RODENTS, RABBITS, AND GRASSLANDS

E. R. Kalmbach

A VARIETY of small mammals fall in the average man's ken of rodents. Among them are the rodents proper (Rodentia), mice, rats, squirrels, prairie dogs, gophers, and others. Hares and rabbits (Lagomorpha) are technically not rodents, but must be included in a consideration of the relation of small mammals to the range because sometimes they are of greater economic importance than the true rodents. Even the insectivorous moles are often associated with the rodents by those who have had to combat them.

Oddly enough there are many species of rodents whose feeding, life habits, or limited distribution make them individually of little significance as hazards in modern grassland agriculture. Some of them are even energetic destroyers of grasshoppers and other insects. With them we need not concern ourselves here; it is more expedient to indicate the few that do most damage.

On the long-grass and short-grass areas of the Plains and Intermountain regions of the West, extensive grazing under conditions of limited rainfall has aggravated the problem of balancing what Nature may produce as well as destroy with what man would like to utilize. In those areas we have spent our greatest efforts to control rodents and tip the balance in our favor.

Throughout those areas the primary problems arising from field rodents are those associated with prairie dogs ( four species), ground squirrels ( more than 20 species) , pocket gophers ( three genera and numerous species), and kangaroo rats ( fully two dozen species classified in two genera).

Not all the varied species of these groups are of paramount importance economically, yet problems connected with any one are essentially the same in their basic causes and in the economics of remedial measures.

Occasionally difficulties are had with other groups of rodents in the management of grasslands, although transgressions by these species are connected usually with other agricultural and horticultural pursuits. Among these rodents are the meadow and pine mice, rice and cotton rats in the South and Southeast. woodrats in the Southwest, and when their numbers increase inordinately even the commensal house mouse and the attractive little deer mouse become liabilities under certain field conditions. Less is known about the effect on grasslands of such species as harvest mice (Reithrodontomys), pocket mice (Perognathus), jumping mice (Zapus), grasshopper mice (Onychomys), and other more obscure forms.

Indirectly the valued beaver may play a part in grassland agriculture through impoundment and flooding of pasture land. The muskrat and its close relative, the round-tailed muskrat or water rat (Neofiber), may have the reverse effect through their undermining or perforating of irrigation structures adjacent to agricultural land.

The hares and rabbits may enter into grassland economy whenever their cyclic increase has aggravated their pressure on the range. This is true of both the black-tailed and white-tailed species that affect the range and cultivated crops, especially when drought or early spring growth has concentrated them in the well-vegetated sections. This is also true of the antelope jack rabbit of the Southwest, but the related snowshoe and arctic hares, though capable of inflicting damage to hay crops at high altitudes or in northern latitudes, seldom become pronounced range-land pests. The diminutive but abundant cottontails, and brush and swamp rabbits, though capable of inflicting damage to young orchards, forest plantings, and truck crops, likewise are not looked upon as outstanding liabilities to range lands or hay crops, and their compensatory value as game animals must always be considered.

Whereas rodents and the associated rabbits have affected grasslands since time immemorial, their role in the ecology of modern agriculture and range use is far from being fully understood. These numerous vegetarian mammals always have been and will continue to be an influence on the range and farm. Their preferences for plant species may vary, however, and in that manner they may exert an influence on the succession of vegetative growth. The result may be for good or harm when judged in the light of man's prevailing range use.

Inseparately associated with the rodent-range complex is the degree of livestock use to which the grasslands have been subjected. This factor in itself may vary the rodent problem, according to circumstances, from one Of little consequence to one of transcendent importance. To approach sound range administration and particularly range rehabilitation by giving consideration merely to one of these two elements (rodents or livestock) may be likened to a person, who, for some unaccountable reason, Persists in merely dressing a wound without attempting to remove the disease. To carry the simile a bit further, it may be pointed out that, in the opinion of many who have studied such problems, the primary infection usually has its origin and persistence in excessive use by livestock. This has left a lasting wound, which rodent populations may be keeping in a state of constant irritation.

Some present-day conditions of depleted range are the result of many years of overuse. Whatever may have been their basic cause, it is evident that changes in the vegetative pattern did not take place suddenly. Even under abuse Nature often reacts slowly, and now that we are trying to rectify the trends that are against our best interests, we must not overlook the fact that readjustment also may be a slow process. This is particularly true at high altitudes and in and regions where the struggle for readjustments is confined to short growing seasons or brief periods of rainfall.

Whereas rather rapid readjustments have been observed in experimental plots from which all controllable pressure elements have been excluded, it must be remembered that this drastic remedial treatment is not attainable on ranges that are being used. On most private and public lands there will be a certain degree of range use by livestock and it is under such conditions that we must appraise the role of field rodents and the economy of their control.

Prairie Dogs

The colonial prairie dog has been fought ever since the competition for forage between it and livestock became a matter of concern. More success has been attained in controlling it than any other group of field rodents. Being gregarious, its pressure on the range was emphasized wherever it had decided to occupy the land, and for that reason it was vulnerable to control since survivors would segregate at some point within their original colony. Here they would again become subject to attack.

The economic and ecological role of these mammals under modern agriculture and range use has been the subject of study through the use of experimental range plots and through an analysis of their food by stomach examination.

One of the earlier attempts to appraise the effect of rodent pressure on the range was the study carried out by Walter P. Taylor and J. V. G. Loft-field on the damage inflicted to range grasses by the Zuni prairie dog in Arizona. At the time of the work (1918), the authors stated, "Determinations under controlled conditions of the actual damage done by rodents, either in cultivated crops or on the open range, are, however, almost wholly lacking."

To determine quantitatively the damage done by these rodents to forage grasses, experimental areas were established at three points and their maintenance and appraisal became a cooperative project of the Biological Survey ( predecessor of the Fish and Wildlife Service), the Carnegie Institution of Washington, and the Forest Service.

Results of 4 years of study in a wheatgrass forage type indicated that prairie dogs destroyed 69 percent of the wheatgrass and 99 percent of the dropseed (Sporobolus), or 80 percent of the total potential annual production of forage. In a blue grama type of range the loss caused by the rodents was computed to be 83 percent of the annual production. From the experimental testimony the conclusion was drawn that "in some overgrazed areas the total eradication of prairie dogs, as well as the reduction of the number of cattle per unit area, apparently will be necessary if the forage grasses are to continue in profitable quantity."

That "the prairie dog has not been shown to have a single beneficial food habit," though doubtless a true statement at the time it was made, is subject to qualification by later studies of the food habits of this quite generally despised field rodent. Some 20 years after the afore-mentioned field studies, Leon H. Kelso, having examined more than 500 stomachs of three species of prairie dogs collected under a great variety of conditions, disclosed that plants of forage or crop value are not the only ones eaten by these rodents. They did, however, comprise more than 78 percent of the food of the three species and wheatgrasses were highest in preference among the grasses. Of the range plants less attractive to livestock but eaten by the prairie dogs are sage, saltbushes, and Russian-thistle.

Taylor's and Loftfield's work on the Zuni prairie dog is worthy of repetition in the case of the black-tailed and white-tailed prairie dogs under range conditions differing from those in Arizona. Not only is there need for appraising more extensively the role of the other species of prairie dogs on which so much has been and still is being spent in control, but experimental procedures on range appraisal have improved since that earlier work. One would not expect that such later studies would materially change the estimate of the prairie dog's direct relationship to forage production but there is reason to believe that important facts in the concurrent use of the range by these rodents and livestock are yet to be disclosed.

Ground Squirrels

The control of ground squirrels has received attention throughout the West, but studies of their life habits and ecological relations have been carried out largely in the West Coast States, particularly in California and Washington. A treatise by Joseph Grinnell and Joseph Dixon on the California ground squirrels brought together for the first time much of the scattered information on life habits and economic status. Greatest emphasis was placed on the Beechey ground squirrel and its close relatives, the Oregon, Fisher, and Douglas ground squirrels which collectively comprise most of the "squirrel problem" of the State.

These were problems primarily of crop lands and, later, those associated with the curtailment and suppression of plague. Whereas the ground squirrel in relation to grassland agriculture in the sense of range protection entered the picture under many situations, much of the earlier work was conducted with the view of conserving highly valued crops. These early studies did not include detailed appraisals of the effect of squirrels on the range through the use of experimental plots. Estimates of over-all forage consumption were made by computations based on conservative estimates of the number of squirrels and the quantity of forage each would normally consume.