COMMUNITY ORGANIZATION: METABOLISM 



525 



ot their food. These species are exceptions 

 in so far as noraial size relations between 

 predator and prey are concerned. The ex- 

 ceptional relations are made possible since 

 both are specialized plankton-feeders (Fig. 

 175) with structural adaptations for con- 

 centrating their food. 



Large ungulate grazers (bison, cattle) 

 and browsers (giraffe), and omnivores 

 such as man," have bodies that are dispro- 

 portionately large compared with the size 

 of much of their food. As noted previously, 

 the pyramid of numbers is especially appli- 

 cable to predators (secondary consumers) 

 and to the myriads of minute plants and 

 animals at the base of the pyramid (many 

 primary consumers and all tertiary and 

 quaternary consumers). 



Consequently size of food, amount of 

 food, availability of food, structural and 

 functional feeding adjustments (pp. 239- 

 262), territory, methods of locomotion, 

 breeding requirements, and shifts in popu- 

 lations (Chap. 28) between strata and be- 

 tween different communities are involved in 

 the pyramid of numbers, either directly 

 with respect to size-number ratio or indi- 

 rectly. 



Large numbers of organisms, especially 

 in the lower levels of the pyramid, may 

 not contribute directly to the pyramid suc- 

 cession. Many die without having been 

 consumed, and serve as food for organisms 

 in still lower levels. 



The broad outlines of the pyramid of 

 numbers are fairly obvious, but many as- 

 pects require critical future investigation. 



Within the normal range of foods at a 

 given level of the pyramid, the size of the 



• Among omnivores, man is a conspicuous 

 example, and his feeding habits may be condi- 

 tioned by a variety of operating influences. For 

 example, the Solomon Islanders of Malaita, an 

 island on which the natives of the interior are 

 still entirely free from government control, were 

 formerly and are still to some extent sharply 

 divided into the yam-growing tribes of the in- 

 terior and the sea-going and fishing tribes of the 

 coast. Their separation was so much ac- 

 centuated by head-hunting raids that the 

 coastal people lived on fortified offshore islets. 

 This isolating custom was broken by a regular 

 periodic truce of a day, when the two groups 

 met on the coast to exchange their respective 

 special food-stuffs. These data rest on the 

 report to the Crane Pacific Ex-pedition from 

 the resident government officials in 1929. fSee 

 Shurcliff, 1930, Jungle Islands, p. 177.) 



specific foods, as well as their population 

 density, affects the consumer. Loosanoff 

 and Engle, 1947, have shown that 

 in experimental feeding of the oyster 

 (Ostrea virginica) there are rather definite 

 concentrations of food above which the 

 density of the micro-organisms begins to 

 interfere with the oyster's feeding. These 

 authors found that the critical concentra- 

 tions that allowed relatively undisturbed 

 feeding corresponded to 2,000,000 Chlo- 

 rella sp., 70,000 Nitzschia closterium, and 

 3000 Euglena viridis per cubic centimeter 

 of water. Hence size of food is associated 

 with density of food in this and similar 

 cases. That is, many more minute organ- 

 isms, such as Chloreila, were needed to pro- 

 duce the same effect as that caused by 

 Euglena. 



The basic energy relations of the pyra- 

 mid of numbers have been described by 

 Lindeman (1942) in terms of productivity: 



Xo > Xi > X.. 



Xn 



There is much to be desired from an 

 over-all study of the pyramid of numbers of 

 a community. So far no community has 

 been analyzed completely v^dth reference to 

 the body sizes and numbers of individuals 

 for each mesh composing the food web. In 

 view of taxonomic difficulties, the labor in- 

 volved in counting populations, and the 

 lack of information on parasites and the 

 minute organisms which live in each stra- 

 tum, no complete pyramid is likely to be- 

 come available in the near future. 



An average community population is 

 generally in a state of flux, involving sea- 

 sonal, twenty-four hours, and other popu- 

 lation cycles (p. 366; Chap. 28), shifts 

 across its boundaries in intercommunity 

 migrations, emigrations, and accidental 

 straying. Such a population is affected by, 

 and affects, the inanimate physical and ani- 

 mate biological portions of its area. 



Another quantitative approach to an un- 

 derstanding of the food web is the concept 

 of biomass, or weight of a species popula- 

 tion per unit area. Walter Pickles (1937) 

 effectively used this term in a study of the 

 ant Acanthomtjops favtis, in which it was 

 found that this species had a weight of 

 0.008 gm. /square meter of territory, and 

 of 7.037 gm./nest, over a census area of 

 880.51 square meters. 



