70 THE ECOLOGY OF ANIMALS 



It kills the same percentage in either case. The 

 other is related to the actual density of the species 

 and usually acts as an automatic control. This idea 

 of automatic control is important, since it explains 

 to a large extent why animal communities continue 

 to exist from year to year without striking changes 

 in the composition of the species — in other words 

 why species do not more often become locally extinct 

 through chance fluctuations. There are various types 

 of this automatic control. The increased chance of 

 parasites finding or reaching a host as the density 

 of the latter increases has been mentioned. Con- 

 trary to expectation, this does not always take place 

 with parasites in mammals. Certain parasites have 

 an age distribution in their hosts such that increase 

 in numbers of the host causes temporary decrease in 

 parasite density owing to the production of many 

 young hosts which carry fewer parasites (Elton and 

 others, 1931). In parasites such as coccidia that 

 often attack young more than old, or both irrespec- 

 tively, this damping down of automatic control by 

 parasites does not occur (e.g. in the red grouse and 

 the Norwegian willow grouse and the rabbit). 



A very important factor in this general class of 

 factors whose effect is to adjust the density of popu- 

 lation of animals to their surroundings is migration 

 (Elton, 1930). Migration is here used in the general 

 sense of movements of individuals in the population. 

 Heape (1931) has summarized with a valuable 

 bibliography many of the known data on the subject 

 of migration, and he makes a distinction between 

 migration (periodic movements as in certain birds, 

 marine and anadromous fish, and dragonfiies), emi- 

 gration (as in the Norwegian lemming when it leaves 

 the mountains never to return), and nomadism (as 

 in the caribou searching for good grazing grounds, 

 or the bumbl|i-bee searching for nectar-producing 

 flowers). This distinction probably cannot be use- 

 fully upheld in the present state of our knowledge, 



