268 



THE NORTH AMERICAN GRASSLAND 



pidity than in the dry sand areas (Pool, 1914). The ponds are often 

 alkaline, and those that dry up leave white barren flats. A tiger 

 beetle, Cicindcla fulgida (Say), is characteristic of such areas, as are 

 some other insects. 



Reactions and Coactions. The effect of the community on the 

 habitat is marked, owing to the very deep root systems of the prairie 

 grasses and to the large number of burrowing mammals and insects, 

 especially ants. The abundance of pallid ground squirrels and the ex- 

 tensive network of burrows made by each pair bespeaks a consider- 

 able movement of soil materials by this single species. The prairie 



Fig. 61. — Painted box tortoise or sand turtle (Terrapene ornata kg.) in the sand 



hills of the mixed prairie area. According to Cahn (1937), Ortenburger found 



this turtle feeding on grasshoppers, caterpillars, and robber flies, while Cahn 



found only vegetable matter in the stomachs. (Photo by Edith Clements.) 



dog digs deeper, commonly to a depth of 12 to 15 feet, and brings 

 enormous quantities of soil to the surface. Since it is commonly con- 

 centrated in towns, the result is much more conspicuous but less gen- 

 erally distributed than for the ground squirrel. Probably the soil- 

 moving type of reaction originally reached its greatest intensity in 

 these mixed prairie areas. Another type of reaction, formerly of much 

 local significance, was the tramping of enormous herds of bison, which 

 was likewise most marked in this association. 



The outstanding coaction was grazing or clipping of grasses by the 

 large ungulates, the prairie dog, the ground squirrel, and other rodents. 



