Domestic livestock trample
soils wherever they graze, water or loiter. Their weight (800-1000 pounds and
more) destroys fragile "biological crusts"
that stabilize the soil and provide a barrier to invasive weeds on arid western
landscapes. The loss of biological crusts increases soil erosion (contributing
to the massive annual loss of topsoil in North America) and invites
invasion of noxious weeds onto damaged sites.
New research summarizes the importance of livestock grazing to soil damage and erosion--and recommends grazing management that permits restoration of soil crusts, even at the expense of ranch profits:
"Desertification of semiarid and arid rangelands is widespread and costly, impacting 85% of North America's . drylands. The most widespread contributor/accelerator of rangeland desertification is livestock grazing, and one of its main symptoms is accelerated soil erosion."
"We propose that a primary objective of rangeland management should be to slow desertification by preventing rangelands from crossing degradation thresholds beyond which positive feedbacks continue the desertification process and natural succession cannot return a site to its previous state. Economically based decisions regarding establishment of stocking rates should be considered only after this goal has been met."
Bowker, M. A., J. Belnap, M. E. Miller. 2006. Spatial
modeling of biological soil crusts to support rangeland assessment and monitoring.
Rangeland Ecology and Management 59(5): 527 (citations omitted).