758 ANIMALS AND THEIR ENVIRONMENT 



a primary consumer in one chain, eating green plants, but a secondary 

 or tertiary consumer in other chains, eating herbivorous animals or 

 other carnivores. Man is the end of a number of food chains. For ex- 

 ample, man eats a fish such as a black bass, which ate smaller fish, which 

 in "turn ate small crustacea, which in turn ate algae. The ultimate size 

 of the human population, or of the population of any animal, is limited 

 (1) by the length of the food chain, (2) by the percentage efficiency of 

 energy transfer at each step in the chain, and (3) by the amount of light 

 energy falling on the earth. Since man can do nothing about increasing 

 the amount of incident sunlight, and very little about the percentage 

 efficiency of energy transfer, he can increase his supply of food energy 

 only by shortening his food chain, i.e., by eating the primary producers, 

 plants, rather than animals. In overcrowded countries such as India 

 and China, men are largely vegetarians because this food chain is 

 shortest and a given area of land can in this way support the greatest 

 number of people. Steak is a luxury ecologically as well as economically! 



In addition to predator food chains, such as the man-black bass- 

 minnow-crustacean one, there are parasite food chains and saprophyte 

 food chains. The ingestion of organic nutrients derived from decom- 

 posing animal or plant bodies or by-products directly through the body 

 wall, a mode of nutrition known as saprophytic or saprozoic, is not very 

 common in the animal kingdom, being restricted generally to certain 

 protozoa. Parasite food chains are common and may be quite complex. 

 For example, mammals and birds are parasitized by fleas; in the fleas 

 live protozoa which are in turn the hosts of bacteria. Since the bacteria 

 might be parasitized by viruses, there could be a five-step parasite food 

 chain. It is obvious that in general the organisms in a parasite food chain 

 are smaller than their hosts whereas the organisms in a predator 

 chain are larger than their prey. 



Since, in any food chain, there is a loss of energy at each step, it 

 follows that there is usually a smaller amount of protoplasm in each 

 successive step. H. T. Odum has calculated that 17,850 pounds of alfalfa 

 plants are required to provide the food for 2250 pounds of calves, which 

 provide enough food to keep one twelve-year-old boy alive for one year. 

 Although boys eat many things other than veal, and calves other things 

 besides alfalfa, these numbers illustrate the principle of a food chain. 

 A food chain may be visualized as a pyramid; each step in the pyramid 

 is much smaller than the one on which it feeds. Since the predators are 

 usually larger than the ones on which they prey, the pyramid of num- 

 bers of individuals in each step of the chain is even more striking than 

 the pyramid of the mass of protoplasm of the individuals in successive 

 steps: one boy requires 4.5 calves, which require 20,000,000 alfalfa 

 plants. 



344. Communities and Populations 



Each region of the earth— sea, lake, forest, prairie, tundra, desert- 

 is inhabited by a characteristic assemblage of animals and plants which 

 are interrelated in many and diverse ways as competitors, commensals. 



