Synopsis: Comparative Biogeography of Soil-Crust Biota
Biological soil crusts most often occur under harsh conditions that include extremes in temperature and light and at least periodic lack of water. They occur in almost all arid and semiarid ecoregions worldwide, and in local arid micro environments in oth
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12.1 Introduction Biological soil crusts most often occur under harsh conditions that include extremes in temperature and light and at least periodic lack of water. They occur in almost all arid and semiarid ecoregions worldwide, and in local arid microenvironments in other climates (e.g., temperate regions). Despite their widespread occurrence, comparing the biogeography and ecology of soilcrust biota and communities on a global basis is almost impossible. This is because most of the data were collected (1) using different methodologies, (2) using different taxonomic systems [e.g., using Drouet’s (1956) keys, where he combined genera], and/or (3) were focused on a single group of organisms (i.e., lichens, algae, cyanobacteria, or mosses) to the exclusion of other groups. In Chapters 2–11, our present knowledge about species composition and ecology of biological soil-crust biota in the different geographic regions is summarized. This information is the basis for the following synopsis.
12.2 World Distribution of Biological Soil Crusts In Fig. 12.1, our present knowledge of the worldwide distribution of the different types of soil-crust communities is depicted. This map is based upon the information given in Chapters 2–11, upon personal experience, and that of colleagues. Single symbols indicate individual locations from which the crust communities are described; however, their distribution may be much more extensive. Soil crusts composed of cyanobacteria, algae, lichens, mosses, or liverworts, or a combination of these organisms, occur in all biomes of the world containing an arid element that sufficiently decreases competition from phanerogamous vegetation. Only the tropical evergreen rainforest appears to Ecological Studies, Vol. 150 J. Belnap and O.L. Lange (eds.) Biological Soil Crusts: Structure, Function, and Management © Springer-Verlag Berlin Heidelberg 2001
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Fig. 12.1. Distribution of biological soil crusts on Earth (this map is based upon the information given in Chapters 2–11 and upon our own personal experience, as well as that of colleagues). Single symbols indicate individual locations, from which the crust communities are described – their distribution may be much more extended
lack biological soil crusts (Fig. 12.1). As discussed in detail in the foregoing chapters, poor sandy soils are generally dominated by cyanobacterial crusts; the proportion of lichens increases with the carbonate, gypsum, and silt contents of the substrate. If the salt influence of the soil increases, the filamentous cyanobacterium Microcoleus chtonoplastes becomes the main constituent of the crust (see also Chap. 17). Reliable records of mossdominated soil crusts are very rare. Soil crusts dominated by green algae and/or other eukaryotic algal groups appear restricted to soils in temperate regions, where they appear as a successional phase, or in desert soils that are slightly acidic. However, our knowledge of this crust type is far too limited to draw any further conclusions.
12.3 Biodiversity Many floristi
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