i86 ANIMAL ECOLOGY 



useful one would be encouraged by natural selection. It is 

 the indifferent characters with which we are concerned. 



9. There is little doubt that it will be through ecological 

 work upon the numbers of animals that this problem will be 

 finally solved, and what we know already about the subject 

 enables us to make certain suggestions. It has been shown 

 in Chapter VIII. that nearly all animals fluctuate considerably 

 in numbers, some of these fluctuations being very violent and 

 often very regular in their periodicity. For our present 

 purpose the important thing to bear in mind is the fact that 

 at frequent intervals (frequent compared to the time which it 

 would take for a species to change appreciably) the population 

 of many animals is reduced to a very low ebb, and that this is 

 followed by a more or less rapid expansion in numbers until 

 the former state of abundance is reached once more. After 

 a lemming year, with its inevitable epidemic killing off of all 

 but a few of the animals, the arctic tundra is almost empty 

 of lemmings. The same thing can be said of the snowshoe 

 rabbit. One year the country is pullulating with rabbits, the 

 following year you may hunt for a whole summer and only 

 see one. There is usually a rather rapid expansion after this 

 minimum of numbers. In a stream near Liverpool studied 

 by the writer, the whole fauna over a stretch of three miles 

 was wiped out during the summer of 192 1, by a severe drought. 

 Recolonisation took place from some deep ponds connected 

 with the upper part of the stream, and after three or four years 

 the population of molluscs, insects, Crustacea, fish, etc., had 

 regained its " normal " density. A similar destruction of the 

 fauna took place in 1921 in a small branch of the Thames 

 near Oxford, but by 1925 the animals had reached '* normal " 

 numbers again (through immigration and natural increase). 

 The Gammarus pulex were very scarce in 1922, but had reached 

 great abundance by 1925, when they were again practically 

 wiped out, this time by an epidemic. 



10. Now, if you turn back to the diagram on p. 180 you 

 will notice that the argument contains a certain fallacy. The 

 original theory says that all animals tend to increase, and at a 

 very liigh rate, but are prevented from doing so by checks. 



