Terrestrial Environment 253 



abundant. The type of bottom fauna in certain lakes has also been 

 found to be related to the redox potential. In lakes in which the 

 water above the deepest mud had an Eh above 0.4 volt, the mud 

 itself supported a pure or mixed fauna characterized by midge larvae 

 of the genus Tanytarsus, whereas in lakes with an Eh below 0.3 volt 

 a true Chironomtis fauna occurred (Hutchinson et al., 1939). For a 

 further discussion of the ecological effects of redox potentials the 

 reader is referred to ZoBell ( 1946 ) ; it is obvious that the influence of 

 this aspect of the oxygen factor is due for closer scrutiny in the future. 



EflFects of Oxygen Availability 



Terrestrial Environment. We may now examine the ecological 

 effects of the varying amounts of oxygen present in different habitats. 

 Since oxygen is amply abundant in the lower portions of the earth's 

 atmosphere, this factor has no important limiting action in most ter- 

 restrial situations. The reduction in the partial pressure of oxygen 

 at high elevations imposes a restriction on the altitudinal distribution 

 of organisms having a high oxygen requirement. No mammals can 

 live permanently at altitudes at which the partial pressure of oxygen 

 is less than about 45 per cent of its value at sea level. Oxygen short- 

 age as well as the thinness of air imposes a similar ultimate limit on 

 the altitude at which birds can live. The altitudinal range for most 

 types of lower animals and for plants is limited by low temperatures 

 or by other ecological factors long before the diminished oxygen con- 

 centration becomes a serious handicap. 



Organisms requiring free oxygen are excluded from soils and 

 ground debris within which the circulation of air is inadequate. 

 The lack of oxygen in poorly aerated soils becomes seriously harmful 

 to the roots of most plants at concentrations below about 10 per cent. 

 Root penetration for most of the vegetation is stopped by soil layers 

 containing less than 3 per cent oxygen and also by the water table 

 since even less oxygen is available in the ground water. If areas with 

 established vegetation are subjected to prolonged flooding, caused for 

 example by the construction of beaver dams, most of the plants will 

 be "drowned"— or killed by the cutting off of root aeration. 



The roots of some plants, however, possess special adaptations en- 

 abling them to grow in soil layers devoid of oxygen. In some types, 

 including particularly the herbaceous hydrophytes, air passages within 

 the stem and major roots allow oxygen to be brought from the 

 atmosphere, or possibly from internal sources in the upper part of the 

 plant, and delivered to the lower extensions of the root system. In 



