136 



ANALYSIS OF THE ENVIRONMENT 



direct opposition, Uvarov (1931) cites evi- 

 dence from Piotet that for the cabbage but- 

 terfly, Pieris rapae, "in nature the emer- 

 gence of adults in the majority of cases does 

 not take place except on the fall of the 

 barometer, a reduction of one milUmeter 

 being sufficient to cause the emergence of 

 all adults which are ready for it." If a pupa 

 is about to transform when the pressure 

 rises, emergence is said to be retarded until 

 there is a new fall. The completion of pupa- 

 tion with a falling barometer, Pictet 

 thought, may be a result of the greater ease 

 with which the pupal cases may be broken 

 with lowered external pressure. In view of 

 all the other known variables and granting 

 that the facts may be as stated, the ex- 

 planation is too simple. Recent evidence in- 

 dicates that hormones influence such phe- 

 nomena (Wigglesworth, 1939; Scharrer, 

 1948). 



There are many reports of a correlation 

 between animal activity and change in 

 barometric pressure. Again decreased activ- 

 ity has been recorded both for increased 

 and for decreased pressures, particularly for 

 the latter. While recognizing the possibil- 

 ities, we conclude in general, as does 

 Uvarov (1931) for insects, that the influ- 

 ence of normal variations of atmospheric 

 pressure acting alone on the activities of 

 animals has not as yet been critically 

 studied. The relation to humidity is a par- 

 ticularly important matter which has not 

 been properly separated from pressure 

 changes. 



Much greater pressure changes occur 

 with altitude; the limits extend from about 

 800 mm. of mercury for land valleys be- 

 low sea level to about 300 mm. in the high- 

 est mountains. It is extremely difficult to 

 separate effects properly attributed to re- 

 duced pressures in nature from those pro- 

 duced by other environmental factors at 

 higher altitudes. As altitude increases and 

 atmospheric pressure decreases, the partial 

 pressures of atmospheric gases also de- 

 crease. The fall in partial pressure of oxy- 

 gen and carbon dioxide is particularly im- 

 portant; in fact, for man, the decrease in 

 the supply of atmospheric oxygen becomes 

 the most important factor in the study of 

 the effects of high altitudes. There is also a 

 lowering of temperature with altitude, and 

 increases occur in the rate of evaporation, in 

 light intensity, and in products of gaseous 

 ionization. In the biotic environment, food 



supply is greatly reduced. These associated 

 factors are sufficiently important to make 

 it doubtful whether most of the ecological 

 effects found in connection with low at- 

 mospheric pressures in high altitudes are to 

 be attiibuted to low pressures as such. Birds 

 are handicapped in their flight by thinness 

 of the air as well as by the reduced supply 

 of oxygen. Soaring flight is possible for cer- 

 tain birds in high altitudes, but even these 

 have difficulty in taking off. Birds have 

 been recorded as high as 27,000 feet. 

 Kingston (1925, p. 194) saw a chough, a 

 crowlike bird, at that altitude on Mt. Eve- 

 rest, It could take off down hill, but did not 

 fly far. It is worth noting that the Andean 

 condor nests at altitudes up to 16,000 feet. 



INCREASED (Hydrostatic) pressure 



Unlike mountain climbing, or even aero- 

 plane ascents, pressures change rapidly and 

 dramatically with increasing depth of water; 

 an added depth of 10 meters ( 10.07 meters 

 at average density) adds another atmos- 

 phere of pressure. The ocean bottom has an 

 average depth of about 3800 meters, and 

 hence the pressure there is about 380 times 

 greater than that at the surface; in the real 

 "deeps," pressures approximate 1000 at- 

 mospheres. Such pressures acting alone pro- 

 duce important changes on the environ- 

 ment and on the animals living therein. 

 Physical oceanographers use the "bar" as 

 their unit for hydrostatic pressure; this cor- 

 responds to one million dynes/cm.* Their 

 working unit is the decibar (0.1 bar), 

 which approximates the increase in pressure 

 with each meter's added depth. 



EFFECTS OF PRESSURE ON THE 

 ENVIRONMENT 



Compressibility of Water 



Within the ecological range, water is only 

 slightly compressible. Johnstone (1923) 

 writes that if water were wholly incompres- 

 sible, the volume of the sea would be in- 

 creased some 11 millions of cubic kilometers 

 and its level would be raised almost 30 

 meters. Such a rise of mean sea level of 15 

 fathoms would alter the outlines of the 

 land surface to a noticeable degree. Despite 

 these superficially impressive totals, the 

 buoyancy of water is little changed with 

 depth, and an incompressible body which 

 falls readily through the upper levels vidll 

 continue to fall to the bottom; an easily 



