CHAPTER IX 



THE PERIODIC FLUCTUATIONS OF OUR FUR-BEARING 



ANIMALS 



(charts I TO V) 



Throughout the animal world we find that not only is 

 the abundance of the higher predatory animals dependent 

 upon the abundance of the lower forms upon which they 

 prey, or which may indirectly affect their food supply, but 

 this dependence may result in remarkable periodic fluctua- 

 tions in the numbers of the predatory forms. In the insect 

 world this phenomenon of rise and fall in abundance is well 

 known; a striking example occurred in 1915, when, owing 

 to the unusual prevalence of aphids, or plant-lice, through- 

 out Canada, wasps were extraordinarily abundant every- 

 where in the same region, the probable explanation being 

 that the wasps increased abnormally owing to the presence 

 of an unusual amount of food in the form of ''honey-dew," 

 which is excreted by the plant lice. Similarly, the abun- 

 dance of certain species of our food fishes is affected by the 

 amount of food in the shape of small Crustacea, or small 

 fishes, in the sea, such food being variable in quantity. 

 For instance, Bullen* showed that, in the years 1903-1907, 

 there appeared to be a direct correlation between the num- 

 ber of mackerel taken during May and the abundance of 

 the small copepod Crustacea. 



Darwin's illustration of the relation of cats to the abun- 

 dance of clover-seed is well known, but may be recalled. 

 Red clover depends on the visits of bumble-bees for its 

 fertilization. The abundance of bumble-bees in any dis- 

 trict depends very largely on the nimiber of field-mice which 



* Jour. Marine Biol. Assn., vol. VIII, p. 394, 1909. 

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