CARRYING CAPACITY. 293 



the storage in the propagative organs is reduced and seed production is like- 

 wise affected. If the drought continues for a second or third year, these 

 effects become cumulative and the stand diminishes greatly in density as 

 well as in height. During wet phases, the growth of the vegetative organs 

 is favored and this in turn promotes propagation and reproduction, but the 

 former especially. As a consequence, the sun-spot cycle of 11 years is clearly 

 expressed in carrying capacity, and this is often true likewise of the 2 to 3- 

 year cycle, particularly in the more arid Southwest. In short, grass types 

 show a carrying capacity cycle of excess and deficit, which must be taken into 

 account if alternate lack of utilization and overgrazing are to be avoided. 

 Such a cycle has a peculiar significance for overgrazing and range improve- 

 ment and is further discussed under these heads (plate 71). 



Relation to rodents. — While the damage done by prairie-dogs to native 

 vegetation has long been known and the indicators recognized (Pound and 

 Clements, 1898: 299; 1900: 414), it is but recently that the full importance of 

 rodents has been realized. This has led to the extensive campaigns for the 

 eradication of rodents, organized and carried out during the last five years by 

 the Biological Survey, and to the cooperative studies of the kind and amount 

 of damage to different grazing types. The plans for the first of these were 

 drawn up by the writer, and they have been carried out on the Santa Rita 

 Range Reserve near Tucson through cooperation with the Forest Service, the 

 Biological Survey, and the University of Arizona. The results have already 

 demonstrated the serious and often critical effect which jack-rabbits have 

 upon the range and have added the kangaroo-rat to the list of rodent pests 

 of the first importance (Vorhies, 1919). While prairie-dogs, ground-squirrels, 

 jack-rabbits, and kangaroo-rats are the most important, pack-rats and pocket- 

 gophers also do much damage, and there are doubtless other rodents which 

 must be reckoned with. The reduction of carrying capacity by rodents is a 

 serious matter at all times, but it becomes critical during drought periods. 

 This is due to its added effect upon a range which is already overgrazed by 

 the stock. The frequent occurrence of drought in the Southwest has greatly 

 magnified this effect, and in some areas the grass (and even the desert scrub) 

 has been almost completely destroyed as a consequence. It is probable that 

 there is a rodent cycle, due to the effect of dry and wet phases upon vegetation 

 as the food-supply, but in a local area this must be more or less modified by 

 the effects of migration. Rodents resemble grazing animals in showing a 

 preference for certain life-forms and dominants, as well as in adjusting them- 

 selves to less palatable species under the spur of necessity. The general 

 features of the methods by which their habits are studied and their effects 

 measured are given under the discussion of range improvement (plate 72). 



Relation to herd and management. — The recognition of the proper meth- 

 ods of handling stock to secure the maximum carrying capacity was first made 

 by Smith (1899), and the importance of such methods has since been em- 

 phasized by Griffiths (1904), Davy (1902), Wooton (1908), and others. Their 

 development into a practical system is due chiefly to the work of the Forest 

 Service in connection with the grazing problems of the national forests (Jar- 

 dine, 1908; Sampson, 1908; Barnes, 1913). The most complete discussion of 

 the system for handling cattle has been given by Jardine and Hurtt (1917), 



