COMMUNITY ORGANIZATION: METABOLISM 



523 



on the vertical coordinate, and numbers of 

 organisms on the horizontal coordinate, a 

 triangular figure, now commonly known as 

 the pyramid of numbers, is the result. In 

 Figures 172 and 173 typical pyramids of 

 the macroscopic invertebrates of the forest 

 floor Utter, in two widely separated com- 

 munities, are examples of this concept. 



The Eltonian pyramid serves to clarify 

 and coordinate a number of aspects of com- 

 munity food relationships. It represents the 



able, and (2) that the predator had the 

 necessary feeding adjustments to collect 

 and eat the food while staying within the 

 margin of expendable energy. For example, 

 from a biochemical point of view, a musk- 

 ellunge might be able to thrive on ameba 

 protoplasm; from a physiological point of 

 view, the expenditure of energy required 

 to obtain this protoplasm would be fantastic 

 contrasted to the energy yield of the food; 

 from the ecological viewpoint, this fish has 



NUMBER Of ANIMALS 



V 500 10 00 



Fig. 173. Pyramid of numbers of the metazoan fauna of the forest iloor stratum of the 

 tropical forest on Barro Colorado Island, Panama Canal Zone. Based on quadrats 25 cm. 

 square. (From Williams.) 



food web in a quantitative sense, and its 

 geometric configuration serves to accen- 

 tuate the size groups in the community. 

 These size groups may or may not be simi- 

 lar to the food niches and to the trophic 

 levels present, depending upon the type 

 of investigation undertaken, but such 

 groupings do give another aspect to fre- 

 quency. 



Such pyramids are the natural result of 

 two tendencies, (1) that smaller animals 

 have a higher reproductive potential than 

 larger animals as a rule, and (2) that 

 smaller animals usually are the prey of lar- 

 ger animals. 



The pyramid of numbers is especially 

 applicable to predators. 



Organisms expend energy to obtain 

 nourishment. As between animals, any 

 predator could probably be nourished by a 

 variety of species-protoplasms if the latter 

 could be obtained in sufiicient amounts. 

 This would imply (1) that food was avail- 



no special adjustment to enable it to feed 

 upon ameba protoplasm in sufficient 

 amounts, and this large, fresh-water preda- 

 tor must obtain its food in larger pieces to 

 conserve energy and grow. The muskel 

 lunge must feed upon the next size group 

 or so in the food web, and so obtain its 

 protozoan protoplasm at third or fourth 

 hand or more. 



Where a predator has special adjust- 

 ments for obtaining certain foods, much 

 more specific feeding relationships are pos- 

 sible, and several size groups in the pyra- 

 mid may be eliminated. For instance, 

 the aye-aye (Chiromys madagascariensis) 

 hunts for beetle grubs that it finds beneath 

 the bark of trees in the forests of Mada- 

 gascar. After gnawing away the bark, it ob- 

 tains the larvae by inserting its attenuated 

 middle finger into their burrows (Fig. 

 174). Here the size difi^erential between 

 predator and prey is less exceptional than 

 that of the anteaters. These mammals open 



