RANGE IMPROVEMENT. 317 



Special studies were made of the life history and food habits of the kangaroo- 

 rat in particular, and the latter were found to have a decisive bearing upon the 

 dominance of the grasses (Vorhies, 1919). Burrows excavated in the winter 

 of 1917-18 showed that the kangaroo-rat stored large amounts of food and 

 that this consisted chiefly of the spikes of Bouteloua. Those excavated in 

 1918-19 exhibited no grass spikes, owing to their failure to form during the 

 drought of the preceding summer, but the bulk of the stored food consisted of 

 the crowns of Bouteloua rothrockii. When the size of the denuded area abput 

 each burrow is taken into account, it is readily seen that the damage done by 

 kangaroo-rats is exceedingly serious, especially in periods of drought. While 

 they occur throughout the desert scrub, it seems probable that the majority 

 have migrated upward into the desert plains as the grasses were eaten off at 

 the lower levels. The persistence of the grasses in sheltered spots and their 

 reappearance in fenced areas in the desert scrub suggests that they can be 

 successfully reintroduced into the deserts about Tucson after the kangaroo- 

 rats have been exterminated. The rats which persist in the desert now live 

 in part upon the shrubs, it seems, and in consequence the latter are now being 

 killed out over great stretches from Ajo to Yuma and beyond. 



In addition to the major pests, the pocket-gopher, wood-rat or pack-rat, and 

 several of the field mice do more or less damage to the range. The gopher 

 damages the range by eating grass roots and disturbing the soil with his 

 burrows, but the injury is usually restricted to small areas. The pack-rat 

 lives on leaves of Yucca by preference, especially 7. radiosa and Y. arborescens, 

 and may do considerable damage in regions where these are important sup- 

 plementary forage. In the desert scrub and plains, it seems to feed largely 

 upon various species of Opuntia, which are heaped up about its nest (plate 72). 



Eradication of poisonous plants. — The loss of range stock from eating poison- 

 ous plants is so evident and often so serious that the importance of its pre- 

 vention requires no argument. The chief difficulty in the way lies in the 

 general ignorance of poisonous species and of the best methods of dealing with 

 them. Quite apart from their poisonous properties, such species are usually 

 undesirable weeds which compete successfully with the grasses and thus re- 

 duce the carrying capacity of the range. As a consequence, eradication is 

 theoretically the most satisfactory way of dealing with them, but this is per- 

 haps economically possible only in small pastures and other local areas. 

 Where they grow over hundreds of square miles, as in the case of the loco- 

 weed, Aragalus lamberti, on the plains and foothills along the eastern front of 

 the Rocky Mountains, eradication is practically impossible under existing 

 conditions, and controlled grazing is the only practicable method of preven- 

 tion. Marsh (1918: 21) has stated: 



"Most of the losses from poisonous plants occur at times when the animals 

 are short of feed and the larger part of the stock poisoning is indirectly due 

 to scarcity of proper forage. This fact of the intimate relation of scarcity 

 of feed to stock poisoning can not be too strongly impressed upon the people 

 who handle range animals in the West. There is apparently a popular idea 

 that range animals will voluntarily seek out poisonous plants and eat them by 

 preference. It may be stated as a general fact that this is not true. Animals 

 seldom eat poisonous plants except as they are driven to do so by the lack of 

 other food. Almost all poisonous plants are actually distasteful to five-stock 

 and under ordinary circumstances will be avoided. The only exception to 



