634 THE MONTHLY BULLETIN. 



the experience of the United States Public Health Service in their 

 eradication work. This appeals to us as a fair estimate on well-popu- 

 lated 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 squir- 

 rel" 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 popula- 

 tion 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 nat- 

 ural 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 per- 

 centage 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 



