BIOLOGICAL CONSERVATION 136 (2007) 388– 397 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/biocon Range size variation, nestedness and species turnover of orchid species along an altitudinal gradient on Re´union Island: Implications for conservation Hans Jacquemyna,*, Olivier Honnayb, Thierry Paillerc aDivision Forest, Nature and Landscape Research, University of Leuven, Celestijnenlaan 200E, B-3001 Leuven, Belgium bDivision of Plant Ecology and Systematics, Biology Department, University of Leuven, Kasteelpark Arenberg 31, B-3001 Heverlee, Belgium cUniversite´ de la Re´union, UMR C53 Cirad-Universite´, Peuplements Ve´ge´taux et Bioaggresseurs en Milieu Tropical, 15 avenue R. Cassin, BP 7151, 97 715 Saint-Denis Messag. Cedex 9, La Re´union, France ARTICLE INFO ABSTRACT Article history: Tropical oceanic islands contribute disproportionably for their area to global biodiversity and Received 17 July 2006 this is especially true for islands with strong altitudinal gradients. On these islands, species Received in revised form richness and composition usually change with altitude, but the way in which they do may 8 November 2006 vary from one system to the next. Better insights in how species richness and composition Accepted 8 December 2006 are related may have far-reaching conservation implications. At the one extreme, species Available online 26 January 2007 composition of a species-poor site may be completely different from that of the most spe- cies-rich site. In this case, conservation of the species-poor site should be encouraged as it Keywords: contains species that are not to be found elsewhere. At the other extreme, species composi- Altitude tion of the species-poor sites may be a subset of the species-rich sites (i.e. nestedness or Nestedness nested subsets). Focus on the most species-rich site may be sufficient to effectively conserve Orchid conservation a maximum of species. In this study, we determined changes in species richness and compo- Orchidaceae sition of orchid species along an altitudinal gradient spanning more than 2500 m on Re´union Stevens’ rule 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 suffi- cient to effectively conserve the extremely high orchid diversity on Re´union Island. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction this is especially true for islands with strong altitudinal gra- dients (Whittaker, 1998). On the other hand, islands are For particular taxa, many tropical oceanic islands contribute specifically vulnerable to many threats, including habitat disproportionably for their area to global biodiversity and loss, habitat degradation and fragmentation, and invasion * Corresponding author: Tel.: +32 16 32 97 73; fax: +32 16 32 97 60. E-mail address: [email protected] (H. Jacquemyn). 0006-3207/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2006.12.008 BIOLOGICAL CONSERVATION 136 (2007) 388– 397 389 of exotic species. Tropical islands have therefore been con- In this paper, we studied the altitudinal range size of 121 sidered biodiversity hotspots containing many plant and orchid species on Re´union Island, a recent volcanic island in animal species that are not found elsewhere, but that are the western Indian ocean. We investigated whether orchid under severe threat (Myers et al., 2000). To effectively con- assemblages located at different altitudes exhibited a signifi- serve a large number of species, the design of nature re- cant nested subset pattern, and whether this nested subset serves representing the biodiversity of the island may be a pattern can be used as a tool for the establishment of conser- first, crucial step. However, a well planned, effective design vation guidelines and for reserve selection. Previous research of a reserve network implies knowledge on the distribution (Jacquemyn et al., 2005) has shown that species richness of of species, patterns of species richness and composition, orchid assemblages significantly decreased with increasing and the way they are interrelated (Margules and Pressey, altitude. In this study, we more specifically addressed the fol- 2000). lowing questions: (1) How are orchid species distributed along On most volcanic islands of recent origin, altitude is one of a steep elevational gradient spanning more than 2500 m and the major gradients that strongly affect the distribution of how does altitude affect the distribution of orchid species plant and animal species. Whereas in most cases the altitudi- with a different life form (i.e. epiphytic vs. terrestrial orchids)? nal range increases with increasing altitude (reviewed by (2) Do orchid assemblages show a nested subset pattern and MacArthur, 1972; Stevens, 1992), species richness and similar- is it related to the altitudinal gradient? (3) If nestedness is pre- ity in species composition mostly decrease significantly with valent, can it be used as a practical tool for the design of a re- increasing altitude (e.g. Kitayama, 1992; Kityama and Mueller- serve network that aims to conserve as many orchid species Dombois, 1994; Lieberman et al., 1996). However, at present as possible? functional analyses between altitudinal range sizes, local spe- cies richness and species composition along an altitudinal 2. Methods gradient have rarely been done in a conservation context. Nevertheless they may reveal important information on 2.1. Study area which to base conservation actions and to derive rules for re- serve selection. At the one extreme, species composition of The study was conducted on Re´union Island (55°390E; 21°000S), species-poor sites may be completely different from that of which is situated in the western Indian Ocean some 1200 km the most species-rich site. In this case, conservation of the off the African mainland and 800 km from Madagascar. Of the species-poor site should be encouraged as it contains species three islands that make up the Mascarene Islands (Mauritius, that are not to be found elsewhere. At the other extreme, spe- Rodriguez and Re´union Island), Re´union Island is the biggest cies composition of the species-poor sites is a subset of the of the three (2512 km2) and it also contains the largest propor- species-rich sites; this pattern is known as nestedness or tion of intact habitat types (ca. 30%) (Doumenge and Renard, nested subsets (Patterson and Atmar, 1986). Focus on the 1989). The island is characterized by a steep altitudinal gradi- most species-rich site may then be sufficient to effectively ent ranging from 0 to 3070 m in the centre (Piton des Neiges) conserve a maximum of species (Patterson, 1987; Cutler, and 2631 m in the southeast (Piton de la Fournaise). Re´union 1991; but see Boecklen, 1997). Island has a tropical climate with a rainy season from Novem- Formal nested subset analysis has been shown to be a ber to April and a cooler, drier season from June to September. useful tool for conservation as it quantifies the degree of Mean annual rainfall is high in the eastern part of the island nestedness of species assemblages. Significantly nested ranging from 1500 mm on the coast to more than 8000 mm at communities may point to predictable extinction sequences high altitudes. On the western part, rainfall is remarkably (Atmar and Patterson, 1993; Lomolino, 1996), provide lower varying between 500 and 1500 mm along the south- insights in the factors determining species composition in western coast. Mean annual temperatures vary between fragmented habitats (e.g. Honnay et al., 1999; Butaye 24 °C along the coastline to 12 °C at 2000 m altitude. et al., 2001; Hylander et al., 2005) and highlight species that Along the altitudinal gradient, originally the following ma- do not conform to the nested pattern and therefore may jor vegetation types could be distinguished on the island (Ca- need special conservation approaches (Simberloff and Mar- det, 1980): (1) a savanna-like vegetation dominated by palms tin, 1991; Patterson et al., 1996). However, recent research (Latania lontaroides) (0–200 m); (2) a semi-dry lowland forest has shown that due to the likeliness of large differences be- mainly consisting of sclerophyllous tree species (200–750 m); tween a perfectly nested system and a significantly nested (3) tropical lowland rainforest with high tree species diversity system, great care has to be taken when applying insights (750–1100 m); (4) mountain rainforest (1100–1800 m) domi- derived from nested subset theory to real ecosystems. A nated by a few trees (Acacia heterophylla, Phillipia montana significant nested subset pattern may show many unex- and Pandanus montanus); (5) at high altitudes (above 1800 m) pected presences and absences. For this reason, a signifi- shrubland dominated by endemic taxa of Ericaceae and cantly nested community does not imply that only the Asteraceae. These major vegetation types are primarily struc- most species-rich site needs to be protected to conserve tured along a gradient of altitude and rainfall (Cadet, 1980; most species. Other approaches that take the complemen- Dupouey and Cadet, 1986). tarity of patches into account may be more useful than conservation measures based on nestedness analysis alone 2.2.