January, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 23 



nut blight fungus (Endothia parasitica) and on white spruce, balsam fir, and 

 tamarack by the spruce bud moth {Tortrix fumifcrana) and other insect pests. 



Other biotic influences might be mentioned, such as the prevention of 

 forest reproduction in the Rocky Mountain region by rodents which destroy 

 the seed ; but these influences are so numerous and varied that they can be 

 merely suggested. 



4. Anthropeic Habitat Factors 



These are separated from biotic factors chiefly by reason of the fact that 

 they are subject to rational control. The modifying influence of man on the 

 nature of the habitat is expressed in various ways : through lumbering and 

 clearing the land for agriculture, through cultivation and the introduction of 

 grazing animals, through irrigation and forest management, etc. 



5. Pyric Habitat Factors 

 Fires may be caused by lightning, in which respect pyric factors might be 

 classed as climatic, or they may be set by man. For reasons which are 

 obvious, however, it seems best to recognize the influence of fire as belonging 

 in a distinct category by itself. The outstanding effect of fire on vegetation 

 is seen in the actual annihilation of existing plant communities, but scarcely 

 less important may be the modification of the habitat brought about by the 

 destruction of the humus, whereby all of the factors associated with the pres- 

 ence of humus are done away with, and, to a less extent, by the removal of 

 shade. 25 



[To be concluded] 



25 The brief outline of factors here presented is far from complete ; it is intended to 

 be suggestive rather than exhaustive. Furthermore, in attempting to group the various 

 factors into particular categories it is unavoidable, if for no other reason than the diffi- 

 culty of distinguishing between direct and indirect effects, that certain lines should be 

 somewhat arbitrarily drawn. Thus, peculiarities in topography are largely responsible 

 for the existence of localized conditions of climate, and in so far as this is true these 

 conditions may well be looked upon as being in the nature of indirect topographic influ- 

 ences. Again, the humus factors, notwithstanding the organic nature of humus, might 

 well be considered in connection with edaphic rather than biotic influences — as indeed 

 they frequently are. Shade is not invariably due to biotic influences; and so on. In 

 practise, here as elsewhere, individual judgment must be exercised in seeking the logical 

 correlation between cause and effect, and the exact treatment of material will necessarily 

 vary with different points of view. 



