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 individuals 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 _spectabilis_ 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.