348 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



years, even then the total amount of turnover of the ground has 

 been enormous. At the rate determined on one tract of land to 

 be one-tenth of an inch per year, it would in that period of time have 

 amounted to 1,700 feet. This is equivalent to 3,400 plowings to a 

 depth of 6 inches. 



Computations of this sort may be carried on endlessly, and it is 

 rather good fun to do so for a while ; but beyond certain limits, they 

 are not particularly profitable In this case, especially, there is a wide 

 margin of inaccuracy when an attempt is made to apply the initial 

 figure over large areas. 



Interestingly enough, our studies have shown that the average 

 depth at which pocket gophers run their burrows beneath the sur- 

 face of the ground is 6 inches. And this, I am informed by Prof. 

 John S. Burd, is the usual depth reached by the farmer of California 

 when he plows the land for his crops. Not only does the gopher 

 bring raw soil to the surface to be further weathered, thus releasing 

 the mineral food that the plants require, but it is continually bury- 

 ing vegetation beneath the earth which it throws up at the mouths 

 of its burrows; and furthermore, vegetation is cut into pieces and 

 carried below ground in large quantities by gophers, much of which 

 is not eaten but remains there, just as does the vegetation which is 

 turned under by the plow, to add to the humus content of the soil. 

 All the excreta of a pocket gopher are deposited underground in 

 special branches of its tunnel system, also at about the same critical 

 depth — 6 inches. The nitrogenous supply from this animal is thus 

 not wasted in any such proportion as it is in the case of those her- 

 bivores which live altogether on the surface of the ground. It must 

 be emphasized that it is on wild land, land that is untouched by the 

 farmer, that the gophers thus serve in a valuable way as enrichers of 

 the soil. 



While the gopher and ground squirrel when they eat grass admit- 

 tedly come into direct competition with horses, cattle, and sheep, the 

 story does not stop there. An important function, it seems to me, 

 performed by burrowing animals is that of counteracting the pack- 

 ing effect of large mammals on uncultivated grazing lands. The 

 impact of heavy feet on the soil, especially when wet, crowds the 

 particles together and renders the earth less suitable for plant 

 growth. Close tamping tends to exclude the air and hence to suffo- 

 cate the plant roots, to which oxygen is as essential as it is to animal 

 life. One has but to observe the conditions on mountain meadows 

 outside the limits of national parks to appreciate the point here 

 made. Often where the countrv has been overstocked with cattle or 



