Plant species richness and spatial organization at different small scales in western Mediterranean landscapes J.M. De Miguel 1, * , L. Ramírez-Sanz 1 , I. Castro 2 , M. Costa-Tenorio 3 , M.A. Casado 1 and F.D. Pineda 1 1 Dept. interuniversitario de Ecología, Universidad Complutense, 28040 Madrid, Spain; 2 Dept. interuniversitario de Ecología. Universidad Autónoma, 28049 Madrid, Spain; 3 Dept. de Biología Vegetal I, Universidad Complutense, 28040 Madrid, Spain; * Author for correspondence (e-mail: demiguel@bio.ucm.es; fax: +34 913945081) Received 3 April 2003; accepted in revised form 9 February 2004 Key words: Ecological boundary, Grasslands, Mediterranean ecosystems, Portugal, Shrublands, Spain, Spatial niche width, Vegetation physiognomy Abstract The relationship between plant species richness and the space organization of the community at different small scales was studied. The study was based on 51 sites distributed along a belt from Central Spain to Portugal. Each site was analyzed with a transect cutting across the boundary between two neighboring patches of shrubland and grassland. Local spatial organization of vegetation was analyzed at different levels of detail and each transect was divided into successively smaller portions. The first division coincides with a physiognomic perception of the site in two patches shrubland and grassland. The average spatial niche width of the species was used to calculate the spatial organization of the vegetation of each division in each site. The correlation between species richness and spatial organization depended on the block size under consideration. A physiognomic criterion, sec- torizing the sites into patches of shrubland and grassland, determines noteworthy floristic changes but does not enable us to express satisfactorily the variability in plant richness. In order to account for this variation, other factors must be taken into account which act at a more detailed small-scale and which determine the internal variability of these patches. In the case studied, the species richness of the sites increases along with an increase in the percentage of species whose occupation of the space is relatively restricted within the site. Many of these species are, however, frequent within the whole of the territory studied. The results highlight the importance of the level of local scale at which the factors influencing occupation of the space, and consequentially, plant rich- ness, preferentially act. This circumstance ought to be taken into consideration in strategies for the conservation of biological diversity, and based on the delimitation of protected spaces with criteria frequently linked to the physiognomy of the vegetation. Nomenclature: Follows T.G. Tutin et al. 1964-1980. Flora Europaea. Cambridge University Press, Cambridge Introduction The causes of variation in biological diversity and the mechanisms regulating or accounting for it have been of interest to ecologists for quite some time Grime 1979; Huston 1979, 1994; Shmida and Wilson 1985, Peet et al. 1983; May 1986; Huston and Smith 1987; Tilman 1988; Tilman et al. 1996; Margalef 1997; Gaston 2000. Scale is an interesting aspect Allen and Starr 1982; Pickett and White 1985; Allen 1987; Plant Ecology 2005 176:185-194 © Springer 2005