706. THE MONTHLY BULLETIN. 



11. The general habits of ground squirrels are such that they were 

 able to hold their own in the face of a host of natural enemies which 

 habitually preyed upon them before the white man's advent. The 

 squirrels are eminently successful in the battle for existence. They 

 inevitably prosper when any natural check is removed. 



12. The recuperative powers of ground squirrels are great. It is 

 shown that if the population of one square mile (if estimated at 640 

 as in the case of the California Ground Squirrel) were subjected t.o two 

 successive control campaigns, each of 90 per cent effectiveness, there 

 would still remain six squirrels; these three pairs of squirrels would 

 theoretically at the end of the third breeding season give rise to the full 

 normal population of 640, with a good margin for natural death. It 

 would seem that, if absolute extermination prove not possible over any 

 large area, eternal vigilance must be exercised to prevent the quick 

 return of the squirrel population to the danger point. The squirrels 

 must be looked after like weeds, which have to be dealt with year 

 after year. 



13. Ground squirrels breed upon uncultivated or waste land, from 

 which they invade the cultivated fields within reach as well as such other 

 lands as are not already fully populated. There is progressive emigra- 

 tion of a certain portion of the squirrel population each year, in August 

 and September, involving chiefly or entirely the young of the year just 

 coming to maturity. By a process of gradual infiltration, land once 

 thoroughly rid of squirrels may thus be reinfested from more or less 

 distant areas of dissemination. Lands successfully poisoned in the 

 spring may be found repopulated the following fall from some adjoining 

 territory. 



14. Since the squirrels if not interfered with by man are stopped in 

 their emigrations only at some natural barrier, it seems clear that control 

 campaigns should not be limited by political or civil boundaries such 

 as state, county, district or property lines. Rather should natural 



Fig. 30. Diagram showing estimated relative importance, as regards economic status, 

 of the different species of ground squirrels in California, a, California (Beechey) 

 Ground Squirrel ; b, Oregon Ground Squirrel ; c, Fisher Ground Squirrel ; d, Douglas 

 Ground Squirrel ; e, all other species of ground squirrels in the state put together. 

 The estimated ratios are, respectively, 10-5-4-3-1. 



114 



