Zonation pattern, succession process and invasion by aliens in species-poor insular vegetation of the Galapagos Islands.
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出版者原稿の一部改訂（p.42, p.43, p.49）Rivised from published version(p.42, p.43, p.49)The Galapagos Islands, located 1,000 km west of the South American coast on the equator in the easternmost Pacific, are of volcanic origin. The vascular plant flora is poor and disharmonic, comprising ca. 200 endemic taxa. The vegetation is altitudinally arranged, roughly related to the increase in precipitation, from the maritime and dry zones in the lowlands, through the transition and moist zones, to the treeless highland zone. The zonation is deflected upward in elevation on the leeward side of each island and on leeward islands, due to the rainshadow of the southeastery trade winds which bring moisture to the islands. The treelessness of the highlands is attributed to the fluctuation of weather conditions between extreme dryness and extreme moisture saturation, to which no tree species adapted to such fluctuation has migrated and in which no endemic trees have evolved. Of the flora, an endemic genus Scalesia (Compositae) has attracted special attention. The genus consists of 15 heliophilous species, of which 12 are shrubs distributed in the dry lowlands and three are trees found in the moist zone. All the species are allopatric in distribution. Scalesia pedunculata, the largest tree of the genus, 12 m high and 15 cm in DBH, predominates in the moist zone of Santa Cruz Island. The canopy population of the dense forest is a cohort of the same age, and nearly all the trees die synchronously at maximum maturity, triggered by a large amount of El Nino rainfall or by extreme drought in La Nina years. Such a stand-level dieback resets the cohort generation, and then self-cyclic succession starts with no successors or shade-tolerant trees. The same was observed in an S. cordata forest on Sierra Negra Volcano of Isabela Island. Alien plants, introduced intentionally or accidentally to the islands, are spreading into semi-natural and natural areas as well as in disturbed habitats. Aliens have increased since the first half of the 19th century and now exceed 600 in number of species. Naturalized aliens include tree species that establish self-recruiting populations filling gaps in habitats and tree niches, since the flora is poor in tree species. El Nino rainfall creates favorable conditions for invasive aliens and accelerates their expansion to semi-natural and natural vegetation. Some of the endemic taxa are declining to endangered status due to invasion by alien plants and grazing by introduced animals like goats and donkeys. The vegetation is changing not only in agricultural and town areas but also in part of the Galapagos National Park despite its strict regulations for protection and conservation.
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