Technical Report No. 3, Ecological Investigation of Natural Community Status: Organ Pipe Cactus National Monument

Steenbergh, Waren; Warren, Peter

Ecological investigations on four major biotic communities at Organ Pipe Cactus National Monument were conducted from September 1974 to June 1976 to document and evaluate the status of natural desert communities that have been subjected to more than 50 years of intensive livestock grazing pressure. Data on plant community structure and composition and rodent populations were obtained from preexisting livestock enclosures, and a series of permanent study plots established along transects in four representative biotic communities. Aerial and ground photography was employed to provide supplemental data on plant community status. Large-scale color-infrared aerial photographs are a useful.tool for documenting and monitoring the status of desert plant communities. Estimates of plant density and ground cover derived from aerial photographs. are lower than those provided by ground truth. Accuracy of estimates decreases with increased complexity and stratification of the plant community. Differences in the complexity of the four biotic communities sampled are characterized by significant differences in plant community structure and composition. Both plant and rodent species diversity increases with plant density and cover. Soil texture directly limits the distribution and abundance of some rodent species. Intensive livestock grazing directly reduces perennial plant cover and species diversity, and indirectly alters rodent species diversity and abundance. Existing biotic communities have been grossly altered in both structure and species composition under long continued intensive livestock grazing pressure. Grazing-caused erosion in some localities has progressed to a point where reestablishment of original natural biotic communities is impossible. Progressive deterioration of unique biotic resources is occurring under the pressure of continuing livestock grazing. Such consumptive use is wholly incompatible with the legislated purposes of the monument. Fulfillment of the intended purposes of Organ Pipe Cactus National Monument urgently demands the complete elimination of domestic livestock grazing at the earliest possible date.