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58 INITIAL CAUSES.
minima and volcanic ash in the atmosphere, two facts are evident. The first
is that the period between extremes is several years. Whatever the effect may
be in sorting out the population, or in producing adaptation, it is clear that
the intensities known, when spread over several years, are quite insufficient to
destroy plant communities and thus denude habitats. The second fact is that
there is no record of the destruction of vegetation at such periods, though
doubtless the effects of frosts were then most marked. In consequence it
seems impossible to regard changes of climate as initial causes of succession.
They are effective only in modifying existing seres.
BIOTIC CAUSES.
General relations.—In considering the influence of animals and plants upon
succession, it is necessary at the outset to distinguish clearly between biotic
causes and biotic reactions. The former, like all initial causes, produce bare
areas on which a new sere can develop. Biotic reactions, on the contrary,
have nothing to do with the production of initial areas, but represent the
modifying action of each stage upon the habitat. They are continuative,
since they induce and control the successive waves of invasion which mark
the various stages. A plant or animal parasite which produces a bare area by
killing ail the plants of a community, as may readily occur in families or pure
stands of trees, is a biotic initial cause. Holophytes and saprophytes can only
react upon the habitat by changing the factors of air or soil. Earthworms
react upon the soil conditions, while rodents such as prairie-dogs both react and
initiate new areas. It is the reactions of the plant communities upon the habi-
tat which are of paramount importance. With the possible exception of
Sphagnum, plants very rarely play the r61e of initial causes. The reverse is
true of man and animals. They are initial causes of great frequence and widespread
distribution, but only a few have a definite reaction upon the habitat.
Like climatic factors, biotic agents may change the existing vegetation, as
well as initiate new vegetation. In both cases they have to do with development,
but they can be regarded as causes of succession only when they produce
bare areas in which invasion occurs. It is probable that animals change
the course of development more often than they start it, while the activities of
man lead largely to denudation (plate 10).
Action and effect.—Man, and animals to a certain extent also, have at their
command the initial processes already considered under topography. These
are removal, deposit, drainage, and flooding. In addition, they may destroy
the vegetation, but affect the soil slightly or not at all. In the case of man, in
particular, the most various activities result in similar processes and areas. It
seems most natural to group them accordingly, rather than to consider them
from the standpoint of the, activities themselves. This is illustrated by the
fact that fallow fields, roadsides, prairie-dog towns, and ant-hills in the prairie
region exhibit essentially the same condition and initiate similar or identical
developments. The most suggestive grouping in consequence is the following
(1) activities that destroy vegetation without greatly disturbing the soil or
changing the water-content; (2) activities which produce a dry or drier habitat,
usually with much disturbance of the soil; (3) activities which produce a wet
or wetter soil or a water area. There is clearly no sharp line of demarcation
between the three groups, but this is evidence that the distinction is a natural