668 Ecology 



populations thus continue to grow. Such overpopulated countries as 

 India do, however, illustrate the truth of Malthus's ideas ; such a 

 country is commonly called a Malthusian country. 



Fluctuations of Aliundance. — Once a population has reached its 

 upper limits, it does not necessarily stay at this exact level. It may 

 fluctuate in numbers from year to year or season to season. At times, 

 these fluctuations may even produce sudden spurts of abundance or again 

 drop off nearly to extinction. Most fluctuations, however, are not this 

 drastic and are simply arythmical variations in abundance. In some 

 ways the most remarkable of all are populations that fluctuate in a 

 more or less definitely cyclic manner. This has been demonstrated for 

 some mammals and a few birds. 



The population of the varying hare in the Hudson Bay area of 

 Canada has been recorded for a number of years by the Hudson's Bay 

 Company and other observers. There is a cyclic abundance of the 

 varying hare with the peaks of abundance occurring at nine- or ten-year 

 intervals. Similarly the Canada lynx which depends to a great ex- 

 tent upon the varying hare for its food has a cycle of abundance with 

 the mean time between peaks of 9.7 years. Similar cycles but with dif- 

 ferent periods have been demonstrated for mice, lemmings, collared 

 fox of Labrador, and several species of birds. The causes of these 

 cycles are not clearly understood, although a number of theories have 

 been postulated. Some of these include such things as inherent rhythms, 

 sunspot cycles and radiation effects, disease, and the shock syndrome 

 of crowding. 



This latter theory postulates that the effect of high populations is 

 to increase the activity of the adrenal cortex so that the animals easily 

 go into shock. They not only die in large numbers, but are probably 

 less fertile. Certainly the best explanation at present is that such cycles 

 are due to a great variety of factors which are not clearly understood. 



Food Networks. — The factor which most frequently limits the 

 growth of any population is that of available food. Within any commu- 

 nity there is a complex of food webs which embraces all organisms. It 

 is so complex that it can best be shown by means of a very simplified, 

 at least partially true, diagram. 



Darwin was one of the first to call attention to the importance 

 of these food webs which he termed "webs of life." In a somewhat face- 

 tious moment, he pointed out that perhaps the economic life of England 

 was in some way correlated with the number of old maids. He rea- 



