300 



ECOLOGICAL FACTORS: 



except competition, a species encounters. Therefore, 

 any stage of a species" life history may be subjected 

 either to periods of optimum conditions and ready ad- 

 justment to the environment or of unfavorable condi- 

 tions and bare adjustment. Bare adjustment most 

 likely is related to periods of environmental extremes, 

 circumstances that are usually limiting. For this rea- 

 son environmental extremes are generally more im- 

 portant than average conditions. However, one can- 

 not extend this concept to imply that optimum en- 

 vironmental conditions automatically mean great 

 success for a species. Although optimum conditions 

 might allow maximum vigor, numbers, and develop- 

 ment, this is not necessarily the case. Again, com- 

 petition with some other species in an otherwise 

 "best" habitat can come into play and hinder the 

 organisms. 



GENE POOL- 



-SUCCESS 



.ECOLOGICAL. 

 AMPLITUDE 



FAILURE 



Figure 17.2 Features in the tolerance of a single gene pool (species 

 or population of o species) to its total environment. Tfie gene pool is the 

 basis of an ecological amplitude, or over all tolerance to the environ- 

 mental complex. The effects of age of individuals, season of the year, 

 fluctuations in habitat, and so on, upon the ecological amplitude deter- 

 mine success or failure. 



It should not be surprising that different gene pools 

 can lead to the organism variations that characterize 

 various phenomena. This can be appreciated in refer- 

 ence to altitudinal and latitudinal zonation of life. 

 Both kinds of distribution reflect the zonation of many 

 ecological factors, principally temperature and mois- 

 ture. Therefore, when a variety of ecological ampli- 

 tudes (actually, kinds of organisms) react to environ- 

 mental zonation, the outcome is a zonation of the 

 organisms. Moreover, many other zonations repre- 

 sent the same kind of phenomena. For example, there 

 are distributions brought about by zonation of soil 

 salts near the ocean or salt lakes, and those from the 

 annual progression of climatic conditions. The prog- 

 ress of climate can be likened to zonation, because 

 each segment of the year is associated vsith unique 

 phases of each organism's life history. 



ECOLOGICAL FACTORS 



Plants often are used as examples in discussion of 

 ecological factors, because they usually reflect their 

 environment more closely than do animals. In addi- 

 tion, the reactions of organisms to land conditions are 

 more complex, but often more precise, than are reac- 

 tions to water habitats. 



Ecological factors were said to fall into two cate- 

 gories, physical and biotic. The physical factors are 

 climate, soil, and topography. Climate, the most in- 

 clusive category, includes general climate, tempera- 

 ture, water, light, and atmosphere. Soil and topog- 

 raphy each have unique features but cannot be segre- 

 gated into distinctive subunits. Biotic factors are 

 further subdivided on the basis of kinds of interorgan- 

 ism relationships. An additional factor is fire, a prod- 

 uct of climate or of biotic factors (including man); its 

 action is physical. 



Physical factors can be classified as primary, sec- 

 ondary, or tertiary. Primary factors, the only ones 

 directly influencing life, are general climate, tempera- 

 ture, water (specifically, availability, including stand- 

 ing water and humidity), light, atmospheric gases, 

 some actions of wind, and nutrients (generally only 

 soil minerals for plants). Secondary factors are pre- 

 cipitation, atmospheric wind, and soil structure (es- 

 pecially texture), all features directly affecting the 

 primary factors and only indirectly affecting organ- 

 isms. The only tertiary physical factor is topography. 

 Land forms directly affect only secondary factors. 



Biotic factors are not always primary, secondary, 

 or more remote, because each factor has varying ef- 

 fects upon different organisms. Only in reference to a 

 particular organism at a particular time is a biotic 

 factor primary, secondary, or less direct (Table 17.1). 



Climate is the most important ecological factor. 

 This is true because, even where local minute varia- 

 tions must be appraised, climate includes most of the 

 primary ecological factors of plants. In addition, cli- 

 mate is an important animal control, acting either as 

 a direct check or as an indirect one through plants. 

 Therefore, plants, at least, exist only where climate is 

 suitable, and animals never are really independent of 

 climate. On the other hand, local fluctuations in 

 individual climatic factors, soil, topography, and 

 biotic features are important. Any of these single fac- 

 tors can prove limiting locally to a plant or animal. 

 For this reason, all ecological factors must be 

 examined. 



