92 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



larger forms under occasional circumstances. In the air itself, animal 

 reaction upon light usually occurs as a secondary effect in the case of 

 phytophagous insects, which destroy foliage, and of other organisms 

 residing upon leaves. 



Reaction upon Humidity, Temperature, and Wind. These three 

 factors are intimately associated in a complex that determines the 

 water relations of the community in the higliest degree. The energy 

 required to bring about transpiration and evaporation is supplied by 

 incoming radiation, the capacity for taking up the moisture concerned 

 is determined by humidity, and both are modified by the moving and 

 cooling action of wind. The reaction upon all three is practically 

 confined to the plant matrix, since that of animals is essentially negli- 

 gible in spite of the fact that mammals in particular intercept radia- 

 tion and wind, and give off moisture in the form of perspiration. 



As with light, the reaction upon temperature is due to absorption 

 and interception, together with more or less reflection, and this like- 

 wise increases in intensity from the upper to the lower layers of a 

 community. Conversely, the plant cover, living or dead, may also 

 serve as a blanket to retain the heat that has entered. The effect 

 upon wind is even more obvious as a consequence of interception and 

 correspondingly influences both temperature and humidity. Finally, 

 lunnidity fluctuates directly with the transpiration, and to a less extent 

 with evaporation from plant surfaces of the water condensed or inter- 

 cepted by them. As the ultimate result, water loss is reduced and the 

 holard relatively increased under a canopy, with characteristic effects 

 upon tlie function and structure of shade plants. 



Reaction upon Carbon Dioxide and Oxygen. Since plants absorb 

 and emit both these gases and animals give off the one and take in 

 the other, it is to be expected that the biotic community will exert 

 some reaction upon the composition of the air. This is an important 

 effect in both soil and water in which diffusion and air movement are 

 much reduced. The mobility of the atmosphere, coupled with the 

 rapidity of diffusion, prevents the accumulation of gases given off and 

 at the same time readjusts the conditions arising from consumi:)tion. 

 Hence, it is only where air movement and diffusion are hindered or 

 the emission of carbon dioxide excessive that accumulation can occur. 

 In nature, such requisites are found only in dense forests and thickets, 

 where a tendency exists to increase the carbon dioxide just above the 

 soil layer especially. This is not true of the oxygen content, since 

 photosynthetic activity is weak in the lower layers, owing to deep 

 shade, and also because this gas is replaced by the augmented carbon 

 dioxide in some degree. 



