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Tropical oceanic islands contribute disproportionably for their area to global biodiversity and this is especially true for islands with strong altitudinal gradients. On these islands, species richness and composition usually change with altitude, but the way in which they do may vary from one system to the next. Better insights in how species richness and composition are related may have far-reaching conservation implications. At the one extreme, species composition of a species-poor site may be completely different from that of the most species-rich site. In this case, conservation of the species-poor site should be encouraged as it contains species that are not to be found elsewhere. At the other extreme, species composition of the species-poor sites may be a subset of the species-rich sites (i.e. nestedness or nested subsets). Focus on the most species-rich site may be sufficient to effectively conserve a maximum of species. In this study, we determined changes in species richness and composition of orchid species along an altitudinal gradient spanning more than 2500m on Réunion Island (Mascarenes). A formal nestedness analysis was performed to investigate whether species-poor sites were a subset of species-rich sites and whether this was related to altitude. Differences in species composition along the altitudinal gradient were determined using two different measures of community similarity. The species-by-sites matrix was significantly nested when sorted by species richness, but not when sorted by altitude. Mean similarity indices were low at both low and high altitude sites, indicating significant turnover of orchid species. Our results suggest that reserve selection should be based on two basic principles: (1) maximize the number of vegetation zones in which reserves are to be placed and (2) within each zone, select the sites with the highest species richness. Given that at present only two nature reserves are present on the island, our results further suggest that they are not sufficient to effectively conserve the extremely high orchid diversity on Réunion Island. [Copyright &y& Elsevier]