4 ATLAS OF BIODIVERSITY RISK CHAPTER 1 for the number of native vascular plant families in Europe (169) which is much lower as compared to North America (excl. Mexico: 210) or China (260). European plant diversity Europe is home to some 11,500 of the estimated 320,000 vascular plant spe- cies on earth. The Flora Europaea lists c. 10,600 flowering plants, c. 160 of the c. 13,000 global fern species, and c. 40 of c. 1,000 species of gymno- sperms. While these species numbers are much lower than those of the top global centres of plant diversity listed in Table 1, parts of the European mountain regions or the Mediterranean show a level of richness comparable with, e.g., tropical Africa (Figure 2). The non-vascular plant flora of Europe is relatively well documented compared to many other regions. It harbours more than 30,000 document- ed species of bryophytes, algae, fungi, and lichens. Especially for algae and fungi, there are still many species to be discovered. On a European scale, highest spe- cies richness can be found in the Mediterranean and the Caucasus. Especially the geodiverse mountainous areas surrounding the Mediterranean like the Balkans region, the Alps, the Pyrenees and the mountain ranges of SE Spain are important centres of plant species richness and endemism (Araújo et al., 2005). The Balkan, the Iberian Peninsula, and Italy have been The distribution of plant diversity across the Earth is highly uneven (Barthlott et al., 2005, Kier et al., 2005, Mutke & Barthlott, 2005, Figure 1). For instance, the small South American country of Ecuador which has a sur- face area comparable to the British Isles harbours some 30-40 percent more species than continental Europe. Generating a world map of plant diversity Plant diversity is documented in thou- sands of inventories such as floras and checklists world-wide. Based on this lit- erature, we compiled a dataset with numbers of native plant species of c. 3,000 operational units such as coun- tries, provinces, islands, mountain ranges, and conservation areas. As these units differ very much in area, only a subset of c. 1,400 of these was used for our mapping approach. Species richness figures of the selected units were standardized using classical models of the relation of area and spe- cies richness. To interpolate between areas with suitable raw data, additional datasets of environmental parameters have been used (Barthlott et al., 2005, Mutke & Barthlott, 2005, Figure 1). The same dataset was the basis for fur- ther macroecological analyses (e.g. Mutke & Barthlott, 2005, Kreft & Jetz 2007, Figures 2, 4), Global Centres of plant diversity Global centres of species richness are located in the humid tropics and sub- tropics, especially in areas with a high heterogeneity of the abiotic environ- ment (‘geodiversity’) like mountains and regions of steep climatic gradients. Five global centres of plant species richness reach species densities of more than 5,000 vascular plant species per 10,000 km² (compare Table 1, Figure 1). In total, there are 20 centres of plant diversity with more than 3,000 species per 10,000 km². Important extra-tropical centres are the Mediterranean-type climate areas of the world with hot and dry summers and cool, wet winters: the Mediterranean Basin, California, cen- tral Chile, the South African Cape Region, and South and Southwest Australia. These regions are character- ized by comparatively diverse and high- ly endemic floras and are considered as Biodiversity Hotspots by Conservation International. High numbers of endemic species can be found on the oceanic islands of the world. Some 70,000 species of vas- cular plants or 20 % of the world’s flora are endemic to islands – thus, occurring nowhere else (Kreft et al., 2008). Environmental and historical controls of plant diversity Plant richness changes systematically along environmental and latitudinal gradients (Mutke & Barthlott, 2005, Kreft & Jetz, 2007, Figure 2). At high latitudes, where tempera- ture and the length of the thermal veg- etation period are limiting factors, spe- cies richness is closely correlated with measures of thermal energy like poten- tial evapotranspiration (PET). On the other hand, water availability and the spatial heterogeneity of the environ- ment appear to be more important at lower latitudes (Kreft & Jetz, 2007, compare Figure 4). In addition to rich- ness-environment relationships, the regional history of the environment, especially of the climate, has consider- ably influenced today’s diversity pat- terns. Due to severe impacts of the harsh climate during the ice ages, the woody plant flora of Central Europe is highly impoverished compared to simi- lar vegetation in East Asia or eastern North America. The same holds true European Plant Diversity in the Global Context JENS MUTKE, HOLGER KREFT, GEROLD KIER & WILHELM BARTHLOTT  Figure 1. World map of species richness of vascular plants (Barthlott et al. 2005, Mutke & Barthlott, 2005). The map is based on species richness figures for c. 1,400 geographical units word-wide. Figure 2. Latitudinal gradient of vascular plant diversity in Europe and Africa. Each dot represents the number of native plant species of a geographic unit (e.g. flora, checklist). Species numbers were standardized for disparities in area size (modified from Mutke & Barthlott 2005).