22 



GEORGE E. NICHOLS Vo1 - IV > No - I 



mated only in terms of geological time. Examples of relatively rapid pro- 

 gressive physiographic change are seen in the erosion of ravines and coastal 

 bluffs in sand and clay, and in the building up of flood plains and coastal 

 swamps. Relatively slow changes are exemplified in the emergence, sub- 

 sidence, and base-leveling of land areas and in the erosion of solid rock along 

 streams and seacoasts. Relatively rapid changes in physiography may exer- 

 cise a relatively immediate effect on the habitat ; the immediate effect of rela- 

 tively slow changes is inconsequential. In the absence of evidence to the con- 

 trary, and with reference to its proximate effect on the habitat, the physi- 

 ography of any area can be regarded as essentially stable. 



Other physiographic influences are seen in the direct mechanical effect on 

 vegetation of water currents, of wave and spray action, of ice, of land- 

 slides, etc. 



5. Bio tic Habitat Factors 



These include influences which are associated with the activity or effect 

 of plant or animal agencies (exclusive of man). The more important biotic 

 habitat factors may be summarized as follows : 23 



Shade. Shade may be produced by other than biotic agencies (e.g., the 

 shade cast by the walls of a ravine), but its effect is most generally associated 

 with the influence of vegetation. The influence of shade may be expressed 

 in various ways : it hinders or prevents the growth of various light-requiring 

 plants; it results in increased atmospheric humidity, which in turn means 

 decreased transpiration by plants and decreased evaporation from the ground ; 

 it modifies the temperature relations of both the air and the soil. 



Root Competition. Competition for water below ground may be quite as 

 influential in its effect on vegetation as competition for light above ground. 

 This is especially true in areas of insufficient soil water. 



Humus. Humus arises through the decay of dead organic matter. The 

 accumulation of humus may variously affect the water relations, the aeration, 

 the food supply, the temperature, the acidity, and the toxicity of the soil ; it 

 may afford conditions favorable or otherwise to the existence of various soil 

 organisms. 



Micro-organisms in the Soil. The precise significance of the complex 

 population of bacteria, fungi, protozoans, and other organisms which inhabit 

 the soil is not yet completely understood, but enough is known to indicate that 

 these organisms may be of the highest importance in their relation to plant 

 communities in general. 24 



Plant or Animal Invasion. The influence of these is exemplified in the 

 widespread destruction wrought on the chestnut in recent years by the chest- 



23 In this connection, see Cowles 'n, pp. 171-179. 



24 For comprehensive discussion of the micro-organic population of the soil and its 

 relation to the growth of plants, see E. J. Russell: Soil conditions and plant growth 

 (fourth edition). London. 1921. 



