48 ANIMAL ECOLOGY 



and Labrador, and its northern limit coincides exactly with the 

 northern limit of the aspen {Populus tremuloides)^ which is its 

 favourite and, in those regions, most easily available food. 

 The southern range of the beaver overlaps that of the aspen 

 enormously, and in that part of America the beaver uses, or 

 once used, other trees instead. "^^ This example shows the 

 way in which factors are often only limiting to distribution in 

 one part of an animal's range. 



1 8. A species may depend on a plant and an animal, both 

 of which may act as limiting factors in its distribution. The 

 elf owl {Micropallas zvhttneyi), which inhabits parts of the 

 deserts of CaHfornia and Arizona, is only found in places 

 where the giant cactus {Cereiis giganteiis) grows, since it nests 

 exclusively in that plant. But it is also dependent upon two 

 species of woodpecker {Centurus uropygialis and Colaptes 

 chrysoides mearnsi) which also nest in the cactus, and whose 

 old nesting-holes are used by the owl. Buxton 20 says : 

 ** Less strictly dependent upon the cactus and woodpeckers 

 are a screech owl {Otus asio gilmani)^ a sparrow-hawk {Falco 

 sparverius), a flycatcher {Myiarchus c. cinerascens)^ and other 

 birds. None of these are confined to the area inhabited by 

 the Giant Cactus, but they all inhabit that area, and within it 

 they all use old woodpecker holes as nesting sites. Frequently 

 a single trunk of the Giant Cactus contains nests of one or 

 other woodpecker, and also of one of the birds which use the 

 old woodpecker holes. Honey-bees also use these excavations 

 as hives. One must remember that the number of living 

 creatures which eventually depend upon the Giant Cactus 

 includes the scavengers in the birds' nests and bees' nests, 

 the insects, few though they may be, which devour it or fre- 

 quent its blossoms, and many others. All these organisms 

 depend upon the growth of Cereiis giganteiis for their existence 

 in certain areas." 



19. The example that has just been given illustrates the 

 difference between an environmental factor which acts on an 

 animal in the ordinary way, and one which acts as a limiting 

 factor. The woodpeckers are both affected by the presence 

 of the giant cactus, and yet not, as species, absolutely dependent 



