neighbors. Changes through the seasons and the years affect not only the com- 

 position of the population, but also the composition of the soils. And there is 

 also some effect upon the climate. For example, the conditions of moisture, 

 light, temperature, and air movement close to the ground in a forest are quite 

 different from the corresponding features on a prairie; and they are made 

 different as the plant-and-animal community develops. 



Of two species living side by side, one may grow faster and shoot up into 

 the air. But the other may mature more quickly and shoot a thousand seeds 

 into the air before the first one has started to flower. The quick grower may 

 fill an acre in the second year; the other, however, may spread over six acres. 

 These differences are, of course, not the only ones. Nor do they tell us which 

 species will in the end survive. 



Through the interactions of plants and animals, of organisms and the soil 

 and the immediate atmosphere, the composition of a population gradually 

 reaches an optimum for the region. There is a balance between the chloro- 

 phyl organisms and the others. There is a balance between plant-eaters and 

 flesh-eaters, between insects and birds feeding on insects, between plants that 

 supply nectar to insects and insects that pollenate the flowers, between the 

 number of nuts and the number of squirrels. 



When this state is reached, it may continue indefinitely. It is called a 

 climax of life development, since it represents the fullest continuous yield of 

 Hfe for the region. And because particular types of plants are characteristic 

 in such situations, various formations are usually designated by the names of 

 "dominant" plant species — for example, a pine forest, a tamarack swamp, a 

 scrub-oak mountaintop, a maple-birch community, and so on (see illustrations, 

 pp. 204 and 564). 



Moving Equilibrium In a stabilized, or cHmax, formation all the vari- 

 ous species are mutually adjusted in equilibrium. And the whole living popu- 

 lation is in equiUbrium with the physical conditions. Soil, cUmate, plants and 

 animals make up together a complete whole. All the parts are related to each 

 other in such a way that the "whole" remains pretty much the same, although 

 changes are going on in every part all the time. 



When the climax has been reached, each species reproduces itself at a rate 

 that keep.s its numbers about the same year after year. Many species that 

 were conspicuous early in the development have disappeared, and new ones 

 seldom make their appearance. Moreover, the kinds of organisms that thrived 

 at one stage of the development cannot thrive in a later stage. Weeds are 

 usually tougher plants than our cultivated varieties, and they are always 

 pushing out into unoccupied spaces. But they are generally not so efficient 

 where the soil and the other inhabitants have become adapted to a more ad- 

 vanced stage. 



Because the larger plants are always the most conspicuous features of such 



570 



