Volcano Watch — Evolution and island hopping

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The rapidly changing geology in the Hawaiian Islands has profound consequences for the plants and animals in Hawai'i on several time scales.

The rapidly changing geology in the Hawaiian Islands has profound consequences for the plants and animals in Hawai'i on several time scales. At the shortest time scale, that of days to years, each new lava flow creates isolated habitats which are crucibles for the creation of new species and for evolution. On the longer time scale of hundreds of thousands of years, each new volcano grows from the ocean floor, through sea level, and eventually reaches some height above sea level. The larger volcanoes, such as Mauna Loa and Mauna Kea, are home to a wide range of ecosystems because of the many climate zones that they include. The "high islands" have the widest species diversity.

The plants and animals that inhabit each new island have arrived mainly from the adjacent older volcanic islands, although the ancestors of all originally arrived as waifs from distant continents. Sometimes their journey from one volcano to the next is easy because the volcanoes have actually coalesced into single composite islands, like Hawai'i, which is made up of five separate volcanoes. At other times, the journey down the island chain is long and difficult, because the volcanoes are widely spaced, and because the channels are wide, such as between Kauai and Oahu. One can readily see the history of any single volcano by looking at the different volcanoes in the Hawaiian volcanic chain. Each of the old low islands was once a high island like the present Hawaiian Islands. The few species on these low islands are relics of a much wider diversity of species that developed when they were high islands.

The ecosystems on any volcano do not remain static once the volcano has formed because, on the longer time scale of millions of years, the islands slowly sink and erode. This subsidence takes place rapidly while the island is growing but slows down after the volcano has stopped erupting. The early sinking is caused by the great weight of the volcanoes, which flexes the Earth's nearly rigid outer layer (the lithosphere) downward. The elevation of the islands is decreased by as much as 7,000 feet in the first million years. Each plant and animal must adjust to the changes associated with this sinking, changes such as different temperatures and rainfall. Some high elevation environments cease to exist during this period of rapid change, and the plants and animals must either adapt or perish. Eventually, nearly all the great variety in environments is eliminated as the island is reduced to sea level.

A period of slower subsidence, after the short period of rapid subsidence, lasts for as long as 30 million years, until the island has subsided completely beneath the sea. Eventually each of the high islands is reduced to a low volcanic island, such as Necker and Nihoa Islands, or La Perouse and Gardner Pinnacles in the Leeward Islands. The oldest of the low volcamic islands in the Leeward Islands is only about 12 to 13 million years old. With continued subsidence, these low volcanic islands become coral islands such as Laysan, Kure, and Midway. This slow decrease in elevation gradually, but relentlessly, eliminate ecosystems and species on these islands, and eventual creates a community with few remaining species.

Each volcano has a series of geologic stages and a correlative sequence of biologic stages. The biologic stages are an initial stage during which colonizations from older adjacent volcanoes vastly outpace extinctions, an equilibrium stage in which colonizations and extinctions roughly match, and an old-age stage in which extinctions are no longer compensated by new colonizations. Colonization is most often from the older adjacent volcanoes; thus, the species on O'ahu arrived from Kaua'i, and the species on West Moloka'i arrived from O'ahu, etc. This chain of colonizations works fine as long as there is an older volcano that still rises above sea level when the next volcano forms. A few times in the past there were apparently no islands to provide the species to colonize the next island. The last time this occurred was about 30 to 35 million years ago (before Midway Island formed). At that point, all terrestrial species that had evolved during the previous 30 million years were eliminated, and new colonizations from distant continents and island groups had to begin again to populate the Leeward Hawaiian islands.

Active lava flows on Kīlauea continue to create new land for colonization. One of the first species to show up is Homo sapien, who is, once again, provided with good lava viewing at the end of Chain of Craters Road in Hawaii Volcanoes National Park. A lava flow began cascading over the small pali near the end of the road on Thursday afternoon. Another large surface flow is proceeding down the eastern side of the Kamoamoa flow field, where it has threatened one of our seismometers. This flow is now advancing towards the coast and may cover the last small section of road between the Kamoamoa and Waha'ula flow fields. The ocean entry at Highcastle has dwindled sharply, as these two surface flows have drawn off lava that was previously flowing in an underground tube to the ocean.