48 GENERAL BIOCHEMISTRY 



absence of grazing animals. In the absence of suitable vegetation the 

 vegetarians cannot invade the environment, and in turn the carnivores 

 are excluded. 



Since all environments contain more than one population, there are 

 always interactions. The study of these interactions is called ecology 

 and is now pursued partly as a scientific problem and partly in the 

 hope of financial returns from improvements in crop yields, soil con- 

 servation, livestock industries, fishing for both pleasure and profit, and 

 lumbering. Man is, of course, one of the earth's greatest ecological 

 factors, owing to his size, broad range of activity, and great nimierical 

 abundance. 



As cxainj)lcs (^f ecological phenomena consider the epidemics of 

 disease resulting when popidalion density rises high enough for con- 

 tagion to spread by contact. Note also the multiplication of partic- 

 ular insects attacking certain crops cultivated intensively. A still more 

 far-reaching example involves the rabbits of northern Canada. In this 

 region rabbits multiply rapidly, the population following cycles of about 

 seven years, whereupon the nimibers are great enough to support an 

 epidemic which sweeps off a high proportion of the individuals. The 

 remainder reprochice rapidly, starting another cycle. However, shortly 

 after the expansion of the rabbit popidation begins, the lynx popula- 

 tion also starts to increase since the food supply is plentiful. But when 

 the majority of the rabbits die of disease, introducing a great competi- 

 tion among lynx for the remainder, the situation becomes so acute 

 that lynx die, migrate to other areas, and simply fail to produce 

 young. Hence the lynx population also declines a year or two after the 

 epidemic that decimates the rabbits. Other animals depending directly 

 or indirectly on the rabbits show similar cycles, shifted from that of the 

 rabbits according to the time required for pressure to develop in each 

 population. Even man is affected, since trappers find regions profitable 

 or not depending on the cycles of the fur-bearing animals and go else- 

 where when fox, lynx, wolverine, marten, and all the others are few 

 in number. 



REFERENCES 



Physical Biocheviistry, second edition. H. B. Bull. John Wiley & Sons, New York, 



1951. 

 A Short Textbook of Colloid Clieinistry. B. Jirgensons and M. E. Straumanis. John 



Wiley & Sons, New York, 1954. 

 Fundamentals of Physical Chemistry. H. D. Crockford and S, B, Knight. John 



Wiley k Sons, New York, 1959. 



