PROBLEM 



1 What Makes Possible the Continued 

 Existence of Plants and Animals? 



Food chains. When you study the activ- 

 ities of organisms, you find that hving 

 things affect one another constantly. In 

 one way or another all kinds of organ- 

 isms become the food of other organisms, 

 either while they are alive or after their 

 death. Lions eat antelopes; antelopes eat 

 green plants. A hawk may eat a king- 

 bird which had eaten ladybird beetles 

 which had fed on scale insects which got 

 their food by sucking the cell contents 

 from an orange tree. These are examples 

 of what may be called food chains. 



Similar food chains exist in the sea. In 

 the surface waters of the ocean there are 

 microscopic green plants known as dia- 

 toms. Thev are present in countless num- 

 bers. Darting through the same waters 

 are water fleas and tiny Crustacea which 

 eat the diatoms. The small water fleas 

 serve as food for shrimp or small fish, 

 which in turn make good eating for the 

 hungry herring or the mackerel. These 

 are caught and eaten by men. 



The organisms at the beginning of the 

 food chain are usually smaller but much 

 more numerous than those at the end. 

 While ten shrimp may make a good meal 

 for a herring, it takes many more water 

 fleas to feed even one shrimp, and count- 

 less diatoms to feed one water flea. Fur- 

 thermore, the food chain, whether it has 



few or many links, has a green plant at 

 the beginning, for green plants are the 

 only organisms that manufacture food, 

 This must be true of every food chain. 

 Animals do not manufacture food; they 

 only consume it. Try now to do Exer- 

 cise I. 



Special relationships. In many food 

 chains an animal that is larger or fiercer 

 preys on a smaller animal, but sometimes 

 a smaller organism fastens itself to a 

 larger one and feeds on its living tissues. 

 You have read before that this relation- 

 ship is called parasitism. The parasite 

 feeds on the host. You know that man is 

 a host to many kinds of animal and 

 plant parasites, such as worms, protozoa, 

 the fungi which cause athlete's foot and 

 ringworm, and many species of bacteria. 

 All animals are hosts to a variety of para- 

 sites. Plants, too, are hosts to many kinds 

 of bacteria and molds or even to larger 

 plants. See Figure 324. Parasitism is a 

 very common relationship. Sometimes 

 the parasite lives within the host over a 

 long period of time, but sometimes it 

 kills its host quickly; by so doing the 

 parasite ends its own food supply and 

 dies of starvation unless it can move to 

 another host. To see some parasites that 

 may be found in various organs of ani- 

 mals, do Exercise 2. 



