Phytogeography and Ecology of Scalesia (Compositae) Endemic to the Galapagos Islands

Abstract

Scalesia (Compositae), a genus endemic to the Galapagos Islands, consists of 12 shrubby species distributed in the lowland dry zone and three tree species found in the mid-elevation moist zone. They are completely allopatric in distribution. All the species have herbaceous traits: fast growth, soft wood, large pith at the center of trunk, and flowering within 1 yr after germination (in greenhouse). The tree species Scalesia pedunculata Hook. f. is shade-intolerant and heliophilous, and predominates as a monoculture in the moist zone of the four larger high-elevation islands. In ecological succession, it functions as pioneer, successor, and climax canopy plant. Even at climax or maturity of this monodominant forest, the canopy is not accompanied by young generations beneath owing to its shade-intolerance. The canopy population of postmature forest dies back nearly synchronously. A new generation then develops to build new forest. The progression from germination to maturity, and further to senescence and die back, is a self-cyclic succession, without change of dominant species. Over much of its range, S. pedunculata is endangered by the effects of past agricultural exploitation or heavy browsing by free-ranging goats, pigs, and donkeys; however, the population on the north side of Isla Santa Cruz has been preserved in good condition in the Galapagos National Park.