• David J. Eldridge
• Matthew A. Bowker

Disturbances to biocrusts may occur either due to human activities or natural forces. Many of the human activities that are most likely to result in biocrust disturbance are linked to agriculture including livestock production. Intensive agriculture includes many activities which cause changes to the soil surface, either mechanically, while plowing, planting, etc., or chemically, by use of herbicides, fertilizers, etc. Mechanical disturbances negatively affect biocrusts, whereas the effect of herbicides depends on the chemical used. Grazing, another mechanical disturbance caused by hoof action, usually results in reduction of biomass and changes in biocrust community structure, leading to a long-term degradation of soil function. Other major mechanical anthropogenic disturbances include military training, e.g., tank maneuvers, recreational activities, hiking, and off-road vehicle use. Biocrusts are easily destroyed by vehicles and trampling that crush or bury biocrust organisms and expose the soil to erosion. Mining is perhaps the most intensive anthropogenic disturbance, as it involves complete removal of the soil surface and even subsurface soils. Natural disturbance agents act via various mechanisms such as addition of heat, sedimentation, or limitation of resources. High- intensity fires generally cause a universal reduction in abundance of biocrust organisms, while frequent fires may select for fire-tolerant organisms. Deposition of sediments may favor groups of organisms that have a greater tolerance of burial, such as filamentous cyanobacteria. Altered precipitation patterns, including amount, seasonality, and frequency of precipitation, can have a negative or positive impact on biocrusts.

Part of the Ecological Studies book series (ECOLSTUD, volume 226)