THE ANIMAL AND ITS ENVIRONMENT 



95 



Nutrition: food chains 



All animals with the exception of a few 

 Protozoa depend ultimately on plants for 

 their food. This is illustrated in Fig. 5-6, 

 where we see that the plant manufactures 

 fats, carbohydrates, and proteins, and the 

 animal breaks these down for its own use. 

 The plants, therefore, are continually build- 

 ing up the organic world while animals are 

 constantly tearing it down. It is apparently 

 a well-established relationship, and one 

 that we hope will endure for a long time 

 to come. 



Energy passes from the plant where it 

 has been stored from the sun to the animal 

 that eats the plant. However, it does not 

 always expend itself completely in a pas- 

 sage that involves only two organisms. 

 Often there are many intermediates which 

 transfer the energy through a food chain, 

 in which one animal after another is eaten 

 until the energy can be released only by 

 the death of the last animal in the chain. All 

 of the food chains in a given community 

 constitute a food cycle. Let us consider 

 several food chains. 



In an abundantly populated fresh-water 

 pond, plants and animals are constantly dy- 

 ing, falling to the bottom, and decompos- 

 ing. This disintegrating organic material 

 forms a source of energy for the growth 

 of bacteria. In addition many algae, simple 

 plants, grow by the utilization of simpler 

 substances, just as all plants do. These two 

 then, bacteria and unicellular plants, form 

 the basis of food for tiny organisms such 

 as Protozoa. Small Protozoa are eaten by 

 larger ones, these in turn are eaten by ro- 

 tifers, then Crustacea, aquatic insects, and 

 finally by fishes — first smaller fish, then 

 larger ones. The latter either die or are 

 eaten by fish-eating mammals such as mink, 

 bear, or man. In the first case the chain 

 ends with the death of the fish, in the 

 second by the death of the mammal. A 

 somewhat shorter pond cycle would be one 

 starting with a snail eating a leafy plant, 



such as is depicted in Fig. 5-7. The snail 

 is then eaten by a crustacean, the crusta- 

 cean by a small fish, the small fish by a 

 larger one. Here the food chain ends un- 

 less, as before, the fish is eaten by bird or 

 mammal. 



On land a food chain may follow a simi- 

 lar pattern (Fig. 5-7). In this case the zebra 

 feeds on plants, and is then eaten by a 

 leopard. This may end the chain, providing 

 the leopard dies a natural death, which is 

 highly unlikely. As it grows older and loses 

 some of its faculties, sooner or later it falls 

 prey to another carnivore. This transfer of 

 energy may go on almost endlessly. 



ANIMAL RELATIONSHIPS 



Some very interesting interrelationships 

 between animals have been established, 

 primarily on the basis of obtaining food, 

 although some seem to have other purposes. 

 Collectively these relationships are spoken 

 of as symbiosis. They range all the way 

 from a loose, more or less haphazard, as- 

 sociation to a closely knit relationship in 

 which the two or more animals are forced 

 to live together. These interrelationships 

 have fascinated bioloo;ists and should also 

 be of particular interest to beginning zool- 

 ogy students. 



Between individual animals 



Commensalism. This is a loose associa- 

 tion of two animals in which one derives 

 benefit from the combination while the 

 other may or may not. There are many de- 

 grees of such associations. For example, a 

 flatworm, Bdelhira, can usually be found on 

 the body of the king crab ( Fig. 5-8 ) . From 

 this association the flatworm is able to pick 

 up bits of food which are dispersed into the 

 sea water as the crab tears up its prey. 

 There seems to be no benefit to the crab 

 from the association. 



Another interesting association is that of 

 certain jellyfishes {Phtjsalia, Fig. 5-8) and 

 several species of small fish. The fish live 



