Biogeodiversity and pedodiversity islands in arid lands of Europe (Almería Province, Spain)

Plant and soil landscapes across bioclimatic belts and drainage basins were studied using georeferenced databases in arid lands of SE Spain, the driest area of Europe. The syntaxonomic system was used to analyze phytocenoses and bioclimatic belts, as well as the concept of potential natural vegetation (PNV), a common approach in many countries of continental Europe. Soil types included in pedological databases were classified using the World Reference Base for Soil Resources international system (FAO 1998). Both bioclimatic belts and drainage basins effectively discriminate soil and plant assemblages in the study area of the Almeria province. The syntaxonomic perspective permits distinguishing between PNV dependent on (i) climate (climatophylous), (ii) climate and lithology, and (iii) soils (edaphophylous). Richness-area relationships of plant and soil assemblages fit well to power law distributions, showing few idiosyncratic differences. PNV, lithological associations, and soil richness are clearly correlated with the area of each climatic beltand watershed. PNV and pedotaxa richness (understood as a number of taxa at a given hierarchical level) increases from the mountain tops to the coastal lands. Around 59% of the PNV units are edaphophylous and 87% of these are edaphohygrophylous that require water supply or tolerate water excess in riverbed ramblas (dry watercourses). Edaphohygrophylous PNV are distributed in small patches within a very arid matrix. They can be considered as plant “biodiversity islands”, a concept different from that of “fertility islands” used by ecologists in arid land studies. The spatial dispersion of these phytocenoses prevents adequate preservation in the frame of conservation biology policies. At landscape level, the extent of plant communities is as follows: PNV climate dependent > PNV climate-lithology dependent > PNV soil dependent. The diversity of plant communities follows an opposite trend: PNV soil dependent > PNV climate-lithology dependent > PNV climate dependent. The PNV most conditioned by soil properties are located along the streambeds of ramblas. These fluvial sediments are not reported as soil materials in soil maps. PNV, soils and lithological associations by drainage basins conform to the predictions of the statistical tool termed nested subsets theory. However, lithological associations by climatic belts depart from this spatial pattern.