1914] Grinnell: Mammals and Birds of the Colorado Valley SI 



Larrea was at least the most conspicuous plant, occurred mostly on 

 the more level upland desert mesa. The accompanying distinctive 

 assemblage of mammals justifies the recognition of what may therefore 

 be appropriately called a creosote (or mesa) association. 



Tongues of typical creosote association often run up from alluvial 

 slopes into the rough hill country, following ravines and terraces. On 

 the other hand, as already noted, the creosote bush and some of its 

 companions in places invade the saltbush association. Along desert 

 washes the two associations may be blended in all particulars to such 

 an extent that definite diagnosis as one or the other is difficult. 



As will have been observed from the lists, there are only two 

 breeding birds of the creosote association proper, Chordeiles acuti/n nnis 

 texensis and Amphispiza bilineata deserticola; and neither of these 

 are abundantly represented. But graminivorous mammals are plenti- 

 ful in species as well as individuals. Not all the species, however, 

 are found in exactly the same places. As elsewhere shown, identical 

 ground on opposite sides of the river may possess distinct species 

 because of the action of the river as a barrier. 



Furthermore, segregation of rodent population on the basis of 

 ability to find or dig safe retreats is evident. The nature of the 

 ground thus bears a controlling relationship. Parts of the desert mesa 

 are swept clean of fine sand by the prevalent winds, the resulting sur- 

 face consisting of packed gravel, or wind-worn pebbles (pi. 12, fig. 18). 

 Other parts of the desert have a sandy soil ; and in places accumula- 

 tions of sand transported by the wind have grown into sand-dunes, 

 having much the character of those described for the saltbush associa- 

 tion on a previous page. 



On sandy ground a common small shrub was the sandbur (Fran- 

 seria dumosa) ; the rattle-weed (Eriogonum inflatum) was ever sum- 

 moning startled attention ; and in the vicinity of Pilot Knob a species 

 of Ephedra was common. On stony ground often no other plant was 

 to be seen than the creosote bush ; but everywhere remains of short- 

 lived sporadic vegetation gave clue to the source of supply of the seeds 

 upon which depended the rodent population. In the vicinity of Pot- 

 holes and Pilot Knob the mesa association included scattering clumps 

 of round-stemmed cactus, and ocotilla (Fouquieria) . 



Each of the two physical types of ground here noted, namely. 

 sandy and stony, possesses certain peculiar species of mammals, as 

 well as of plants. This segregation is indicated in the list, and might 

 again serve as basis for recognizing two separate associations, the 



