36 BULLETIX 10f>l, r. S. DEPARTMENT OF AGRICULTrRE. { b^Ii.^nJ;'*!, 



ABUNDANCE. 



One's first impression of a well-occupied spectccbilis area is that 

 a large family must inhabit each den, but. as previously mentioned, 

 we have gradually been compelled to shift from this conception to 

 the idea of but a single animal to a mound, exce^Dt when the young are 

 present. Therefore a census of the adult kangaroo rat population 

 can readily be made, simply by counting the mounds. Such a censii.s 

 affords at least a conseryatiye estimate of the number of adult indi- 

 viduals occujDying a given area. 



The first estimates of abundance on the Range Reserve were from 

 actual counts of dens on areas measured off for experimental fencing, 

 and gave the figure of about two mounds to the acre. From time to 

 time rough estimates were made on different portions of the pastures, 

 and these checked well with the above. Later still, a careful comit 

 showed 300 mounds on approximately 160 acres (see p. 8). or 1.87 

 mounds per acre. Xine areas of 2 acres each, representing different 

 environmental conditions, were later selected in different portions of 

 the Range Reserve, and the dens accurately counted. The number of 

 dens per 2 acres varied from none to a maximum infestation of 12. 

 neither extreme occurring over large areas. The total number of dens 

 was found to be 43 on the 18 acres, or an average of 2.38 dens per acre. 



From all these estimates it may fairly be concluded that two 

 mounds, or two animals, per acre is a conservative estimate for the 

 infestation of the entire Range Reserve, with the possible exception 

 of small areas at its upper edges, where the altitude limit of spectahilis 

 is passed. It is. however, impossible to estimate the area of the State 

 infested with kangaroo rats, for some large stretches of fine grassland 

 show no kangaroo rats whatever, while others have more than are 

 present on the reserve : and we have no estimates of the extent of 

 either type. 



ECONOMIC CONSIDERATIONS. 



In ^lay. 1891. Fisher fomid a ranchman at AVillcox. Ariz., who com- 

 plained more bitterly of the depredations of spectoJjilU than of those 

 of any other mammal. 



On the United States Range Reserve the food material appro- 

 priated by the kangaroo rat during good years is inappreciable. 

 There is such an excess of forage grass produced that all the rodents 

 together make very little difference. But with the periodic recur- 

 rence of lean years, when drought conditions are such that little or 

 no grass grows, the effects of rodent damage not only become appar- 

 ent, but may be a critical factor determining whether a given number 

 of domestic animals can be grazed on the area (PI. VIII. Fig. 2^. 



