BIOTIC FACTORS 87 



of biotic factors, it must be distinctly understood that these are not properly 

 factors of the habitat as a physical complex, but that they are rather to be 

 considered as reactions exerted by the effect, or formation, upon the cause 

 or habitat. This is most especially true of plants. 



129. Animals. The activities of man fall into two classes: (i) those 

 that destroy vegetation, and (2) those that modify it. There are rare in- 

 stances also where the work of man has changed a new or already denuded 

 habitat. In the cases where the vegetarion is destroyed, the habitat itself 

 is sufficiently changed to permit the effect to be measured by physical factor 

 instruments. Otherwise, the influence is felt only by the formation, as when 

 man makes possible the migration of weeds, and it can be measured in 

 terms of invasion by the quadrat alone. It becomes especially evident, 

 then, in the case of man's activities, that where they produce a denuded 

 habitat they are to be regarded as factors in the habitat ; when they merely 

 affect the formation, this is not strictly true. The changes wrought by 

 other animals are essentially the same as those produced by man. They 

 are not so marked nor so important, but their relation to habitat and forma- 

 tion is the same. As a rule, however, they aft"ect the habitat much less 

 than they do the formation. 



130. Plants. As a dead cover, vegetation is a factor of the habitat 

 proper, but it has relatively little importance, since it occurs regularly dur- 

 ing the resting period. Its chief eft'ects are in modifying soil temperature, 

 and in holding snow and rain, and thereby increasing the water-content. 

 By its gradual decay, moreover, it ;iot only adds humus to the soil, but it 

 thereby increases the water-retaining capacity of the' latter also. The 

 cover of living vegetation reacts upon the habitat in a much more vital 

 fashion, exerting a powerful effect upon every physical factor of the 

 habitat. The factors thus affected are distinctly measurable though it is 

 often impossible to determine just how much of the factor is directly trace- 

 able to the vegetation. This is a simple problem in the case of most aerial 

 factors, especially light, but it is extremely difficult for soil factors, such 

 as water-content and soil texture. In the case of all habitats covered with 

 formations, by far the great majority, it is impossible as well as unneces- 

 sary to separate the physical factors of the habitat proper from the re- 

 action upon them which the plant covering exerts. Indeed, the great differ- 

 entiation of habitats is largely due to the universal principle that in 

 vegetation the effect or formation always reacts upon the cause or habitat in 

 such a way as to modify it. As fundamental causes of succession, the discus- 

 sion of the various reactions of vegetation is reserved for another place. 



