Demographic studies of three indigenous stand-forming plant taxa (Scalesia, Opuntia, and Bursera) in the Galápagos Islands, Ecuador

Abstract

Mortality and turnover of some of the most characteristic woody plants of Galápagos have been studied during 25 to 32 years on the islands of Santa Cruz, Santa Fe and Pinta. Scalesia pedunculata, S. helleri subsp. helleri and S. baurii subsp. hopkinsii display comparable patterns of high mortality, rapid turnover and relatively short life expectancy, but occur in different habitats and islands in the archipelago. In contrast, Opuntia echios var. gigantea, O. echios var. barringtonensis, O. galapageia var. galapageia, and Bursera graveolens display low mortality (especially of adults), slow turnover and long life expectancies, and occur in similar habitats on different islands. Conditions that favour one taxon may at the same time negatively affect another taxon, and adults, juveniles and seedlings within the taxa may react differently to the same environmental factors. In the humid zone species S. pedunculata, diebacks may be triggered by strong El Niño events, and mass seedling events are associated with El Niños and a subsequent year of drought. In the other taxa investigated, which are arid zone species, the response to strong El Niño events is less pronounced and, in some respects, also different. The differences in mortality, turnover and life expectancy of the species of Scalesia on the one hand and the Opuntia taxa and Bursera on the other, suggest that both humid and arid vegetation types containing species of Scalesia should get higher conservation priority than arid vegetation types without species of Scalesia.