122 COACTION: THE INTERRELATIONS OF ORGANISMS 



which must eat dead grass and forbs, frequently digging away snow 

 to secure them. 



Taylor and Loftfield (1922) have measured the coaction of the 

 prairie dog, Cynomys gunnisoni, in northern Arizona in connection 

 with the grazing exclosures and found that this species may destroy 

 as much as 80 per cent of the total potential annual production of 

 forage where the vegetation still persists. Under extreme conditions 

 of crowding, prairie dogs may consume the entire cover and be forced 

 to move elsewhere. Many social burrowers, especially those that form 

 "towns," often bare the surface more or less completely; this is also 

 true in a large degree of the solitary kangaroo rats (Vorhies and 

 Taylor, 1922) and in some measure of the ground squirrels when 

 closely aggregated. 



Pound and Clements (1898:414) pointed out the characteristic 

 dominants of the extensive "dog towns" in western Nebraska, and the 

 changes involved have been traced in greater detail in and about the 

 grazing exclosures mentioned above (Clements, 1919). Similar stud- 

 ies have been made of the areas denuded by kangaroo rats in the 

 grassland and desert scrub climaxes of southwestern Arizona, where 

 the destruction of the cover is sometimes complete (Clements, 

 1920:90; 1928:297). 



Small Surface Resident Grazers. In this group are included rab- 

 bits, mice, etc., which feed on green grass and forbs in summer and 

 their dead tops in winter. The principal animals of this type in 

 North America are voles and jack rabbits. In localities where shrubs 

 or cacti occur, jack rabbits may feed on these in winter or during 

 drought periods. However, it must often be necessary for them to 

 eat dry grasses, though the food records are few as yet (Vorhies and 

 Taylor, 1933). Certain gallinaceous birds may belong here also, but 

 nearly all those that graze do so only for a portion of the year and 

 then not exclusively. 



In addition to those whose activities have been described, various 

 omnivores such as squirrels, most ground squirrels, many mice, porcu- 

 pines, some^ foxes, and birds compete with the large grazers in a 

 minor way. However, phytophagous chewing insects are often more 

 important in the destruction of herbaceous plants in competition with 

 grazing mammals. This is most strikingly true of grasshoppers, which 

 in general are most abundant in grassland areas, where grazing is the 

 prevalent coaction. 



