ECOLOGY 'JQ'J 



explanation for this proved to be that the sHme secreted by the group 

 of fish was enough to precipitate much of the colloidal silver and ren- 

 der it nontoxic, whereas the amount secreted by a single fish was not. 



Such animal aggregations do have survival value for the species. 

 Allee has called this "unconscious cooperation." When genes governing 

 a tendency toward aggregation arise in a species and prove to have 

 survival value, natural selection will tend to preserve this inherited 

 behavior pattern. The occurrence of many fish in schools, of birds in 

 flocks, and so on, are examples of this "unconscious cooperation" which 

 occurs very widely in the animal kingdom. 



From such simple animal aggregations there may evolve complex 

 animal societies, composed of specialized types of individuals, such as 

 the colonies of bees, ants and termites (section 155). Man is another 

 example of a social animal. 



343. Food Chains 



The ultimate source of all the energy used by living things is sun- 

 light, the energy of which is converted to a biologically useful form by 

 the process of photosynthesis carried on by green plants. Only a small 

 fraction, about 3 per cent, of the light energy striking the leaves of a 

 green plant is transformed by photosynthesis into the potential energy 

 of a food substance; the rest escapes as heat. This loss is not the result 

 of inefficiency of the biochemical processes involved, but of the opera- 

 tion of the laws of thermodynamics. The Second Law of Thermody- 

 namics may be stated as "whenever energy is transformed from one 

 form into another there is a decrease in the amount of useful energy; 

 some energy is degraded into heat and dissipated." In other words, no 

 transformation of energy can be 100 per cent efficient. 



When an animal eats a plant, much of the energy is again dissi- 

 pated as heat and only a fraction is used to synthesize the animal's 

 protoplasm. When a second animal eats the first, there is a further loss 

 of energy as heat, and so on. The transfer of food energy from its ulti- 

 mate source in plants through a series of organisms, each of which eats 

 the preceding and is eaten by the following, is known as a food chain. 

 The number of steps in a food chain is limited to perhaps four or five 

 because of the great decrease in available energy at each step. The per- 

 centage of the food energy consumed that is converted to new proto- 

 plasm, and thus is available as food energy for the next organism m 

 the food chain, is known as the efficiency of energy transfer. 



The first step in any food chain, the capture of light energy by 

 photosynthesis and the production of energy-containing foods by plants, 

 is relatively inefficient; only about 0.2 per cent of the mcident light 

 energy is stored as food. The efficiency of energy transfer when one 

 animal eats a plant or another animal is higher, ranging from 5 to 20 

 per cent. Some animals eat but one kind of food and therefore are 

 members of a single food chain. Other animals eat many different kinds 

 of food and are not only members of different food chains, but may 

 occupy 'different positions in different food chains. An animal may be 



