Cryptogamic soil crusts in Grand Canyon National Park were trampled by hikers, under controlled conditions, to determine how rapidly they were pulverized and how rapidly they recovered. Only 15 trampling passes were required to destroy the structure of the crusts; visual evidence of bacteria and cryptogam cover was reduced to near zero after 50 passes. Soil crusts redeveloped in just one to three years, and after five years the extensive bacteria and cryptogam cover left little visual evidence of disturbance. Surface irregularity remained low after five years, however, suggesting that recovery was incomplete.
Cole, David N.
"Trampling disturbance and recovery of cryptogamic soil crusts in Grand Canyon National Park,"
Great Basin Naturalist: Vol. 50
, Article 5.
Available at: http://scholarsarchive.byu.edu/gbn/vol50/iss4/5