Populations 83 



ent processes to factors of the environment. Each process has a 

 range of tolerance for the environmental factors that must fall within 

 the intensity span of the factors. Organisms do not function unless 

 the ranges of tolerance of all these processes fall within the in- 

 tensity spans of all the environmental factors. These ranges of toler- 

 ance of any organism are, of course, determined by the genotype 

 assembled in the zygote and by the developing epigenotype (see 

 Chap. 4). It is important to note that, until reproduction occurs, 

 there are at most physicochemical, not biological, functions among 

 individuals. Organisms may be considered part of the physical en- 

 vironment of other organisms until they cooperate to reproduce. 



There is no satisfactory way, at present, of dealing with the com- 

 plex interactions of organism and environment. Usually some very 

 rough classification of environmental factors is employed. For exam- 

 ple, Andrewartha and Birch have divided the environment of a 

 given organism into four components: weather, available nutrition, 

 other organisms, and a place to live. These components may be 

 further subdivided as required. All are continually changing in some 

 degree. Just as the range of tolerance of a particular process changes 

 during the course of development (resistance to desiccation, heat or 

 cold sensitivity), so the intensities of environmental factors change 

 cyclically, as well as in complex and Httle-understood patterns. The 

 soil around an oak tree's roots may become leached of mineral ele- 

 ments, which are restored by leaf fall and disintegration. The food 

 plant of a butterfly dies out in a drought year. The required en- 

 vironmental factors for the establishment of seedlings or for the 

 pollination of flowers may be present for a brief period at only one 

 time of the year, and the behavior of the plant must be closely 

 correlated with the occurrence of these factors. 



Interactions of amazing intricacy may be seen in natural popula- 

 tions. For example, "other organisms" in the environment may be 

 classified also as nutrition (host, prey, food plants of herbivores) or 

 as a place to live ( host, trees, etc. ) . A young muskrat may find all 

 suitable burrow sites (a place to live) occupied by older stronger 

 individuals (other organisms) and be forced to migrate. During its 

 migration it may freeze (weather), starve before it finds suitable 

 forage (nutrition), or be killed by a coyote (other organism). 



As part of the environment of an organism, other organisms may 

 change the microclimate (as in the shade, leaf-fall zone, and root 

 range of a tree). They may serve as vectors of genetic material in 

 infection or reproduction, as well as of propagulcs. The flowers, 

 fruits, and seeds of the angiosperms show a great diversity of devices 

 effecting successful pollination and dissemination by specific animal 



