28 The Medium 



pressure. In contrast, for every increase of 10 m (33 ft) in depth in 

 the water, pressure is increased by 760 mm Hg, or 1 atmosphere. The 

 tremendous pressures existing at the average depth of the ocean and 

 in the ocean deeps are indicated in Fig. 2.3. At a depth of only 900 

 m in the ocean Beebe's bathysphere was subjected to a total pressure 

 on its whole surface of more than 7000 tons. It is not surprising that 

 in his explorations of the earth man has not been able to descend into 

 the ocean to depths much greater than 4 km, whereas he has ascended 

 in the atmosphere to heights of more than 22 km. What are the 

 ecological effects of these differences in pressure and in the rates of 

 pressure change in the two media? 



Pressure Reduction with Altitude 



The reduction of pressure with altitude seems to be of little impor- 

 tance for plants, invertebrate animals, and the lower vertebrates. 

 Insects have been subjected experimentally to a reduction of pressure 

 from 760 mm Hg to 0.0001 mm Hg without harmful effect. Similarly, 

 frogs have withstood reduction of pressure down to 100 mm Hg. 

 Beetles reach the highest meadows in the Himalayas and earthworms 

 are found up to the snow line in the Andes. In most situations it is 

 not the reduced pressure which limits the altitude at which plants and 

 cold-blooded animals can exist. The distribution of these organisms 

 up the sides of mountains is ordinarily stopped by other adverse fac- 

 tors, such as low temperature, unsuitable soil, or lack of food, long 

 before the influence of the reduced pressure is felt. 



For warm-blooded vertebrates the reduction of pressure with alti- 

 tude becomes important primarily because of the lesser amount of 

 oxygen present. It is true that at very high altitudes the thinness 

 of the air renders flying more difficult, but the chief limitation imposed 

 on birds, and also on mammals, is the impairment of respiration. 

 Distinctly harmful effects are observed for man when the pressure of 

 the atmosphere has been reduced to about half that normal at sea 

 level. The highest permanent human settlement occurs in Tibet at 

 an altitude of about 5000 m. Even the best adapted of other mam- 

 mals are not found living permanently much higher than this altitude 

 (Hesse, Alice, and Schmidt, 1951, Ch. 24). Although vultures and 

 eagles have been reported at about a thousand meters higher, they 

 probably remain at such altitudes for only short periods of time. 

 Insects, which often abound near the rocky or snow-covered peaks of 

 mountains, are undoubtedly blown there by the wind, as they could 



