774 



ANIMALS AND TH£IR ENVIRONMENT 



on the hare, it is obvious that the lynx cycle is related to the hare cycle. 



A three to lour year cycle ot abundance is shown by lemmings and 

 voles, small mouselike animals living in the northern tundra region. 

 Every three or lour years there is a great increase in the number of 

 lemmings; they eat all the available food in the tundra and then migrate 

 in vast numbers looking for food. They invade villages in hordes and 

 finally many reach the sea and drown. The numbers of arctic foxes and 

 snowy owls, which feed on lemmings, increase similarly and when the 

 lemming population decreases, the foxes starve and the owls migrate 

 south— there is an invasion of snowy owls in the United States every 

 three or four years. 



Although some cycles recur with great regularity, others do not. For 

 example, in the carefully managed forests of Germany the numbers of 

 four species of moths whose caterpillars feed on pine needles were esti- 

 mated from censuses made each year for the period from 1880 to 1940. 

 The numbers varied from less than one to more than 10,000 per thousand 

 square meters. The cycles of maxima and minima of the four species 

 were quite independent and were irregular in their frequency and dura- 

 tion. 



Attempts to explain these vast oscillations in the numbers of a 

 species on the basis of climatic changes have been unsuccessful. At one 

 time it was believed that the cycles were caused by sunspots, and the 

 sunspot and lynx cycles do appear to correspond during the early part 

 of the nineteenth century. However, the cycles are of slightly different 

 lengths and by 1920 were completely out of phase, with sunspot maxima 

 corresponding to lynx minima. Attempts to correlate these cycles with 

 other periodic weather changes or with cycles of disease organisms have 

 been unsuccessful. 



The snowshoe hares, for example, die off cyclically even in the ab- 

 sence of predators and in the absence of known disease organisms or 

 parasites. The animals apparently die of "shock," characterized by low 

 blood sugar, exhaustion, convulsions and death, symptoms which re- 



160 



HARE 



LYNX 



1675 

 TIME 



1685 1895 



IN YEARS 



l«05 



1915 



1935 



Figure 37.8. Changes in the abundance of the lynx and snowshoe hare, as in- 

 dicated by the number of pelts received by the Hudson's Bay Company. This is a 

 classic example of cyclic oscillation in population density. (Redrawn from MacLulich, 

 1937.) 



