NUMBERS : DYNAMICS 63 



British coast by cold winters in 1916-17 and 1928-9 

 (Orton and Lewis, 1931). Stenhouse (1928) found 

 that house sparrows in the Shetlands mostly died 

 from epidemic disease during the years 1926-8. 

 The studies of Harrisson and Wynne Edwards (1932) 

 on the island of Lundy show that the bird population 

 of a limited area fluctuates greatly in different years, 

 both in numbers and in species (i.e. some species 

 become extinct, and others arrive). We are there- 

 fore left with a picture of animal communities as 

 liable to many disturbing factors. Populations never 

 remain constant for very long, and tend all the time 

 to oscillate about a theoretical optimum point for 

 the species. We shall have to inquire what happens 

 when species depart very far from this theoretical 

 optimum, what in general are the causes of these 

 great fluctuations, and what eff"ects they have upon 

 associated animals — for we have already stressed 

 the close interdependence of one form of animal upon 

 another. When we reflect that most species are kept 

 down in numbers in great measure by the attacks of 

 enemies or parasitoids or parasites, we can see what 

 profound influence the fluctuations in one animal 

 must have upon others in the same community. 



The actual recording of periodic changes in numbers 

 can be carried on by two methods. In the flrst 

 place, periodic censuses of the type described in the 

 last chapter provide the most accurate measure of 

 changes in absolute numbers of the population over 

 a series of years. Such censuses have been carried 

 out for rooks in certain parts of Great Britain 

 (Alexander, 1933), and sampling censuses are widely 

 used in plankton studies from year to year or in 

 order to show seasonal changes and in the study of 

 soil protozoa. Generally, it is not practicable to 

 carry out complete censuses often enough or on a 

 sufficiently large scale to provide absolute flgures of 

 density. Fluctuations can, however, be studied by 

 another method : by recording the relative differences 



