70 ANIMAL ECOLOGY 



world, and to it we have applied the term " pyramid of num- 

 bers." It results, as we have seen, from the two facts (a) that 

 smaller animals are preyed upon usually by larger animals, 

 and (b) that small animals can increase faster than large ones, 

 and so are able to support the latter. 



The general existence of this pyramid in numbers hardly 

 requires proving, since it is a matter of common observation 

 in the field. Actual figures for the relative numbers of different 

 stages in a food-chain are very hard to obtain in the present 

 state of our knowledge. But three examples will help to 

 crystallise the idea of this " pyramid." Birge and Juday ^2 

 have calculated that the material which can be used as food 

 by the plankton rotifers and Crustacea of Lake Mendota in 

 North America weighs twelve to eighteen times as much as they 

 do. (The fish which eat the Crustacea would weigh still less.) 

 Again, Mawson^s estimated that one pair of skuas (Megalestris) 

 on Haswell I. in the Antarctic regions, required about fifty to 

 one hundred Adelie penguins to keep them supplied with 

 food (in the form of eggs and young of the penguins) ; while 

 Percival '^^^ states that one lion will kill some fifty zebras per 

 year, which gives us some idea of the large numbers of such a 

 slow-breeding animal as the zebra which are required to produce 

 this extra margin of numbers. 



