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Ecology influenced by island growth, subsidence, and isolation
The Hawaii hot spot is beneath the southern end of the island chain. The Emperor Seamounts, Northwest Hawaiian Islands (Hawaiian Ridge), and the main Hawaiian Islands were built in succession over the hot spot. The northwest motion of the Pacific Plate slowly draws them away from the hot spot. Removed from the source of lava, they cease to erupt and erosion whittles them away until they disappear beneath the sea. The growth and subsidence of the islands as they pass over the hot spot influences the distances between islands, the climate zones and ecosystems available on the islands, and the evolution of the animal and plant species.
Our research on biogeography of the Hawaiian Islands
The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.
How old is the island biota?
HAWAIIAN CHAIN - The long-term landscape changes in the Hawaiian archipelago impact dispersal, speciation and extinction of species. To quantify this, models were developed of elevations of and spacing between the islands for the last 32 million years, accounting for volcano growth, subsidence and erosion. The size, spacing, and total number of volcanic islands have varied greatly over time. The current landscape of large, closely spaced islands was preceded by a period with smaller, more distantly spaced islands. Considering that rates of dispersal and speciation must also have changed, much of the present species pool is probably the result of recent colonization from outside the archipelago and divergence within the islands now present, with limited dispersal from older islands. This view is consistent with abundant phylogenetic studies of Hawaiian organisms. Twelve out of fifteen multi-species lineages have diverged within the lifetime of the current high islands (last 5 million years). Three of these, and an additional seven (mostly single-species) lineages, have colonized the archipelago within this period. The timing of colonization of other lineages remains uncertain.
Reference: J.P. Price and D.A. Clague (2002) How old is the Hawaiian biota? Geology and phylogeny suggest recent divergence, Proceedings of the Royal Society of London, 269: 2429-2435.
Island growth and subsidence: influence on biologic evolution
HAWAIIAN CHAIN - The 6,126 km long, 85 million-year old (Ma) Hawaiian-Emperor volcanic chain has always been distant from any nearby continents throughout its formation. Of the 129 volcanoes in the chain, 104 reached sea level to become islands and only 30 became islands taller than 1 km. Many of the islands were in existence for less than 1 million years, and in many cases the previous volcano would have already subsided below sea level before the next emerged. Therefore, the idea that biologic colonizers could have simply migrated from one volcanic island to the next for the last 85 million years is not correct. The highest islands, however, had life spans up to 16 million years and, despite their distant locations, were presumably important sources of colonizers. Between about 70 and 34 Ma, a group of colonizers had a chance to establish themselves on more than one island for the first time, but then this group of colonizers was eliminated when the islands all subsided. Again from 30 until 23 Ma, several islands were present for colonizers to propagate from older to younger islands, but they were all low islands resulting in a low species diversity. Since the formation about 16 Ma of the Gardner Pinnacles, the first high island in the Hawaiian chain, the high species diversity characteristic of the high islands became possible, emigrating to each new island even though the source island may have been as far as 800 km from the newly formed one.
Reference: D.A. Clague (1996) The growth and subsidence of the Hawaiian-Emperor volcanic chain, In: The origin and evolution of Pacific Island biotas, New Guinea to Eastern Polynesia: patterns and processes, A. Keast and S.E. Miller (eds), SPB Academic Publishing, Amsterdam, pp.35-50.
Endemic species radiate when they occupy new territory
HAWAIIAN CHAIN - Distances between islands and elevations of older islands influences the probability that species can emigrate to younger islands and find suitable habitat to colonize. Based on submerged shorelines in bathymetric data and the slope of the active volcanoes, none of the earlier islands of the archipelago attained the height of Mauna Loa, or of Haleakala or Mauna Kea in their prime, but many were nearly as high. Some coalesced into single islands while the volcanoes were active, providing land bridges between volcanoes (much like the island of Hawaii now is a single island with 5 volcanoes) and they became separate islands as they subsided and eroded. Distances between the older islands are greater now than in the past because of subsidence and landslides, so channels between islands were not always the barriers to species emigration as they appear today. Area and habitat diversity were lost from older islands as they subsided, and localized species could have been lost as landslides removed sections of islands. There were times when no high islands were present, and there never was a continental mass from which to derive immigrants that was substantially closer to the archipelago than there is today.
The high islands have extraordinary biological diversity compared with lower islands, due to elevation, rainfall patterns (which are influenced by elevation), and heterogeneous topography. Species endemism is very high: many are entirely restricted to the present islands and not found naturally anywhere else. The high islands are geologically very youthful, so the recentness of so many unique living species is striking.
Genetic evidence suggests that some endemic species inhabiting the modern high islands evolved from lines that colonized the older islands, rather than arriving recently to the archipelago from distant continents. Once pioneers landed on a new island, they radiated quickly into many habitats. A modern analog of this is populations that have colonized recent lava flows: these are sites of dynamic genetic change.
Reference: H.L. Carson and D.A. Clague (1995) Geology and biogeography of the Hawaiian Islands, In: Hawaiian Biogeography: Evolution on a hot spot archipelago, W.L. Wagner and V.A. Funk, eds, Smithsonian Institution Press, DC., 14-29.
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