Plant Ecology & Diversity, 2014 Vol. 7, Issue 3, 433–444, http://dx.doi.org/10.1080/17550874.2013.802050 The effect of harsh abiotic conditions on the diversity of serpentine plant communities on Lesbos, an eastern Mediterranean island George C. Adamidis a , Elena Kazakou b , Alan J.M. Baker c , Roger D. Reeves c and Panayiotis G. Dimitrakopoulos a * a Biodiversity Conservation Laboratory, Department of Environment, University of the Aegean, Mytilene, Lesbos, Greece; b Montpellier SupAgro, UMR Centre d’Ecologie Fonctionnelle et Evolutive, Montpellier, France; c School of Botany, University of Melbourne, Parkville VIC, Australia (Received 13 October 2011; final version received 30 April 2013) Background: Diversity patterns of plant communities are related to the environment, including productivity and patchiness of habitat. Aims: To determine differences in diversity patterns between serpentine and non-serpentine communities. Methods: A two-year study was conducted in native eastern Mediterranean grasslands. For each year 40 0.25 m 2 plots were sampled across four pairs of sites, each of which contained a serpentine and an adjacent non-serpentine plant community. Alpha and beta diversity (variation in species composition among plots within localities), species composition and biomass production were determined. Total soil elemental concentrations and pH were also measured. Results: Serpentine habitats were shown to support a lower alpha diversity relative to non-serpentine habitatas on a per plot basis. Differences in alpha diversity between the two substrates were associated with variation in soil chemistry rather than above-ground biomass production. Serpentine habitats also exhibited lower beta diversity, which was unrelated to variation in biomass production. The two contrasting communities presented distinct species composition. Conclusions: Differences in diversity patterns between serpentine and non-serpentine communities in the eastern Mediterranean are influenced by soil chemistry rather than biomass production. Keywords: alpha diversity; beta diversity; floristic composition; heavy metals; productivity; stress; ultramafic soils Introduction Environments under high physical stress (or stressful, sensu Grime 1977) often support low species richness (Huston 1979; Begon et al. 2006). Since stress is consid- ered at the individual plant species level (Körner 2003), plants colonising harsh environments tolerate extreme non- resource-related conditions (e.g. temperature, wind, salinity or pH) as well as shortages of water and nutrients, and excess insolation through special physiological and struc- tural adaptations (Van der Meulen et al. 2001; Begon et al. 2006). The set of species traits permitting colonisation, survival and competitive success is restricted in harsh envi- ronments, often leading to dominance of one or a few specialised species, such as those in salt deserts (Hillebrand 2008). Alpha diversity of the plant communities developing in harsh environments can be affected by their low levels of productivity (reviewed by Waide et al. 1999 and Mittelbach et al. 2001; Gillman and Wright 2006; Adler et al. 2011). Low productivity can reduce alpha diversity as a result of physiological stress and resource limitation (Chesson and Huntly 1997). On the other hand, because competitive inter- actions are dominant in productive communities – while facilitation is more likely in less productive communities – community saturation is expected to occur commonly in productive habitats. In addition, unproductive communi- ties have greater heterogeneity of resources than productive *Corresponding author. Email: pdimi@env.aegean.gr communities on the small scale, and more species niches can thus be represented (Rosenzweig and Abramsky 1993). The lower productivity of communities facing extreme con- ditions may therefore permit more species to coexist by increasing heterogeneity and reduced competitive inter- action for limited resources (Grime 1979; Huston 1979; Tilman and Pacala 1993; Michalet et al. 2006; Maestre et al. 2009). Beta diversity, i.e. the variation in species composition among plots within localities, can be lower in harsh envi- ronments than in those under benign conditions (Harrison 1999a; Harrison and Inouye 2002), as a result of the dif- ferences in productivity levels between them (Chase and Leibold 2002; Harrison et al. 2006). Increased mean pro- ductivity of benign environments can be associated with greater micro-environmental heterogeneity (e.g. topogra- phy, rockiness), leading to increased small-scale dissimilar- ity in species composition or beta diversity (Huston 1979; Tilman and Pacala 1993; Chase and Leibold 2002; Harrison et al. 2006). Communities facing harsh environmental conditions could present higher stability over time in terms of species composition, and hence greater resistance to environmen- tal fluctuations (e.g. climate change; Grime et al. 2000, 2008). The patchiness, abiotic stress, and the presence of species with specific functional traits could serve as potential mechanisms for explaining the greater temporal © 2013 Botanical Society of Scotland and Taylor & Francis