Plant species diversity of Adriatic islands: An introductory survey TONI NIKOLIC ´ 1 , OLEG ANTONIC ´ 2 , ANTUN L. ALEGRO 1 , IVA DOBROVIC ´ 1 , SANDRO BOGDANOVIC ´ 1 , ZLATKO LIBER 1 , & IVANA RES ˇ ETNIK 1 1 Faculty of Science, Department of Botany, University of Zagreb, Croatia and 2 RudjerBosˇkovic ´ Institute, Zagreb, Croatia Abstract The European Mediterranean region is one of the world’s major centres of biodiversity. Research on plant species diversity in this region has neglected the area along the eastern part of the Adriatic basin in comparison to the other Mediterranean areas. The main aim of this study was to focus on this neglected area, by supplying data which is at present lacking in order to discuss the species-area relationship (SAR), floristic richness and endemism of the Adriatic islands and coast. Floristic data for 106 Croatian islands collected by different authors were integrated, systematised and presented in a form usable by other researchers worldwide. The Power (Arrhenius) function was used for modelling (by non-linear regression) the SAR. Residuals around the regression curve (as indicators of floristic diversity without the influence of island area) were calculated for each island. The proportion of endemics in the total island flora varies from 0% up to 28.6%, and 17.5% for narrow endemics. Floristic richness of the broader region was estimated (on the basis of SAR extrapolation) on 1807 species for all Croatian islands, and on 2797 species for the entire Mediterranean area in Croatia. Keywords: Adriatic basin, Arrhenius function, endemism, floristic diversity, species-area relationship, species richness Introduction All five of the world’s Mediterranean climate regions (i.e. California, Chile, South Africa, Australia and European Mediterranean basin) are concentrated within the latitudes 308 and 458, a little less than halfway from the equator to the poles. The Medi- terranean biome, only 2% of the world’s surface area, contains 20% of the total floristic richness (Me ´dail & Que ´zel 1997). The European Mediterranean basin is a major contributor to the biodiversity of the Mediterranean biome, because 10% of the all known higher plants are found there in only 1.6% of the Earth’s surface (Me ´dail & Que ´zel 1997). There is a significant number of papers concerning the flora and vegetation in this area, and also a series of contributions with more or less general overviews (e.g. Cowling et al. 1996; Dallman 1998; Blondel & Aronson 1999; Greuter 2001; Thompson 2005; etc.). All surveys reach a similar conclusion: the circum-Mediterranean region is one of the world’s major centres for plant differentiation, located at the crossroads between Europe, Asia and Africa, and playing a role as biodiversity reservoir. The main processes facilitating plant evolution are related to geological history, past and present climate, and, in more recent times, human activities, often in complex interactions. The diversity is not uniformly distributed, and the areas with the highest biodiversity rate present very complex historical and paleogeographical factors (Thompson 2005). These areas, called ‘‘hot-spots’’ (Me ´dail & Que ´zel 1997, 1999; Mittermeier et al. 1997), ‘‘megadiversity countries’’ (Mittermeir et al. 1997) or ‘‘centres of plant diversity’’ (Davis et al. 1994), which are also very important from the point of view of conservation strategies, include the Canary Islands and Madeira, the High and Middle Atlas mountains, the Beatic-Rifan complex, the Maritime and Ligurian Alps, the Tyrrhenian Islands, southern and central Greece, Crete, southern Anatolia and Cyprus, the Syria-Lebanon-Israel area and the Cyrenaic Mediterranean (Me ´dail & Que ´zel 1999). All mentioned reviews and papers, and others not cited here, have as background the floristic data collected all around the Mediterranean basin, during centuries of botanical investigations. Correspondence: T. Nikolic ´, Faculty of Science, Department of Botany, University of Zagreb, Rooseveltov trg 6, HR-10000 Zagreb, Croatia. Tel: þ3851 489 8064. Fax: þ3851 489 8093. Email: toni@botanic.hr Plant Biosystems, Vol. 142, No. 3, November 2008, pp. 435–445 ISSN 1126-3504 print/ISSN 1724-5575 online ª 2008 Societa ` Botanica Italiana DOI: 10.1080/11263500802410769