=128. Influence and importance.= Biotic factors are animals and plants. With respect to influence they are usually remote, rarely direct. Nevertheless, they often play a decisive part in the vegetation. Their effect is, as a rule, felt directly by the formation rather than the habitat, but in either case the one reacts upon the other. Such factors are not themselves susceptible of exact measurement, but their influence upon the habitat is usually measurable in terms of the physical factors affected. In the case 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 instances 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. 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 formation is the same. As a rule, however, they affect the habitat much less than they do the formation.