Lichens are the result of a symbiotic relationship between algae and fungi. At Carlsbad Caverns National Park, lichens can be seen on rocks and tree trunks to a limited extent. More prominent are the black, brown, and white "crusts" that can be seen on the soil surface. These crusts are living organisms, composed of various lichens, algae, mosses, and liverworts. Previously referred to as "cryptogamic crusts," researchers prefer to use more scientifically correct terms, such as "microphytic" (tiny plants) or "microbiotic" (tiny organisms) to describe these organisms.
The crusts don't look very impressive when they're dry, but when it rains they come out of dormancy quickly. Rapidly absorbing moisture, they develop the deeper pigments (especially greens) for photosynthesis. During this time, the plants take advantage of the moisture to grow and reproduce by producing spores.
Microphytic crust can be an important component of arid and semi-arid ecosystems. For example, they prevent soil erosion. This happens two ways. First, the root-like structures of lichens and mosses, along with the filaments of some algae, bind the soil particles together. This often creates an irregular soil surface that interrupts wind patterns, reducing wind erosion and trapping wind-borne soil particles. Second, the crusts physically protect the soil by covering it with their thalli ('bodies'), thereby reducing rain-caused erosion and removal of sediment.
Crusts also improve the moisture content of soils by increasing the depth of water penetration and the total soil moisture content. Crusts may also decrease evaporation from the soil surface, enhancing the higher rain infiltration rates found in crusted soils. Crusts enhance seed germination and seedling development, presumably by providing a stable soil substrate and extra nutrients.
The effects of algae and lichens on soil fertility have been studied extensively. Certain crusts contribute high quantities of nitrogen, an extremely limited nutrient in desert soils. Crusts also contribute organic matter (carbon compounds) to soils.
These crusts tend to be rather fragile and are often severely damaged by mountain bikes, hikers, cattle and fire. Studies have documented varying recovery times, usually measured in decades. These are just some of the plants that benefit when hikers stay on trails.