ENVIRONMENTAL FACTORS 49 



upon it, whereas the elf owl is. The other birds mentioned 

 are not strictly dependent upon the woodpecker, although its 

 presence affects them enormously, while the elf owl is entirely 

 linked up in its distribution with the woodpecker holes. It 

 further shows that the working out of biotic limiting factors is 

 far more difficult and complex than similar work on physical 

 and chemical factors, since there are such elaborate inter- 

 relations between the different species. We are therefore 

 compelled to study more closely the relations between different 

 animals, and this leads us on to a consideration of animal 

 communities. 



