Biological soil crusts are an essential part of desert ecosystems throughout the world, as they are important in soil stabilization and soil fertility. Despite their importance, there have been few efforts to examine the population dynamics of the dominant species comprising these crusts or the effect of exotic plant invasions on these dynamics. In this study, we followed changes in lichen and moss cover for 8 years in plots dominated by native grasses or invaded by the exotic annual grass Bromus tectorum and across sites representing a range of land use histories. Our data showed that cover of both lichens and mosses can increase dramatically over short time periods, often going from just above 0% cover to as high as 9% cover in only 6 months. Cover of the nitrogen-fixing lichen Collema declined throughout the study, going from 19% in 1996 to as low as 2% in 2003, likely in response to an increase in monthly maximum temperatures during the study period. Changes in chlorolichen cover (lichens with green algal phycobionts), on the other hand, appeared related to precipitation patterns. Past grazing may be responsible for declines in species richness for both mosses and lichens and decline in cover for lichens. A recent Bromus invasion did not affect species richness in never-grazed plots, but a 50-year-old invasion appeared to be related to lower species richness in the previously intermittently grazed plots. Bromus invasion was related to lower cover of Aspicilia, Collema, Placidium, yellow lichens combined (Caloplaca tominii, Candelariella terigena, Fulgensia bracteata, and Fulgensia desertortum, total lichens, and total mosses in some plots in some years. Extended drought was likely responsible for a large decline of all species in 2003. Loss of lichen and moss cover is expected to affect many aspects of this ecosystem. Of special concern is the loss of Collema, as it is the dominant source of nitrogen for this region. Published by Elsevier B.V.
