56 THE ECOLOGY OF ANIMALS 



to be at an optimum for increase in numbers. Of 

 course such increase beyond a certain optimum 

 density is not necessarily an adaptive advantage for 

 the species, because it brings into action these auto- 

 matic checks (such as epidemic disease) which may 

 actually wipe out the species completely. 



There is another way of studying densities of 

 numbers. It is a general matter of observation that 

 smaller animals make up by their ' numbers what 

 they lack in size. Morris (1922) found the soil fauna 

 of unmanured arable land to contain about 5,000,000 

 individuals per acre. Alexander (1932) has pubHshed 

 figures for the numbers of birds per acre on farm 

 land near Oxford. Examples are : song-thrush and 

 linnet each 8 ; yellow-hammer and wood-pigeon each 

 17 ; blackbird, 35 ; and green woodpecker only 1. 

 These are taken on a somewhat complex mosaic of 

 habitats ana therefore represent the Lowest Densities. 

 Man has a much lower density than most of these 

 birds. We can combine the facts about size and 

 density by working out the weights of animals per 

 acre. This can be done if we know the density of 

 animals and the average weights of an individual. 

 Thus a wood-ant {Formica rufa) weighs about 2 

 milligrams when dry. A nest -colony of 10,000 wood- 

 ants (not an uncommon scale of size for a nest) 

 would weigh about 20 grams. An acre of pine wood 

 with 20 nests would have a total dry weight of ants 

 of 400 grams — about a pound. Alexander (1932) 

 converted his total bird census figures mentioned 

 above into weights (not dry) per acre which came to 

 averages of about 57 kilograms per acre in October, 

 47 in November, and 32 in February, the decrease 

 representing in all probabiUty the gradual winter 

 mortahty. Human density at one person to two 

 acres would give a weight of about 6-5 kilograms per 

 acre dry weight, and some 19-5 kilograms fresh. 

 Morris (1922) estimated the total weight of nitrogen 

 in the bodies of the soil fauna on unmanured land 



