628 General and Applied Biology 



iood, burrowing, protection, etc. Plants requiring a specified amount 

 of moisture may not be able to get it from one soil but are able to do so 

 from some other soil. One plant requires a soil of a certain consistency 

 to give it its necessary anchorage, while the requirements of another 

 plant may be entirely different. The aeration of the soil is also deter- 

 mined to a great extent by its physical make-up. All of these and other 

 similar factors help to determine the ecologic relationships. 



(g) The Slope of the Soil: The slope of the soil and its exposure 

 naturally determine the quantity and quality of light and heat. Some 

 plants require a minimum of light and heat and hence would not find 

 conditions ideal on a slope which is exposed to the hot sun of the after- 

 noon. The opposite slope might be much more favorable for such plants. 

 The slope of the soil also aflfects the drainage and this may also be an 

 important factor in the distribution of certain kinds of plants. In all 

 probability animal distributions are also affected by the environmental 

 conditions of the slope of the soil. The slope may be conducive to erosion 

 which may aflfect plant and animal distributions. 



(h) Pressure: Pressure may be considered from the following stand- 

 points: air, water, and soil. Naturally, the type of environment in which 

 an animal lives will determine which of these pressures will influence 

 its distribution. Air pressure is 15 pounds per square inch at sea level 

 and decreases uniformly as one ascends from sea level to the higher 

 regions. In high elevations the air pressure becomes too low to permit 

 normal respiration in certain animals. This is a very salient factor in 

 determining certain animal distributions. 



Water pressure increases as one descends from the surface. Water 

 pressure in the ocean is equal to the depth in feet multiplied by 0.434. 

 Thus, at 200 feet depth, the water pressure per square inch is approxi- 

 mately 87 pounds. This pressure naturally determines the vertical dis- 

 tribution in deeper bodies of water because not all animals are so con- 

 structed as to withstand such enormous pressures. The rapidity of 

 movement or the quietness of the waters also influences directly or indi- 

 rectly the ecologic relationships of animals living in them. Soil pres- 

 sures also vary according to the depth and physical construction of the 

 soil. The pressure in soils is in some instances so great as to prevent the 

 locomotion of certain types of animals through them. The porosity of 

 the soil, its oxygen and moisture contents, and its food content are addi- 

 tional factors which might influence the distribution of living animals 

 in it. 



