THE ANIMAL COMMUNITY 69 



and Eliot Howard's work ^ has shown that similar territory 

 systems play a very important part in the lives of warblers. 

 We can approach the matter also from this point of view : 

 the smaller an animal the commoner it is on the whole. This 

 is familiar enough as a general fact. If you are studying the 

 fauna of an oak wood in summer, you will find vast numbers 

 of small herbivorous insects like aphids, a large number of 

 spiders and carnivorous ground beetles, a fair number of 

 small warblers, and only one or two hawks. Similarly in a 

 small pond, the numbers of protozoa may run into millions, 

 those of Daphnia and Cyclops into hundreds of thousands, 

 while there will be far fewer beetle larvae, and only a very few 

 small fish. To put the matter more definitely, the animals 

 at the base of a food-chain are relatively abundant, while those 

 at the end are relatively few in numbers, and there is a pro- 

 gressive decrease in between the two extremes. The reason 

 for this fact is simple enough. The small herbivorous animals 

 which form the key -industries in the community are able to 

 increase at a very high rate (chiefly by virtue of their small 

 size), and are therefore able to provide a large margin of 

 numbers over and above that which would be necessary to 

 maintain their population in the absence of enemies. This 

 margin supports a set of carnivores, which are larger in size 

 and fewer in numbers. These carnivores in turn can only 

 provide a still smaller margin, owing to their large size which 

 makes them increase more slowly, and to their smaller numbers. 

 Finally, a point is reached at which we find a carnivore {e.g. 

 the lynx or the peregrine falcon) whose numbers are so small 

 that it cannot support any further stage in the food-chain. 

 There is obviously a lower limit in the density of numbers of 

 its food at which it ceases to be worth while for a carnivore to 

 eat that food, owing to the labour and time that is involved 

 in the process. It is because of tliese number relations that 

 carnivores tend to be much more wide-ranging and less 

 strictly confined to one habitat than herbivores. 



22. This arrangement of numbers in the community, the 

 relative decrease in numbers at each stage in a food-chain, is 

 characteristically found in animal communities all over the 



Lu , L I 8 R A R Y % 



