16 The Plant World. 



AN ANIMAL FACTOR IN PLANT DISTRIBUTION. 



By J. C. Blumer. 



The Rincon mountains of Arizona possess an ecologic factor 

 of no small importance in the form of a small rodent (Thomomys 

 sp.). It inhabits in large numbers the Canadian pine region, 

 especially on the west side of the mountains, and extends down 

 into the Transition zone. Its work was also found to a lesser 

 extent in the high pine region of the Chiricahua mountains. 



The animal appears to be somewhat variable in color and 

 form, and may represent a variable species or else more than one 

 sub-species. This timid, circumspect, and industrious little 

 brown mammal does a large amount of work in the autumn be- 

 tween the close of the rainy season and the advent of cold wea- 

 ther. Over large areas, both on ridges and slopes and in canyon 

 bottoms, its mounds of earth are conspicuous. Frequently, es- 

 pecially on top of ridges, the soil is so honey-combed with its 

 runways that in walking over it, one sinks through as though 

 walking on thinly crusted snow. In the sunny days of October, 

 especially before a change of weather,it is very busy with its 

 burrowing, making many surface openings, always from below, 

 along its tortuous and much-branched underground runways. 

 A part of these, oblique in direction, it uses to expel the excavated 

 dirt, therewith building its conspicuous chains and groups of 

 mounds. Other shafts, usually more vertical, are opened for 

 foraging purposes, gathering for its winter nest dry leaves and 

 stems, grass, and other loose material that may be within a 



radius of about twice the length of its body. If grassy tussocks 

 art at hand the rodent digs into these, utilizing the grass roots for 

 food or nesting purposes. It probably lives largely on the roots 

 of various perennials encountered along the route of its diggings 

 and is responsible for the killing or crippling large numbers of 

 these by cutting the roots or burying the plants. On the other 

 hand, it is probably instrumental in favoring the germination 

 of many seeds and the establishment of many young plants that 

 otherwise would not come into existence. This is brought 

 about by the throwing up of the loose soil in which become 

 imbedded during the autumn, chiefly by the aid of wind and 



