A major impact of desertification is biodiversity loss and loss of productive capacity, for example, by transition from land dominated by shrublands to non-native grasslands.
In the semi-arid regions of southern California, many coastal sage scrub and chaparral ecosystems have been replaced by non-native, invasive grasses due to the shortening of fire return intervals.
This can create a monoculture of annual grass that cannot support the wide range of animals once found in the original ecosystem.
In Madagascar’s central highland plateau, 10% of the entire country has been lost to desertification due to slash and burn agriculture by indigenous peoples. In Africa, if current trends of soil degradation continue, the continent will be able to feed only 25% of its population by 2025, according to UNU’s Ghana-based Institute for Natural Resources in Africa.
Causes
Sand dunes advancing on Nouakchott, the capital of Mauritania.
Desertification is induced by several factors, primarily anthropogenic causes, which began in the Holocene era and continue at the highest pace today.
The primary reasons for desertification are overgrazing, over-cultivation, increased fire frequency, water impoundment, deforestation, overdrafting of groundwater, increased soil salinity, and global climate change.
Deserts may be separated from surrounding, less arid areas by mountains and other contrasting landforms that reflect fundamental structural differences in the terrain. In other areas, desert fringes form a gradual transition from a dry to a more humid environment, making it more subtle to determine the desert border. These transition zones can have fragile, delicately balanced ecosystems.
Desert fringes often are a mosaic of microclimates. Small pieces of wood support vegetation that picks up heat from the hot winds and protects the land from the prevailing winds. After rainfall the vegetated areas are distinctly cooler than the surroundings.
In these marginal areas activity centres may stress the ecosystem beyond its tolerance limit, resulting in degradation of the land. By pounding the soil with their hooves, livestock compact the substrate, increase the proportion of fine material, and reduce the percolation rate of the soil, thus encouraging erosion by wind and water.
Grazing and collection of firewood reduce or eliminate plants that bind the soil and prevent erosion. All these come about due to the trend towards settling in one area instead of a nomadic culture.
Sand dunes can encroach on human habitats. Sand dunes move through a few different means, all of them assisted by wind. One way that dunes can move is through saltation, where sand particles skip along the ground like a rock thrown across a pond might skip across the water’s surface. When these skipping particles land, they may knock into other particles and cause them to skip as well.
With slightly stronger winds, particles collide in mid-air, causing sheet flows. In a major dust storm, dunes may move tens of meters through such sheet flows. And like snow, sand avalanches, falling down the steep slopes of the dunes that face away from the winds, also moving the dunes forward.
0 comments:
Post a Comment