24 



The Mediwn 



in the amount of intermixture play a part in subdividing the terrestrial 

 environment into arid and humid climates and the aquatic environ- 

 ment into stagnated and aerated water. Transition areas of special 

 interest exist— such as swamps and the tidal zone— where sometimes 

 one medium and sometimes the other dominates the scene. Dams 

 built by beavers occasionally result in the flooding of large tracts of 



1 mm 



Pennak, 1939, AAAS Pub. No. 10 



Fig. 2.1. Enlarged diagram of sand in a beach habitat showing water-filled spaces 



between the grains. 1 = rotifers, 2 = gastrotrichs, 3 = tartigrade, 4 = nematode, 



5 = harpacticoid copepods. 



land, transforming them from terrestrial to aquatic habitats (Fig. 

 2.2); conversely, the growth o£ vegetation often tends to fill up a 

 shallow pond, gradually converting it into a swamp, and eventually 

 into dry land. Ordinarily, however, the medium is a highly inde- 

 pendent factor, for rarely do the activities of organisms cause a change 

 from one basic medium to the other. 



CONTRASTING QUALITIES OF AIR AND WATER 



The two fundamental media are very different in nature, and this 

 difference has important ecological consequences. Air is composed 



