the experience of the United States Public Health Service in their eradication work. This appeals to us as a fair estimate on well-populated territory, such as that around San Francisco Bay.
It is the authors' belief that if the entire area in California occupied by this species be taken into consideration a population of one squirrel per acre, or 640 per square mile, at the conclusion of the breeding season, would be a fair average. At this rate there would be a total population of 32,000,000 California Ground Squirrels in the state in July, and one-fourth this, or 8,000,000, in March, before the young of the year are out. If the closely allied Fisher and Douglas ground squirrels be included, as from an economic standpoint might well be done, the "digger squirrel" population of the state in summer, when crops are maturing, may be put at between 40 and 50 millions, in this state.
As for rate of increase, we are dealing with a prolific animal. As already shown, the average size of the litter in the California Ground Squirrel numbers practically eight. Males and females are present in a general population in about equal numbers. Even though but one litter is reared by each female squirrel each year, this would mean that for each pair of squirrels at the beginning of the breeding season there will be ten individuals at the close of the breeding season. The evidence we have examined goes to show that all the squirrels breed the first year of their lives—that is, when each is not quite one year old—as well as subsequently, and that the life-time of a squirrel, if it dies of old age, is five years. If we do not count upon any fatalities, one pair of squirrels can be reckoned on to give origin to a population in five years of 6,250!
In recent efforts to eradicate squirrels a 90 per cent efficiency has been currently estimated. This means that, if no follow-up campaign be waged, ten squirrels out of each original 100 will be left, to form a nucleus of future increase. At the end of the second year the population would be back to normal. Supposing, further, that a follow-up campaign is waged at the end of a suitable interval before the next breeding season, also with a 90 per cent effectiveness; then only one squirrel per original hundred would be left. Even then, when only six squirrels are left on one square mile, these in the third breeding season will produce, barring normal fatalities, the original 640, with a good margin to spare.
The factors limiting the population of ground squirrels under natural conditions, that is, as not affected by human agency, include the following, in the order of probable importance: (1) Quantity of food available at the season of the year when food is scarcest; (2) natural enemies, including predatory mammals, birds and reptiles; (3) adverse weather conditions, recurring rather infrequently, as when territory is inundated during exceptionally heavy rains; (4) disease; (5) old age. The rate of increase, through long ages, has been adjusted to more than meet the expected death rate from all causes combined. This rate of increase, fourfold each year, is now inherent and we have no reason for expecting any abrupt and permanent change in it either way.
With the arrival of the white man and his accessories in California, the natural balance has been upset. Man has destroyed a large percentage of the natural enemies of the ground squirrel. Cultivation of the land has, on the other hand, in portions of the state improved the food supply. The general tendency is for the squirrels to breed up on
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