Approach to the Study of Ecology 21 



A^ B 



It might very well be that no direct causal relation exists between 

 A and B whatsoever, but that both are controlled by a third influ- 

 ence, C. 



A B 



\ / 

 C 



Or factor A may influence C, which in turn influences B, thus: 

 A B Temperature Plant 



^ ^ \. . ^ 



C Moisture 



In considering the factors of the environment separately in order to 

 distinguish and to measure the influence of each, we must remain 

 thoroughly aware that in nature the factors are never acting alone. 

 Animals and plants are subject to many influences at the same time, 

 and the effect of one factor is often modified by action of other factors. 

 The "real life" of the organism, on which its growth, distribution, and 

 multiplication depend, necessarily involves the simultaneous and con- 

 tinuing impact of all existing factors and also influences that occurred 

 at earlier stages in the organism's experience. 



An important difference exists in the extent to which factors can be 

 modified by living organisms. Some features of the environment are 

 largely unaffected by the activities of the organisms present; these are 

 tinmodifiable or conservative factors. The salinity of the ocean is an 

 example of a conservative factor. The volume of tlie ocean is so great 

 that, although animals and plants living in it are continually adding 

 or withdrawing salts, the amounts have an immeasurably small effect 

 upon the total salt content of the water. The modifiable or non-con- 

 servative factors of the environment are susceptible to change caused 

 by the inhabitants of the area. The oxygen in a small pond, for ex- 

 ample, may be so depleted by the respiration of a large population of 

 fish that an unfavorable or even a lethal condition for the fish is pro- 

 duced; or the concentration of oxygen in the pond may be increased 

 by the photosynthesis of algae— a modification that will benefit the 

 fish. Heather (CaUuna) tends to increase greatly the acidity of the 

 soil in which it is growing, and this condition favors the further de- 

 velopment of this plant, but it is unfavorable to most other plants. 

 Through modification of its own environment, heather often comes to 

 dominate the vegetation in large areas, as may be seen in Jutland. 



No sharp division exists between modifiable and unmodifiable fac- 

 tors. All gradations exist, and a given factor may be modifiable in 



