THE ANIMAL COMMUNITY 



55 



years the birds of both species had increased enormously in 

 numbers. But there is another side to the story. Formerly 

 the grasslands and young sugar-cane plantations had been 

 ravaged yearly by vast numbers of army- worm caterpillars, 

 but the mynahs also fed upon these caterpillars and succeeded 

 to a large extent in keeping them in check, so that the outbreaks 

 became less severe. About this time certain insects were 

 introduced in order to try and check the spread of Lantana, 

 and several of these (in particular a species of Agromyzid 

 fly) did actually destroy so much seed that the Lantana began 

 to decrease. As a result of this, the mynahs also began to 

 decrease in numbers to such an extent that there began to 

 occur again severe outbreaks of army- worm caterpillars. It 

 was then found that when the Lantana had been removed 

 in many places, other introduced shrubs came in, some of 

 which are even more difficult to eradicate than the original 

 Lantana?^ 



6. It is clear that animals are organised into a complex 

 society, as complex and as fascinating to study as human 

 society. At first sight we might despair of discovering any 

 general principles regulating animal communities. But care- 

 ful study of simple communities shows that there are several 

 principles which enable us to analyse an animal community 

 into its parts, and in the light of which much of the apparent 

 complication disappears. These principles will be considered 

 under four headings : 



A. Food-chains and the food-cycle. 



B. Size of food. 



C. Niches. 



D. The pyramid of numbers. 



Food-chains and the Food-cycle 



7. We shall see in a later chapter what a vast number of 

 animals can be found in even a small district. It is natural 

 to ask : *' What are they all doing ? " The answer to this is 

 in many cases that they are not doing anything. All cold- 

 blooded animals and a large number of warm-blooded ones 



