12 ANIMAL LIFE IN THE YO SEMITE 



ASSOCIATIONS WITHIN THE CANADIAN ZONE 



Swift-stream Bed-fir 



Biparian (two types, Willow and Lodgepole-pine 



Comus pubescens) Jeffrey-pine 



Aspen Granite outcrop 



Meadow Cliff 

 Chaparral (three types, Bed-cherry, 



Arctostaphylos patula, and 



Huckleberry-oak) 



ASSOCIATIONS WITHIN THE HUDSONIAN ZONE 



Lake Lodgepole-pine 



Shore Hemlock 



Swift-stream Whitebark-pine 



Biparian (Willow) Talus (or Bock -slide) 



Meadow Cliff 

 Heather 



ASSOCIATIONS WITHIN THE AECTIC-ALPINE ZONE 



Swift-stream Dry grassland 



Willow-thicket Talus (Bock-slide) 



Meadow CUff 



Within each general association there is often plainly to be seen still 

 further restriction in the habitat preferences of species. For example, in 

 the major association, "coniferous forest," in its minor division (within 

 the Canadian Zone) known as the red-fir association, we find several species 

 of birds and of mammals, each adhering closely to a yet smaller division 

 of the general environment. The Sierra Creeper keeps to the larger tree 

 trunks, the Short-tailed Mountain Chickadee to the smaller twiggery, the 

 Western Golden-crowned Kinglet to the terminal leafage, and the Ham- 

 mond Flycatcher to the most prominent twig-ends and the air-spaces 

 between branches and between trees. The Tahoe Chipmunk is largely 

 arboreal, the Allen Chipmunk terrestrial. 



In final analysis, no two species well established in a region occupy 

 precisely the same ecologic space; each has its own peculiar places for 

 foraging, and for securing safety for itself and for its eggs or young. 

 These ultimate units of occurrence are called "ecologic niches." If two 

 species of the same ecologic predilections are thrown into the same environ- 

 ment, one or the other will quickly disappear through the drastic process 

 we call competitive replacement. Thus it comes to pass that the amplitude 

 of the general environment — the number and extent of distinct ecologic 

 niches it compasses — determines the richness of the fauna, both as regards 

 number of species, and the number of the individuals to the unit of area 

 representing each species. This principle may be abundantly verified by 

 any student who will carry on active field observations a season or two 

 over even a small part of the Yosemite section. 



