In arid and semi-arid lands throughout the world, vegetation cover is often sparse or absent. Nevertheless, in open spaces between the higher plants, the soil surface is generally not bare of autotrophic life, but covered by a community of highly specialized organisms (Fig. 1.1). These communities are referred to as biological soil crusts, or cryptogamic, cryptobiotic, microbiotic, or microphytic soil crusts (Harper and Marble 1988; West 1990). Biological soil crusts are a complex mosaic of cyanobacteria, green algae, lichens, mosses, microfungi, and other bacteria. Cyanobacterial and microfungal filaments weave through the top few millimeters of soil, gluing loose particles together and forming a matrix that stabilizes and protects soil surfaces from erosive forces (Cameron 1966; Friedmann and Galun 1974; Friedmann and Ocampo-Paus 1976; Belnap and Gardner 1993). These crusts occur in all hot, cool, and cold arid and semi-arid regions. They may constitute up to 70% of the living cover in some plant communities (Belnap 1994). However, biological soil crusts have only recently been recognized as having a major influence on terrestrial ecosystems.
Globally, this consortium of soil biota has many similarities in function, structure, and composition, in spite of their unconnected and seemingly dissimilar environments. Crusts are found in an astonishing variety of habitats throughout the world: in desert and semi-desert plant communities, ranging from shrub and succulent deserts to open woodlands; in steppe formations in both the northern and southern hemispheres; in the gaps between evergreen shrubs and in forests in the Mediterranean-type climate; and on open ground or between alpine or tundra vegetation. On a small scale, biological soil crust communities are found in open types of vegetation in temperate climatic regions; for example, they are frequently present in (and often restricted to) areas of a few square meters in xerothermic local steppe formations in central Europe and in the pine barrens of the eastern United States.
In rangelands, biological soil crusts can be viewed from functional, structural, and compositional perspectives. They function as living mulch by retaining soil moisture and discouraging annual weed growth. They reduce wind and water erosion, fix atmospheric nitrogen, and contribute to soil organic matter (Eldridge and Greene 1994). Structurally, biological crusts are a rough, uneven carpet or skin of low stature (1 to10 cm in height). Below ground, lichen and moss rhizines, fungal hyphae, and cyanobacterial filaments form a matrix that binds soil particles together (Belnap 1995). Horizontally, soil crusts occupy the nutrient-poor zones between vegetation clumps in many types of aridland vegetation. Compositionally, biological soil crusts are diverse. In many arid and semi-arid communities there are often many more species associated with the biological soil crust at a given site than there are vascular plants (Rosentreter 1986; Ponzetti et al. 1998).
Rangeland managers in North America have historically used key indicator plants for determining the ecological trend and health of vegetation (USDA 1937; Stoddart et al. 1943). Biological soil crusts can also be used as indicators of ecological health. In addition, they act as indicators of abiotic factors, such as the presence of calcareous soils. Crustal organisms read environmental factors differently from and on separate time scales than do vascular plants (McCune and Antos 1982). Most crustal organisms are biologically active during the cool seasons when the soil surface is moist (Rosentreter 1986). In contrast, vascular plants are active in spring and summer when air temperatures are above freezing.
Unlike vascular plants, crustal organisms, particularly lichens, are not greatly influenced by short-term climatic conditions. This makes them ideal indicators of long-term environmental factors. Therefore, each community component can provide information that may complement, explain, or indicate something about a site’s characteristics and disturbance history for rangeland management and evaluation. Just as plants increase or decrease with livestock grazing, many biological soil crust components are good indicators of physical disturbance, such as by livestock, human foot traffic, or motorized NOTES vehicles (Belnap 1995).
Land managers have been slow to use biological soil crusts in rangeland evaluations. Descriptions of vegetation or “habitat types” used by public land management agencies sometimes include biological soil crusts (e.g., Daubenmire 1970; Hironaka et al. 1983) but rarely present them as a dominant factor even when they are prevalent. This is partly because of perceived difficulties with identification (see Chapter 6). The problem of identification is exacerbated by the small size of these organisms, which often lack reproductive structures due to the harsh environments where they grow. However, identification problems can be substantially reduced by grouping organisms by function or general morphological characteristics (see sections 1.3 and 6.1.2).