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 wide- 

 spread 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 develop- 

 ment, but they can be regarded as causes of succession only when they pro- 

 duce 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 



