36 ANIMAL LIFE IN THE YOSEMITE 



THE INTERRELATIONS OF LIVING THINGS 



That forests afford the means of existence for a great number of animals, 

 with reference to both species and individuals, is a trite statement which 

 no one is likely to question. We would offer, however — albeit with some 

 caution — a second statement : Forests depend, for their maintenance in the 

 condition in which we observe them in this age of the world, upon the 

 activities, severally and combined, of the animals which inhabit them. 



Beginning at the root of the matter, in a double sense, as we have 

 emphasized beyond in the chapter on the pocket gophers, mammals which 

 burrow are of importance to forests. The pocket gophers, the ground 

 squirrels, the moles and the badgers, are natural cultivators of the soil 

 (see p. 142), and it is, in considerable degree, the result of their presence 

 down through long series of years that the ground has been rendered suit- 

 able for the growth of grasses and herbs, and even of bushes and trees, 

 particularly in their seedling stages. A host of insects, also, which live in 

 the ground at least part of their lives, contribute to rendering the soil 

 more productive of vegetable life. 



Vegetable materials, leaves, twigs and trunks of trees as well, contribute 

 to soil accretion by reason of their being torn to pieces by animals (see 

 p. 322), their particles scattered hy animals, and these finally overlaid by 

 the earth brought up by animals from deeper substrata. The animals which 

 figure conspicuously in this process are the woodpeckers, chickadees, and 

 nuthatches, the tree squirrels, chipmunks, and porcupines, the burrowing 

 beetles, the termites, and the ants, and then the burrowing and burying 

 mammals already referred to. This process of incorporating humus into 

 the soil, accomplished in large measure by animals, is of direct and lasting 

 importance to the forests. 



We do not make any claim that all animal life is directly beneficial to 

 the forests. For many insects may be seen to feed upon the foliage, the 

 bark, and even the live wood of individual trees, and in so doing such 

 insects shorten the lives of these trees, or even sometimes kill them outright 

 within a single season. It is obvious that a sudden overabundance of such 

 destructive insects would bring serious injury to the forests. 



But observation has led us to recognize, in certain groups of hirds, 

 natural checks to undue increase of forest-infesting insects. Insects of one 

 category inhabit the bark of a tree or the layers of wood immediately 

 beneath ; others pursue their existence among the smaller twigs ; still others 



