36 BULLETIX 1091, U. S. DEPARTMENT OF AGRICULTURE. { ^SfSo^l. 
ABUNDANCE. 
One's first impression of a well-occupied spectabilis 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, except 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 census 
affords at least a conservative estimate of the number of adult indi- 
viduals occupying 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 count 
showed 300 mounds on approximately 160 acres (see p. 8). or 1.87 
mounds per acre. Nine 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 spectabiUs 
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 May. 1894. Fisher found a ranchman at Willcox, Ariz., who com- 
plained more bitterly of the depredations of spectdbilis 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"). 
