296 GRAZING INDICATORS. 



While conclusive evidence as to the degree of overgrazing must be obtained 

 from the failure of a range to maintain the herd upon it, such evidence can 

 rarely be secured except from experimental tests. This is due to many fac- 

 tors, of which variations in carrying capacity with the climatic cycle and differ- 

 ences in management are the most important. As a consequence, it is most 

 satisfactory to draw evidences of overgrazing from the behavior of the plant 

 cover and to determine the degree by means of quadrat measurements. The 

 competition between the individuals and species of the plant community is so 

 keen and the balance so exact that the slightest disturbance can be readily 

 detected. Grazing itself constitutes such a disturbance, and its effects upon 

 growth, propagation, and reproduction can be minutely measured by means 

 of the various kinds of quadrats. Such dominants as the grasses, however, 

 have such an advantage over the subdominant herbs because of their under- 

 ground parts and methods of growth that only a severe disturbance can 

 throw the balance in favor of the herbs. When this happens, the first evi- 

 dence is afforded by the increase in the number and vigor of the latter, which 

 consequently serve as indicators. With increasing disturbance due to over- 

 grazing, the annual members of the native flora appear in the most disturbed 

 areas as the pioneers of minute subseres, and are later followed by introduced 

 weeds. In the final condition the grasses will have disappeared, largely or 

 completely, only the more weedy societies will persist, and the ground will be 

 chiefly or wholly occupied by weedy annuals and biennials. Such a com- 

 munity represents one or more stages of the secondary succession and its 

 tenure depends upon the continuance of the disturbance that initiated it. If 

 the latter ceases, the successional process begins and soon terminates in the 

 original climax if the grass dominants have not been killed out. Under such 

 conditions, succession is universal and inevitable in all climaxes, and this fact 

 lies at the basis of all methods of range improvement (plate 73). 



On the basis of the maximum annual production of forage, overgrazing 

 occurs whenever the yield drops below this point. It is evident that the 

 maximum production can not have a fixed or average value, but that it must 

 be correlated with the periods of the climatic cycle. A degree of grazing 

 which would be disastrous in a drought period would fall far short of adequate 

 utilization during a wet one. Coville (Sampson, 1908 : 5) has applied the 

 term "destructive overgrazing" to the condition in which all or part of the 

 native dominants are killed. It is characteristic of areas overgrazed during 

 the critical drought periods of the double sun-spot cycle. For the sake of 

 clearness, three types of grazing are recognized here. These are overgrazing, 

 close grazing, and reserve grazing. Overgrazing results when the proper 

 maximum yield of a particular year or period is not obtained because of the 

 failure to make enough food for propagation or seed-production, or because 

 the seed-crop has been destroyed. There are varying degrees of overgrazing 

 from a slight reduction in yield to the complete destruction of the range. 

 Close grazing is the type in which the total annual yield is utilized in such a 

 way as to maintain the carrying capacity. Reserve grazing is the process in 

 which part of the annual yield is held in reserve, either by means of a reserve 

 pasture or by understocking. It constitutes an insurance against emergencies 

 and is specially adapted to periods of drought. In actual practice, close 

 grazing is usually preferable for the wet phases of the climatic cycle, and re- 

 serve grazing imperative for the dry phases. 



