Natural History of the Ground Squirrels of California/Summary of Points of Special Importance from the Standpoint of Ground Squirrel Control
SUMMARY OF POINTS OF SPECIAL IMPORTANCE FROM THE STANDPOINT OF GROUND SQUIRREL CONTROL.
From the foregoing account of the natural history of the ground squirrels of California the following facts and inferences stand out as seemingly of special importance in connection with the determination and application of methods of ground squirrel control.
1. Of the eighteen kinds of ground squirrels occurring within the limits of the state, there appear to be only four meriting any particular consideration from an economic standpoint. These four are the California (or Beechey), the Oregon, the Fisher and the Douglas ground squirrels, here named in the estimated order of importance (see fig. 30). All the other fourteen kinds are, for the present at least, negligible, in most of the cases because they inhabit areas not cultivated by man.
2. The Oregon Ground Squirrel is less than half the size of a "digger" squirrel, but it is ordinarily present in much greater numbers per given area within its range (most of Siskiyou, Modoc and Lassen counties) than is any one of the "digger" squirrels in its range. The Oregon inhabits open grass lands and hence comes into sharp competition with cattle interests.
3. The Oregon Ground Squirrel is more of a grass eater than a seed or grain eater, and the most successful method of poisoning should involve the selection of an appropriate bait accordingly. It does not store up food to the extent that the "digger" squirrels do. The Oregon Ground Squirrel, more than any of the "digger" category, is subject to a sharply defined period of hibernation, and this involves all the individuals, of whatever age.
4. The California Ground Squirrel is our species of greatest aggregate numbers and is the one which is most widely distributed over the cultivated parts of the state. Its close relatives, the Fisher and Douglas ground squirrels, are known along with the California as "digger" squirrels; regarding most of the following considerations the three may be classed together.
5. Because of relatively large size the individuals of the "digger" category are able to inflict serious loss. Adults average 1¼ to 1½ pounds in weight, and are easily able to consume ½ ounce of dry grain or 2 ounces of green forage at a meal.
6. On open range and pasture lands these squirrels feed largely on alfilaria and bur clover, two of the most valuable forage plants in the state. The squirrels are then serious competitors for subsistence against the flocks and herds upon which man depends for his own support. On cultivated ground these squirrels feed upon or destroy in other ways grain and fruit crops to a very large extent where present even in numbers not above those reached on wild land. The tendency seems to be to increase to extraordinary numbers on cultivated lands unless effectively checked by man. This is due both to improvements of food conditions from the standpoint of the squirrel, and to removal of its natural enemies by man either purposefully or thoughtlessly.
7. The food preferences of ground squirrels are strongly in evidence and vary from species to species, and sometimes within the same species, from place to place and season to season. It is common testimony of those who have practical experience in poisoning ground squirrels that the Douglas is much more easily handled than the California; in other words, the former takes the strychnine-coated barley more readily. It is obvious that the success of any method of control by the use of poison must depend importantly on the nature of the bait employed. The fact that in some places the California Ground Squirrel has been found to pass up barley altogether for the seeds of bur clover suggests a likely way of improving poisoning methods locally.
8. In the "digger" category of ground squirrels there is evidence that a greater or less proportion of the population hibernates each winter. In the Douglas this feature of the annual activity of the animal is clearly evident, in that the majority, or at the higher altitudes all, of the individuals disappear for weeks or months together during the winter season. In the case of the California Ground Squirrel, however, numerous individuals are to be seen aboveground in the lower country in favorable weather at any time during the winter. But evidence at hand goes to show that these active individuals are chiefly young of the year and that most of the older scpirrels are then lying dormant belowground, in some extreme cases for as long a period as from August to February. During this interval, therefore, any method of poisoning, and probably also of gassing, will obviously be ineffective upon a portion of the population, and this portion which escapes will reappear at the beginning of the next breeding season to reinfest the area concerned.
9. Some obstacles to the success of control by the method of gassing arise through the unequal extent and irregular course of the burrows of the squirrels. It was found that although the volumetric content of the burrows of the California Ground Squirrel excavated averaged 5.2 cubic feet, in one case an extreme of 17.8 cubic feet was reached. This obtained in one of the "colonial" types of burrows in which several establishments supposed to have been originally separate had come to be intercommunicating. It was found that the usual dosage was ineffectual in this case. There is no definite way of distinguishing such "colonial" burrows, from surface appearances alone. Then, again, in some burrows there is an abrupt rise in the underground course of the burrow, which prevents the onward flow of a gas heavier than air, such as carbon bisulphid, and the squirrel is not overtaken. In either of the above circumstances we find a reason for the partial failure of extensive gassing campaigns with current methods.
10. Ground squirrels reproduce rapidly. In the California the average number of young in a litter is 7.2, with 4 and 11 as extremes. There is but one litter reared each year, and the young begin to appear aboveground about the first of May. The sexes are equally divided in a given population, and it is believed that each female breeds the first season of her life, that is, when she is slightly less than a year old, and that she has an "expectation" of rearing four more litters in case she lives to die of old age. Thus a population of 10 per acre in March may be expected to increase to 50 per acre by the last of May. Postponement of attention by the farmer is a losing proposition. A stitch in time actually saves nine.
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.
barriers be hewn to, such as those of climate, seacoasts, rivers, brush lands and deserts. The forest ranger in anticipating the sweep of a potential fire, or in combating any actual fire, outlines his campaign irrespective of any but those lines which will naturally aid most in stopping the spread of the conflagration.
15. If "drive weeks" be advocated, as a popular measure to secure control locally, the time of the year selected should be fixed in accordance with the optimum chances for success, on the grounds of avidity of the squirrels for kinds of bait available, minimum natural population (previously to the time of appearance of the young), and probable weather conditions.
16. The above general remarks must not be construed as in any degree intended to discourage the continued energetic application of the best methods of ground squirrel control now in use. But it is hoped the facts and inferences set forth will convince the reader that the problem is not a simple one, and cannot be solved by casual, half-hearted measures. It is believed that great improvement can be secured both in devising of method and in mode of application.