Biological soil crusts (BSC) (cyanobacteria, lichens and bryophytes) are key elements in dryland ecosystems. BSC enhance soil stability and fertility, and influence hydrological processes. Inadequate land use such as intensive livestock grazing may severely alter BSC diversity and cover with potentially negative feedbacks on ecosystem processes. The degree to which BSC communities may be affected by long-term grazing or able to recover after grazing removal is overall poorly understood. Considering the important role of BSC in arid ecosystems and the severe land degradation associated with grazing, our goal was to examine BSC communities under different grazing regimes in grassland ecosystems of Central Mexico. We addressed the question, how high exposure to and release from grazing would influence BSC species richness, diversity and cover along i) a perturbation gradient with different grazing intensities (moderate/intermediate/ heavy) and ii) a grazing-recovery gradient using differently-aged livestock exclosures (6/11/28 years). We hypothesized that BSC responses would show opposite trends along the two gradients: a decline in BSC biodiversity/cover with increasing perturbation and an increase in BSC biodiversity/cover with increasing time of recovery. In each of the six gradient levels, we established in the interspaces of Bouteloua gracilis tussocks 48 monitoring quadrats (25cmx25cm) for photographic recording (N=288).
Results/Conclusions
BSC communities consisted of cyanobacteria, 16 lichen, three moss, and one liverwort species. As expected, species richness decreased along the perturbation gradient (P<0.05) with 14, 12, and 9 species occurring under moderate, intermediate, and heavy grazing conditions, respectively. Similarly, BSC species richness was higher (P<0.05) in the 28-year exclosure (14 species) compared to the 11- (13 species) and 6-year exclosures (12 species). Unexpectedly, BSC cover was higher (P<0.0001) in heavily grazed (31%) than in moderately (16%) and intermediately (12%) grazed sites. As hypothesized, total BSC cover was higher (P<0.0001) in the 28-year exclosure (72%) than in the 11- (31%) and 6-year (20%) exclosures. Along the perturbation gradient, BSC diversity was higher (P<0.05) in sites with intermediate grazing (H´=2.29) compared to sites with moderate (H´=2.27) and heavy (H´=2.01) grazing. Along the recovery gradient, BSC diversity dropped significantly (P<0.05) comparing the 6-year (H´=2.36) and 11-year (H´=2.35) with the 28-year exclosure (H´=2.22). We conclude that BSC species and cover may be highly vulnerable to grazing intensity. However, from a community perspective, BSC show a remarkable resilience after livestock removal suggesting these organisms play a fundamental role in degraded semiarid ecosystems both from a functional and potential restoration perspective.
