Out of the Indian Ocean blue: Réunion, a discreet analog to the Hawaii's Big Island ?

Abstract

Réunion is a tropical volcanic island formed by a geological hotspot located 700 km eastwards from Madagascar in the SW Indian ocean. Its highest summit, the Piton des Neiges, culminates at 3070 m above sea level. Althgough human settlement was more recent on Réunion (17th century), the geographical characteristics of the island, its geology and natural history make it very similar to several hawaiian islands, but most notably to the Big Island of Hawaii. In this presentation, we present a comparison of Réunion and Hawaii biotas, focusing on evolutionary history, biodiversity features, vegetation dynamics and conservation issues. We show that the two islands share many physical features, such as a large variety of climatic and soil conditions occurring over short geographical distances, strong altitudinal gradients and contrasts in rainfall regimes between the leeward and windward coasts because of annually fluctuating trade winds. They both shelter a unique biodiversity with high levels of endemism, many well-documented cases of evolutionary radiations, and very diverse habitats zonated according to elevation. Mountain habitats at upper elevations represent unique ecosystems like monospecific stands of Sophora denudata, or forest dominated by Acacia heterophylla, a sister species to Acacia koa. Differences between the two islands arise from two contrasting biogeographical contexts, with Réunion biotas showing affinities with both Asia and Africa. We conclude with conservation issues and lessons that can be gained from both contexts.