GRAVITY, PRESSURE, AND SOUND 



131 



Submarine or coastal disturbances set up 

 so-called tidal waves that are not really 

 tidal at all (Macdonald, Shephard, and 

 Cox, 1947). These range up to some 90 

 feet in height and may travel even the 

 10,000 miles from South America to Japan. 

 Such sea waves frequently cause changes 

 along the shore Hne with accompanying 

 destruction of the existing biota. On land, 

 earthquakes may produce avalanches of 

 snow or earth in mountains that may sweep 

 away whole forests and may alter river 

 courses or dam them to form lakes. Level 

 land has been thrown into mild undula- 

 tions, and the underground water level may 

 be disturbed. These and other community 

 effects will be mentioned in a later section 

 (p. 578). 



Finer Stratification of the Environment 



Gravity continues to produce stratification 

 in the environment. In regions where car- 

 bon dioxide escapes from the earth, its 

 greater density causes it to displace the 

 lower air, as in the death valley on the 

 Dieng plateau in Java (Hesse, Allee, and 

 Schmidt, 1937). Animal life is impossible 

 in such areas. Not all the stratification in 

 the atmosphere can be accounted for by 

 gravity. Thus ozone is heavier than either 

 oxygen or nitrogen, and yet, perhaps be- 

 cause it originates as an ionization product 

 in the increased light intensity and de- 

 creased pressure of high altitudes, it is 

 found mainly in the stratosphere. There 

 is little ozone below an altitude of 52,000 

 feet, and 75 per cent of the total ozone is 

 above 72,000 feet (Ellis, and Wells, 1941). 



Stratification also occurs in aquatic en- 

 vironments. As was shown earher (p. 93) 

 in discussing theiTnal stratification, warmer, 

 Ughter water floats on colder strata, often 

 with a fairly sharp boundary between the 

 two. Fresh water is hghter than salt water. 

 The fresh water released by melting glaciers 

 and ice floes in summer overlies the denser 

 cold water of the colder oceans, where sur- 

 face layers may have only half the salinity 

 of the deeper waters. Similar conditions 

 prevail in regions of brackish waters. In the 

 Baltic Sea, the surface waters tend to be 

 less salt than is water from the depths. In 

 the western Baltic, the salinity at the sur- 

 face may be 8 to 12 per mille where the 

 underlying water is 27 "/^^ , as opposed to 

 about 35 °/pp for the water of the open 

 ocean. The surface waters are diluted for 



miles oflF shore near the mouths of large 

 rivers. Depth-tolerant marine communities 

 can exist in deeper, saltier waters even 

 though overlaid by less dense brackish 

 water, which they cannot tolerate, pro- 

 vided that the stratification on the basis of 

 density does not restrict the oxygen supply 

 too greatly. 



Mechanical sorting of solid particles is of 

 common occurrence. The heavier materials 

 settle out of a watery or aerial suspension 

 most rapidly and hence are found nearer 

 their source, while the Hght dust or detritus 

 may be carried much farther. Thus, coarser 

 gravels are deposited near the mouth of a 

 river, while the finer mud settles slowly 

 far out in the lake or ocean. The annual for- 

 mation of varves on the bottoms of the lakes 

 in the temperate zone also results from 

 selective settfing. Coarser materials that ac- 

 cumulate on the ice or are brought in by 

 spring freshets sink rapidly and become 

 overlaid by finer stuff that settles more 

 slowly; the finest particles are finally drawn 

 down in the quiet water under ice in winter 

 (see p. 82). Stratified rocks are a more 

 permanent expression of these same tend- 

 encies. The bottom ooze of the oceans at a 

 distance from coast fines is characteristi- 

 caUy fine. 



The pull of gravity provides food for 

 the animals in the deeper waters of the 

 ocean where food supphes come only from 

 the drifting down of whole or disintegrating 

 bodies, or excreta from above. Similarly, 

 gravity produces the autumnal fall of forest 

 leaves as well as occasional crashing of the 

 trees themselves. The pull of gravity brings 

 rain down to the earth and causes water to 

 run toward lower levels. All the work of 

 running water, which produces much of the 

 dissection of physiographically young land- 

 scapes and causes the peneplain formation 

 characteristic of older ones, results from 

 the force of gravity. Few environmental 

 forces are more important, more difficult 

 to control experimentally, or more neglected 

 in modern ecological study. 



DIRECT EFFECT OF GRAVITY ON ANIMALS 



Animal Structure 



An animal's bulk cannot exceed certain 

 structural and functional limits without en- 

 dangering its fife. The body weight must 

 be adequately supported. Principles of 

 physics indicate that there are natural fim- 



