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.

ECONOMIC CONSIDERATIONS.

In May, 1894, Fisher found a ranchman at Willcox, Ariz., who complained
more bitterly of the depredations of _spectabilis_ than of those of any
other mammal.

On the United States Range Reserve the food material appropriated 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 recurrence of lean years, when drought
conditions are such that little or no grass grows, the effects of rodent
damage not only become apparent, but may be a critical factor
determining whether a given number of domestic animals can be grazed on
the area (Pl. VIII, Fig. 2).

With two kangaroo rats to the acre (1,280 per square mile), there would
be 64,000 animals on the 50 square miles of the Range Reserve. If each
rat stores 4 pounds of grass seeds and crowns and other edible forage
during the season (and in severe seasons we find that more crowns are
stored than under ordinary conditions), a total of 256,000 pounds,