosystem establishment. Continued monitoring of terrestrial primary succession on the island is expected to provide evidence for the existence of a seabird-driven ecosystem.