Potassium-argon dating of five basalts from the three main islands of the Juan Fernández (or Robinson Crusoe) Islands of Chile in the southeastern Pacific gives ages of 1.01 +/- 0.12 and 2.44 +/- 0.14 million years for Masafuera, 3.79 +/- 0.20 and 4.23 +/- 0.16 for Masatierra, and 5.8 +/- 2.1 for Santa Clara. These ages are much younger than that of the underlying oceanic plate and are consistent with the origin of the island-seamount chain from a mantle hot spot beneath the eastward moving Nazca plate. The young age for the archipelago suggests that speciation within endemic genera has occurred within the past 4 to 5 million years. Endemic genera of apparently more ancient origins, such as Lactoris and Thyrsopteris, have apparently dispersed to the islands and survive refugially.

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Summary During the recent Universidad de Concepción–Ohio State University expeditions to the Juan Fernandez Islands, observations were made concerning recent changes in the composition and extent of the native flora. In comparison with the data of Skottsberg from 1916‐17, many of the endemic species have become much rarer, and some perhaps extinct. Several especially aggressive introduced species, e.g., Acaena argentea, Aristotelia chilensis, and Rubus ulmifolius, continue to invade and replace the native vegetation. Increasing areas of dry habitats and increasing proportions of xeromorphic or lithophilic plants now characterize the total vegetation. The total area of native vegetation is now restricted to the central ridges and cliffs. The major causes for the floristic degradation appear to be the introduced animals which have overgrazed and trampled the plants as well as compacted the soil. This has lead to erosion and further loss or degradation of the plant cover.