An Animal Factor in Plant Distribution. 17 



gravity, large numbers of seeds. The mounds weather down 

 and spread out under the influence of wind, rain, frost and melt- 

 ing snow, produce a favorable seedbed, and the following sum- 

 mer gixe rise to colonies of young plants, among which annuals 

 are especially numerous. This animal may thus be responsible to 

 a considerable extent for the unusually large proportion of annuals 

 in the flora of many parts of the high Rincons, and may disturb 

 the balance of competition in this as well as other directions. 



It may be easily inferred that the animal exerts a large in- 

 fluence upon the plant life also indirectly. The many tunnels 

 give access to the air with its daily and seasonal fluctuation in 

 temperature, and probably admit more or less water from the 

 rains and snows, introducing frost action. This aids disinte- 

 gration of the undeilying rock, while the generally loosened soil 

 is more easliy transported. Extensive burrowings are not in- 

 frequently found in gravelly soil so shallow that the runways 

 are in contact with bedrock, yet barely hidden beneath the sur- 

 face. The nesting material decays and increases moisture ca- 

 pacity and fertility, and for this and other reasons subsequent 

 abandonment of an area results in marked benefit to the plants. 

 The animal thus amounts to a geological as well as an indirect 

 botanical factor, and modifies soil temperature, soil moisture, 

 its depth, consistency, aeration, and undoubtedly its bacterial 

 life as well. Its direct harmful effect in the destruction of roots 

 may possibly be offset in a measure by the destiuction of larvae 

 and other animal life harmful to plants. 



The relation of the little mammal to forestry is interesting. 

 In its domain the forest is composed mainly of Pinus arizonica and 

 P. strobiformis. The reproduction is remarkably good, particu- 

 larly on the more open slopes and ridges where usually the rodent 

 work also is or has been prominent. Many young seedling trees 

 are growing in a cover of pine needles under mature trees, places 

 where ordinarily such are rarely seen. Upon close inspection 

 mineral soil may be found under the needles, the remains of 

 mounds of the rodent. Frequently young pines may be seen in 

 juxtaposition with the mounds, whereas they are rare in adjoin- 

 ing places without the exposed mineral soil which is practically 

 indispensable to the establishment of successful young seedling 



