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: (1) 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 vegetation 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. Jt 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 affect 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 effects are in modifying soil temperature, 
and in holding snow and rain, and thereby increasing the water-content. 
By its gradual decay, moreover, it not 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. 
