Metamorphosis of flora and vegetation during ontogeny of the Juan Fernández (Robinson Crusoe) Islands

Abstract

Abstract Colonization, levels of speciation and vegetational metamorphosis during ontogeny of oceanic islands over geological time are central evolutionary and biogeographic concerns. A suitable archipelago in which to examine such events is the Juan Fernández (Robinson Crusoe) Archipelago, located at 33° S latitude off the coast of continental Chile and consisting of three major islands: Robinson Crusoe Island, 667 km west of the coastline; Santa Clara Island, 1.6 km off the south-western end of the former island and Alejandro Selkirk Island, 181 km directly further west into the Pacific Ocean. The two largest islands are c. 50 km2, with Santa Clara Island being only 2.1 km2. Radiometric dating for the three islands reveals the ages of Robinson Crusoe Island and Santa Clara Island to be c. 4 My, whereas that of Alejandro Selkirk Island is c. 1 My. Molecular clock estimates of crown-group divergences of endemic taxa fall mostly in these geological ages. Recent investigations have brought together a detailed inventory of the vascular flora of the archipelago with biological, evolutionary and biogeographic information that now supports examining metamorphosis of the vegetation and flora during the 4 My of island ontogeny. Geological dating and geomorphological models are used to infer changes in the landscape on both major islands. It is suggested that the original Robinson Crusoe Island may have been substantially larger and higher than at present (3000 vs. 915 m). Subsidence and erosion of the island led to loss of vegetation zones, especially impacting plants between 1000 and 3000 m. Some species may have adapted to this restructuring, but many would have gone extinct. Alejandro Selkirk Island, now 1 My, is only slightly smaller and lower than the original island (1319 vs. 2000 m), but it has probably lost some higher elevation habitats. The present vegetation of the two islands correlates with these suggestions, with Alejandro Selkirk Island retaining broad zones, but Robinson Crusoe Island having a flora that has been compacted and substantially intermixed with invasive species in recent centuries. Floristic inventory reveals 11 endemic genera, but most of these occur only on Robinson Crusoe Island, which has been the centre of cladogenesis in contrast to greater levels of anagenesis on Alejandro Selkirk Island. The ferns, due to their high dispersability, show almost no cladogenesis. Total endemic species diversity is higher on Robinson Crusoe Island (87) than on Alejandro Selkirk Island (62), and this difference is probably due to the greater age of the island and initial environmental heterogeneity. It is also probable that during this ontogeny, numbers of endemic species on Robinson Crusoe Island have been lost through island subsidence and erosion, loss of habitat and human impact.