BIOTOPES AND BIOCOENOSES 139 



The whole composition of a biocoenosis, its plant foundation and 

 the competition among the animals which enter it, determines both the 

 species and number of individuals in a given biotope. There is a 

 general balance in a biocoenosis, so that the changes by death or 

 increase cause only slight fluctuations about a mean. This balance, 

 however, is unstable. It is disturbed through the variations in the 

 habitat conditions themselves, which undergo fluctuations. When the 

 numbers of a single species change, the intimate nature of the internal 

 relations of a biocoenosis causes the other members to change. Thus 

 an excessive multiplication of the processionary caterpillar of the oak 

 {Cnethocampa processioned) directly affects the oaks which are eaten 

 bare, and the other forms which are dependent on them, such as the 

 oak tortricid (Tortrix viridana) , and the gall wasps, whose breeding 

 places and food are reduced. In addition, the fertilization of the forest 

 bottom by the caterpillar dung has an effect in a richer growth of the 

 ground cover; the wild animals, which take up with their food the 

 widely disseminated poisonous and irritating hairs of the caterpillars, 

 may leave such forests. Cuckoos, on the other hand, are attracted, 

 since the hairs are not objectionable to them. As the increase in the 

 caterpillars comes in May and June, the concentration of cuckoos must 

 have an important effect on the breeding of the smaller perching 

 birds, in whose nests the parasitic European cuckoo lays its eggs. 

 A notable increase in the plankton of a given part of the sea has an 

 important influence on the numbers of fish in the following years, as 

 the food supply favors the growth of the fry. The number of pre- 

 dacious animals is dependent on the number of food animals; the 

 abundance of lynxes in Canada, for example, seems to depend directly 

 on the abundance of snowshoe rabbits (Fig. 11). 



Such fluctuations in general are rapidly equalized on account of 

 the superabundant fertility of all members of the community. Perma- 

 nent changes in the faunal composition and the numeric relations of 

 the species result when one of the members of an association disappears 

 or when a new one is added. The caribou has been almost exterminated 

 in Labrador by ruthless hunting ; two other species that were dependent 

 upon it disappeared with the caribou — wolves and Indians. 5 The 

 introduction of numerous palaearctic species, such as passerine birds 

 and earthworms in New Zealand, Australia, and other countries, has 

 operated to destroy and drive out the native forms. 



In addition to being conditioned by their biotic interrelations the 

 members of an animal community are dependent on the physical con- 

 ditions of their biotope. These habitat conditions demand a higher 

 degree of adaptation on the part of the animal population the more 



