THE ENVIRONMENT 



311 



Other areas of lower concentration until an over-all 

 uniform concentration of every substance exists 

 throughout the mixture. Osmosis might best be under- 

 stood if one imagines sugar, a solute, dissolved in 

 water a solvent. If such a mixture is placed in a 

 particular kind of membranous bag and the bag in 

 turn is placed in pure water, the process of dif- 

 fusion will take place. The pure water (the area of 

 highest concentration of water) will move into the 

 sugar solution (the area of lowest concentration of 

 water) through the membranous bag.' Notice that 

 the bag prevents sugar diffusion into pure water; only 

 the solvent does the moving. Osmosis, then, is 

 diffusion of a solvent through a differentially 

 permeable membrane from a region of higher con- 

 centration of the solvent to a region of lower con- 

 centration of the solvent. 



Now consider an animal in fresh water. You 

 should know that an animal's body fluids are salty 

 and that most animals have only about 70 per cent of 

 their weight in water. In this situation water is the 

 solvent and salt the solute. With this in mind, it is 

 obvious that the water is of lower concentration in the 

 animal's body than in fresh water. Therefore, fresh- 

 water animals should swell and finally burst from the 

 continuous intake of water. Fresh-water fishes are 

 good examples of why no such explosion occurs. 

 Nothing prevents the water intake, but the excess 

 water is removed by a continuous secretion of urine. 

 However, this creates another problem. Most animal 

 urine contains salts, so if fishes were to continuously 

 lose salts in large quantities, their life processes 

 would not continue properly and death soon would 

 occur. Therefore, fish adaptations that absorb salt 

 from the urine are essential and do exist. In ad- 

 dition, most fresh-water fishes have specialized gills 

 that extract salts from the surrounding water, even 

 though these salts are in weak concentrations. 



Marine fishes provide good examples of water re- 

 lationships in the ocean. Their bodies contain a 

 higher concentration of water than does the sea, so 

 marine fishes should lose their body waters to the 

 lesser salt concentration in their surroundings. 

 Fishes do lose water and for this very reason! How- 

 ever, adaptation again makes the difference. These 

 animals drink large quantities of salt water, much of 

 the salt being excreted by the gills, and they form 

 very little urine. 



Both marine and fresh-water areas provide a 

 myriad of conditions for life. A brief introduction to 



intertidal and fresh-water habitats will illustrate 

 some of these conditions. In addition to examination 

 of ecological factors, some mention will be made of 

 general trends of biotic succession, the sequence of 

 life from its first invasion of an environment through 

 subsequent changes and to final conditions of sta- 

 bility {climax). 



INTERTIDAL ENVIRONMENT 



The ocean is a much less severe habitat than the 

 land. This can be appreciated if one compares 

 marine and terrestrial environments in relation to 

 usual animal responses. The environmental differen- 

 ces may best be illustrated through comparisons of 

 ectotherms, especially those whose body tempera- 

 tures closely approximate that of their environment. 

 The marine animals reflect ocean conditions by hav- 

 ing a more constant and generally lower body tem- 

 perature. Because of their low temperature, these 

 marine creatures have a slower living rate and are 

 less active; and because of relatively constant ocean 

 temperatures, they have no need for special mech- 

 anisms to escape heat extremes. In addition, marine 

 life leads a less rigorous existence than terrestrial 

 life because water and food are more plentiful. Even 

 reproduction requires less energy in the ocean. For 

 example, most eggs neither need cases (drying is im- 

 possible) nor yolk (microscopic food is available to 

 the larvae). Also, water buoyancy enables slower 

 movement with less energy than is needed for land 

 movement; however, fast movement requires more 

 energy in water than on land. Finally, even pressure 

 is of little concern. Slow movements from the depths 

 to the surface are possible. 



The large brown algae called kelp are examples of 

 the relative ease of marine existence. Such large algae 

 presumably do not grow on land partly because they 

 lack the necessary supportive tissues for erect growth; 

 any large land plant lacking sufficient supportive 

 tissues would be reduced to an entangled mass that 

 would have difficulty accomplishing its life processes. 

 Also, kelp require no specialized structures such as 

 roots, stems, and leaves. Although kelp have special- 

 ized anchoring devices called holdfasts, the entire algal 

 structure tends to carry on the various life processes. 

 Owing to this lack of special areas such as those for 

 food making, large marine algae need not have veins 

 to transport materials from one part of the plant to 

 another. In short, one can say that kelp, like marine 



