398 



ECOSYSTEM ECOLOGY: 



amount of growth as a result of consumption of the 

 materials in the next lower layer. Each level of the 

 pyramid shows neither numbers nor weight of the life 

 represented, but rather depicts growth, or produc- 

 tion, as a consequence of consuming the next lower 

 level's production. A biomass pyramid has the same 

 components with the same relative positions and im- 

 plications as the production pyramid, but the graph 

 indicates the weight of all living creatures in each 

 step of the food pyramid. A pyramid of numbers also 

 has the same food-chain basis, but displays the num- 

 ber or organisms in each link of the chain. Hence, the 

 food-pyramid concept treats in different ways — either 

 in terms of materials formed, weight of creatures, or 

 number of individuals — the fact that it takes more 

 life in one link of a food chain to support the next 

 link. 



FOOD 



Various factors influence the proportion of the total 

 food in an ecosystem that is usable or available to the 

 life of the area (Figure 20.3). Of most importance in 

 relation to this proportion is the efficiency with 

 which different organisms obtain energy from food. 

 This efficiency in the energy of a given ecosystem is 

 directly related to the quantity, kind, preference, 

 and productivity of the food. 



QUANTITY AND KINDS 



A certain amount of food in the form of plants, ani- 

 mals, and water is absolutely necessary for an energy 

 cycle — hence, an ecosystem — to operate. Moreover, 

 this total biomass must exist in definite proportions 

 of unlike kinds of food. Without such a balance in the 

 form of a functioning food pyramid, some organisms 

 will starve and dramatic changes will occur in the 



preference, 



life needs 



seasonal 

 or annual 



suitability 



nutritional 

 value 



availabi lity 



stored food 



Figure 20.3 Factors determining the food available to an organism. 



composition of some of the populations within the 

 ecosystem. Such imbalance probably is the basic 

 cause of ecosystem collapse and of community 

 extinction. 



Certain factors, especially with reference to ani- 

 mals, affect the availability and balance of food. 

 First, the already-mentioned possibilities of sea- 

 sonal and annual variations in population numbers 

 naturally cause fluctuation in the composition of an 

 ecosystem. It is not likely that any species of animal 

 can eat the same food throughout its lifetime. Rather, 

 if animals are to survive, seasonal and often annual 

 variations in food consumption must be common. 



Second, the nutritional value of certain foods, es- 

 pecially plants, is known to vary. This may mean 

 that individuals feeding upon less nutritious food 

 must either eat more, assume a functionally inactive 

 state (dormancy), or change their food habits. 



Third, the storage of food by many animals makes 

 food available when otherwise there would be serious 

 famine. 



Fourth, only certain kinds of food are suitable for 

 each species. For example, carnivores have both a 

 lower and upper size limit upon their prey. This is 

 not entirely a disadvantage, because it means that 

 the young usually do not compete with the adults of 

 their species for the same kind of food. 



Fifth, in spite of limitations most animals can use 

 a variety of foods. 



Finally, many animals can survive for brief peri- 

 ods on emergency foods that are not sufficiently nu- 

 tritious to maintain proper health, but that do pre- 

 vent starvation. 



PREFERENCE 



The food eaten by an animal regularly depends 

 upon preference and availability. Definite preference 

 for or prejudice against certain foods is implied by 

 many situations where a nutritious but minor food 

 item is easier to obtain and more abundant than the 

 mainstay of an animal's diet. In such circumstances 

 it seems possible that the basis for preference and 

 prejudice is psychological. At least there is no reason 

 to believe that many cases of choice of food represent 

 anything other than an expression of "likes" or "dis- 

 likes." Insofar as preference is concerned, foods are 

 highly preferred, moderately eaten, occasionally 

 eaten, or refused; however, the easiest available 

 foods still tend to be most used. 



Closely allied with preference and availability in 



