Trophic Levels and Relations 471 



to diatoms, often less than 30 ^ in diameter, and green flagellates or 

 photosynthetic bacteria of still smaller dimensions. 



The possible size of consumers is generally influenced by the size of 

 their food, but the possession of specialized food-getting equipment 

 enables some consumers to be very different in size from the organism 

 that provides their nutriment. Herbivores may be either larger or 

 smaller than the vegetation upon which they feed. Insects inhabiting 

 forests are characteristically only a minute fraction of the size of their 

 food plants. Some rodents are larger and some smaller than their 

 food; crustaceans and mollusks that filter phytoplankton from natural 

 waters are generally very much larger than the plants upon which 

 they feed. 



In the next step in the food chain, namely the subsistence of the 

 carnivores on the herbivores, the predatory species must be stronger 

 in some respect than its prey and is usually larger. Among animals, 

 therefore, a tendency exists for smaller species to form the early links 

 in the food chain and for larger species to form the later links. The 

 maximum size of a prey species that can be attacked successfully de- 

 pends upon the feeding apparatus of the predator. Thus some or- 

 ganisms in the community may be too large and some may be too 

 small for the consumer to catch or to obtain in sufficient quantity. 



Since total biomass tends to become smaller at successive levels in 

 the food chain, and since size of individual generally becomes larger, 

 at least among the animals, it follows that in the typical situation a 

 reduction in the number of individuals comprising successive links 

 in the food chain takes place. A relation of numbers is another con- 

 sequence of the operation of the ecological complex. Among the ani- 

 mals of a community the herbivores are typically the most numerous; 

 they take in food material synthesized by the plant producers and 

 pass it on to the subsequent consumers. Primary carnivores that prey 

 upon these "key industry animals" are less abundant, and secondary 

 and tertiary carnivores generally exist in still fewer numbers. This 

 numerical relationship with the more abundant species near the base 

 of the food chain and the less abundant species near the top is known 

 as the pyramid of numbers. In food chains involving parasites the 

 size relationships are reversed because the parasite is smaller than its 

 host, and hyperparasites must be still smaller. For this reason the 

 pyramid of numbers is reversed for the successive steps of parasite 

 dependency, and the parasites of each link are generally more numer- 

 ous than their hosts. 



An illustration of the pyramid of numbers among mammals and 

 birds inhabiting range land is presented in Table 20. The differences 



