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 to 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 emigration 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.
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