PROBLEM A What Are Our Relationships to Other 



Organisms? 



The web of life. We hav^e seen that hv- 

 ing things are interdependent. Animals 

 and nongreen plants could not live with- 

 out the food made by green plants. 

 Green plants continue to exist because of 

 the organisms which bring about decay. 

 So complicated are these interrelation- 

 ships that one biologist has called the 

 tangle the "web of life." And this web of 

 life becomes even more complicated be- 

 cause of the relationships of organisms 

 to all parts of their physical environment. 

 This relationship of organisms to all parts 

 of their environment, including other 

 organisms, is a special field of biology 

 called ecology (ee-kol'o-gee). 



Relationships in small areas change 

 constantly. If you look at any fairly 

 small area of the earth, you soon dis- 

 cover that the numbers of individuals of 

 a particular species constantly change. 

 Sometimes the change is like a pendulum 

 swinging in one direction and back again. 

 In some years the northern woods are 

 crowded with rabbits. As a result the 

 lynx, which feeds on rabbits, begins to 

 prosper. More of its offspring find food 

 and survive, so that soon large numbers 

 of lynxes feel the pinch of hunger. Some 

 die of starvation and those that survive 

 reproduce less plentifully. This gives 

 rabbits an opportunity to increase again. 

 The pendulum swings back and the 



cycle, in time, is repeated. In realit)% in 

 the tangled web of life, each swing of 

 the rabbit-lynx pendulum affects many 

 other organisms; these changes have not 

 been described. 



Changes of a different nature may also 

 occur. About sixty years ago a farmer 

 moved away from his farm in southern 

 Ohio. Because the hilly farm was very 

 poor, no one could be found even to try 

 to raise crops on it and it was abandoned. 

 Today, on one hillside field which was 

 once a farm there is a grove of pine trees. 

 First, there were weeds and grasses; then 

 scattered shrubs and brambles which 

 grew from seeds blown by the wind or 

 carried by birds. Pine seeds, also, were 

 blown in from a nearby grove. These 

 seedlings grew more slowly but eventu- 

 ally grew taller than the other plants in 

 the field. Some pines grew taller than 

 others and, eventually, those in the shade, 

 along with the shrubs and brambles, 

 were killed by starvation because there 

 was too little light for photosynthesis. 

 The kinds of birds, small animals, and 

 insects changed with the changing plants. 



One could give many examples of 

 changes in animal and plant populations. 

 You may have noticed some examples 

 yourselves. Ecologists through careful re- 

 cording of facts can now predict ap- 

 proximately when some species will 



