tlie Neotropical and Madro-tertiary Horas, and arc- 

 for tlie most part unrelated to luirasian forms. Re- 

 lated species and genera are found, however, in the 

 arid regions of Soutli America. Tlie vegetation of 

 the present Great Basin, Mohave, and Sonoran des- 

 ert regions was largely woodland and chaparral 

 through much of the Miocene and into the Pliocene. 

 Desert vegetation hecame differentiated with the in- 

 creasing aridity of the mid- Pliocene. The Mohave 

 and Sonoran deserts became distinct as cool winters 

 in late Pliocene and Pleistocene restricted the less 

 hardy succulent species to the Sonoran desert. It is 

 probable that the origin and development of deserts 

 elsewhere over the world has followed the same gen- 

 eral pattern (Axelrod 1950, Clements 1936). 



DESERT SCRUB BIOCIATION 



Large mammals, such as the bison and 

 pronghorn. are mostly absent from the desert scrub. 

 The mule deer is present in small numbers both in 

 tiie desert scrub and in the basin sagebrush, and the 

 mountain lion, bobcat, and badger penetrate to some 

 extent. The most common species of animals are the 

 following (Dice 1939, Huey 1942) although addi- 

 tional species occur farther south in Mexico (Burt 

 1938, Van Rossem 1945, Baker 1956) (species with 

 asterisks in this and following lists also found in 

 basin sagebrush biociation ) : 



Mammals 



*BIack-tailed jack rabbit *Botta's pocket gopher 



Desert cottontail Desert pocket mouse 



Rock squirrel Rock pocket mouse 



Spotted ground squirrel Merriam's kangaroo rat 



Round-tailed ground Desert kangaroo rat 



squirrel *Canyon mouse 



FIG. 26-3 In the Sonoran desert of Arizona. Saguaro (tree 

 cactus at left), paloverde trees (in middle distance), tree cholla 

 (cactus at right center), organ pipe cactus (upper right), creo- 

 sote bush (the taller bushes in the foreground), and bur sage 

 (the snnaller bushes in the foreground). 



Cactus mouse 

 *Deer mouse 

 Southern grasshopper 



mouse 

 White-throated wood rat 

 ♦Desert wood rat 



* Coyote 



*Kit fox 



Gray fox 



♦Western spotted skunk 



Collared peccary 



Mountain sheep 



In the mesquite vegetation of New Mexico, the 

 mouse and rat populations are highest in May with 

 about 8.5 individuals per hectare (3.4/acre) and with 

 the kangaroo rats the most numerous species. These 

 small desert mammals tend to have larger home 

 ranges than do comparable species in deciduous forest 

 and grassland (Blair 1943). Several of these species 

 extend their ranges well south through the Chihua- 

 huan faciation (Dalquest 1953). 



:'%.^.>*4f' 



>A FIG. 26-4 Sagebrush with spars 

 grass in Nevada (courtesy U.S. 

 Forest Service). 



Desert biome 335 



