ECOLOGY. 



353 



in weedy associes. Three factor stations have been installed in the reserve 

 for a complete study of the physical factors in the different grass communities. 

 The inclosures in northern Arizona have been studied in much greater detail, 

 and new stations have been established at Kingman and at Prescott. In 

 addition to the isolation transect already established at the Sonora substation 

 in western Texas, in cooperation with the Texas Experiment Station, 2 new 

 exclosures have been installed to show the effect of resting the grass for the 

 four different seasons of the year, as well as successive years. A considerable 

 number of quadrats was located within and without the exclosures, and it is 

 expected to chart these twice a year until the major facts are established. 



Destruction of the Range by Prairie-dogs, by W. P. Taylor and J. V. G. Loftfield. 



Experimental exclosures were installed in three representative areas of 

 northern Arizona in 1918 through cooperation with the Biological Survey of 

 the United States Department of Agriculture. While these were designed 

 to serve several purposes, the most important was to determine the effect of 

 the food-habits of the prairie-dog of the region (Cynomys gunnisoni zuniensis) 

 upon the carrying capacity and composition of the range. The latter is a 

 portion of the mixed-prairie association, usually modified in this region by 

 the suppression of the tall-grasses through overgrazing. The northern area 

 lies in Coconino Wash, about 9 miles south of the Grand Canyon, and con- 

 sists of Agropyrum glaucum and Sporobolus cryptandrus, with small amounts 

 of Bouteloua gracilis, Stipa comata, and other grasses. The exclosure con- 

 sists of two equal parts, proof against both cattle and rodents, but with one 

 containing a group of prairie-dogs, and it lies beside an unfenced area pro- 

 vided with quadrats also. This series has permitted a quantitative study 

 of the vegetation under three conditions: (1) under total protection, (2) 

 grazed by cattle alone, and (3) grazed by a known number of prairie-dogs. 

 Similar exclosures are located at Williams and at Seligman, the former in a 

 community of Bouteloua gracilis and Muhlenbergia gracillima, the latter in 

 one of B. gracilis and eriopoda. 



The growth of the grasses under the three conditions has been measured 

 by means of clip quadrats, and at the Coconino Station has yielded the follow- 

 ing figures, expressed as grams of forage per square meter: 



Agropyrum glaucum shows a consistent increase for the three years of total 

 protection, probably owing to its better utilization of rainfall as a result of 

 its sod habit. As a bunch-grass, Sporobolus cryptandrus appears to be more 

 dependent upon the seasonal distribution of the rainfall, and it is also unable 

 to hold its own in competition with the increasing Agropyrum. Rodents are 

 especially fond of it, as are cattle also; but while the latter merely graze it to 

 the ground, the prairie-dogs destroy it completely by the second year. 



