NUMBERS : STATISTICS 51 



it is possible to follow accurately the changes or 

 fluctuations in the numbers of a species. A good 

 deal of work has been done on the rook in this 

 country. This has been summarized by Alexander 

 (1933). 



Another method of studying animal numbers is by 

 making comparisons from month to month or year 

 to year. If these comparisons can be made according 

 to some standard method they are of considerable 

 value in showing the relative numbers at different 

 times. More will be said about this in the next 

 ahapter. For the moment let us inquire what con- 

 clusions of a general ecological importance have been 

 obtained so far from the census work that has already 

 been done on animals. 



The results of censuses have to be expressed in 

 terms of the number of animals on a given area at 

 a given time, i.e. the density of numbers. But it is 

 not sufficient to study only the crude densities. 

 Suppose we are studying herons. The crude figures 

 might give the number of herons on 50 square miles 

 of country. This we may call the Lowest Density 

 (Elton, 1932). It expresses the number of herons 

 on 50 square miles of country includmg all kinds of 

 habitats, some of which herons do not frequent at 

 all. The Lowest Density expresses in a sense the 

 biological success and adaptive elasticity of the 

 species over a large area of mixed country. If there 

 are a large number of ponds and lakes and streams 

 there will be a chance for more herons to hve in 

 the area. If there are very few streams and ponds 

 there will be very few herons. We therefore require 

 another term which will allow for the patchy dis- 

 tribution of suitable habitats and express the numbers 

 of herons in terms of the actual country that they 

 really inhabit and breed in. This we call the Eco- 

 nomic Density. The Economic Density will tend to 

 be much more uniform in different parts of the heron's 

 range than the Lowest Density. Within the natural 



