Characterization of Relict Communities for Monitoring Park Ecosystems in Glen Canyon National Recreation Area

Butler, Jack; Bich, Brian; Schmidt, Cheryl

A study was conducted in 1990 and 1991 to evaluate the effects of grazing by domestic livestock on selected Indian ricegrass/galleta grass (Oryzopsis hymenoides/Hilaria jamesii) and pinyon-juniper (Pinus edulus - Juniperus osteosperma) communities of Glen Canyon National Recreation Area (NRA). The specific objectives were to: (1) identify and characterize the vegetation, avian, and rodent components of relict-nongrazed communities, (2) identify and characterize the vegetation, avian, and rodent components of selected communities that best represented the end product of light, moderate, and heavy grazing by domestic livestock, and (3) identify and evaluate community characteristics that may serve as indicators in a comprehensive system which can be used to monitor the ecological condition and trend, using the selected relict-nongrazed communities as reference points. A distinct shift in the composition and abundance of the major herbaceous species along the selected grazing gradient was observed in the Indian ricegrass/galleta grass communities of Grand Bench. The density and basal area of Indian ricegrass decreased in response to grazing, with density decreasing by as much as 80% on the grazed sites during both 1990 and 1991. The density and basal area of galleta grass and broom snakeweed (Gutierrezia sarothrae) tended to increase on grazed sites. The little pocket mouse (Perognathus longimembris) was the most abundant rodent species trapped on all sampled sites and demonstrated a 50% decrease in abundance at the heavily grazed site. The deer mouse (Peromyscus maniculatus) was the second most abundant rodent species recorded and tended to increase with increasing grazing intensity. Total bird density and bird species diversity exhibited a significant decrease with increasing grazing intensity. The black-throated sparrow (Amphispiza bilineata) was the most abundant avian species recorded and suffered a 50% decrease in density at the grazed sites. The house finch (Carpodacus mexicanus), observed only at the relict site, appeared to demonstrate the greatest relict site specificity. Several of the selected Indian ricegrass/galleta grass community components examined may serve as useful ecological indicators of community change in response to grazing by domestic livestock. It should not, however, be concluded that grazing will have parallel effects in other habitat types. It is also likely that some of the observed changes in plant and animal species composition and density along the grazing continuum can be attributed to the effects of disturbances placed on these communities by the activities of grazing livestock.