The Channel Islands are covered with a wide range of soil types-from fine sandy loams to clay-with the most diversity found on the larger islands. Until recently, most of what we knew about the islands' soils came from a cursory investigation that was done in the late 1970s; however, the National Resources Conservation Service and the NPS Soils Inventory and Monitoring Program have just conducted a mapping project that has greatly expanded and enhanced our understanding of park soils.
One unique feature of Channel Islands soils are its cyanobacterial crusts-layers of cyanobacteria, lichens, mosses, green algae, microfungi, and bacteria, which are common on the islands. The cyanobacteria's thread-like filaments stabilize the soil and improve water infiltration. The bacteria also convert atmospheric nitrogen into a form that plants can use, increasing soil fertility. Despite the vital roles they play in arid environments like the Channel Islands, these fragile crusts are highly susceptible to disturbance from grazing, hooved animal and human foot traffic, and off-road vehicles. It is important to try to prevent these kinds of impacts, as damaged crusts may take hundreds of years to fully recover.
Soil Disturbance and Erosion
A combination of natural processes and human impacts has caused soil disturbance and/or erosion on all of the Channel Islands. Many of the islands' soils are naturally erodible, and there are also a number of erosion-prone sedimentary geologic formations. This, combined with the rugged topography and steep slopes common to many parts of the islands, and exposure to waves and wind, has caused natural changes to the islands' soils over time.
Humans have also altered park soils by clearing the plants that hold soils in place and reshaping vegetation patterns through development and agriculture. Nonnative animals like sheep, burros, elk, pigs, and rabbits that were brought to the islands over the years have also over-grazed the islands' plants and damaged the soils through burrowing, rooting, and trampling. These disturbed soils are easily compacted, which increases water runoff and limits oxygen exchange with plant roots, hindering their growth and ability to hold soils in place.
The particular human and natural histories of each island have shaped their soils and associated erosion and disturbance problems as described below.
San Miguel Island
Poor historical livestock and agricultural management, compounded by drought, stripped San Miguel of its once abundant native vegetation and caused severe soil erosion. Two-thirds of the island was left largely devoid of topsoil and covered by drifting soil and sand, and its edges gullied and marked with large, unstable dunes. Since the removal of sheep in the 1960s, and feral burros in the 1970s, reemerging plants have stabilized the island's sandy soil surface, markedly reducing erosion in many places.
Santa Rosa Island
Extensive sheep and cattle grazing, along with browsing and rooting by nonnative elk, mule deer, and feral pigs caused serious erosion on Santa Rosa Island. A substantial network of unimproved roads built to support past ranching, oil exploration, archeological excavations, and military installations also continues to cause erosion problems. The windswept ground on much of the far western end of the island is so bare that impermeable caliche (a calcium carbonate hardpan) has been exposed. The soil on many of the ridges was also stripped away by water and wind after protective vegetation was decimated by grazing animals. Within the canyons, particularly in the south and west parts of the island, deep gullies have developed and are still expanding today. The National Park Service has rehabilitated of some of the most damaged areas including road-created gullies that were causing large amounts of sediment to run into coastal waters.
Santa Cruz Island
Santa Cruz Island carries a similar history of heavy livestock grazing, pig rooting, and massive vegetation loss. This, combined with weakly-cemented sedimentary formations, has led to major soil erosion in certain areas. Most steep slopes have succumbed to gravity, depositing significant amounts of sediment into nearby valleys and watershed drainages where deeply incised gullies are also common. The removal of cattle and sheep, and subsequent recovery of native vegetation have helped slow erosion on Santa Cruz Island; however, gully and sheet erosion are still occurring throughout the island, particularly in the erosion-prone sedimentary Monterey formations found on the island's isthmus and east end.
All three islets of Anacapa were grazed (primarily by sheep) in the early 1900s, and their native vegetation and soils suffered as a result. The last sheep were removed from West Anacapa in the 1960s. Vegetation stripping and excavation during the construction of Coast Guard facilities on East Anacapa also caused a great deal of erosion. Aside from some trampling of soil crusts in areas of East Anacapa human-caused erosion has decreased, though natural processes including sea cliff erosion continue to eat away at the island, and in some areas expanding gull colonies are also exposing more bare ground.
Santa Barbara Island
The dramatic, near-vertical sea cliffs on the southwest and north sides of the island are being worn away by waves and are very unstable. As with the other islands, Santa Barbara's vegetation and soils have been damaged by past livestock grazing and farming, but were also devastated by rabbits introduced in the mid-20th century. Two species of invasive iceplant have also altered the soil chemistry in some areas so much that the normal clay particle structure has been destroyed, dramatically increasing its erodibility.
The islands most notable soil damage may be seen in its 12-acre "badlands" where deep gullies crisscross through bare ground and only small patches of grass persist. Based on aerial photographs, these badlands developed sometime between 1943 and 1957-a period when rabbit populations on the island peaked and vegetation was especially heavily grazed. They have since been expanding at the rate of about 1% per year, and their remaining dense clayey subsoil is eroding at the rate of about 6 inches per year. In 1995-97, the National Park Service put erosion control matting over about 2 acres and took measures to encourage vegetation to return.