FIG. 18-2 Growth of populations of Paramecium caudafum and 

 P. aurelia, cultivated separately and in mixed populations (from 

 Sause 1934). 



of it. Rarely, if ever, will they be equally adapted, 

 and ordinarily the one with the better adaptations or 

 greater aggressiveness will win out and occupy the 

 niche to its full carrying capacity. The basic idea of 

 this rule was understood from observations of natu- 

 ral distribution long before it was verified experi- 

 mentally (Grinnell 1904, Jordan 1905). 



In experimental cultures, Paramecium caudatum 

 and P. aurelia maintain separate populations at a 

 high level, but when the two species are mixed, they 

 quickly come into competition. As long as the food 

 supply is ample, both Species increase in biomass, but 

 as the food supply approaches exhaustion, P. aurelia 

 persists and P. caudatum declines until it finally dis- 

 appears. An analysis of the relative adaptation of the 

 two species shows that P. aurelia is capable of faster 

 population growth and is more resistant to the ac- 

 cumulating waste products than is P. caudatum 

 (Cause 1934). Similarly, Daphnia pulicaria in mixed 

 cultures causes the extinction of D. magna when 

 oxygen and food become limited (Frank 1957), and 

 Tetrahymena pyrijormis persists while Chilomonas 

 Paramecium disappears in mixed cultures of these 

 protozoans (Mucibabic 1957). 



When two species of flour beetles with similar re- 

 quirements for food, space, and other conditions are 

 cultured together in the same volume of flour, one 

 species always becomes extinct and the surviving 

 species then establishes a stabilized population. In 



cultures free of parasites, Triholium castaneum is the 

 successful species, but in cultures containing the 

 sporozoan parasite Adelina tribolii, T. castaneum be- 

 comes extinct and T. confusum persists, since it is 

 less susceptible to the parasite (Park 1948). Para- 

 sites or predators may then influence the success of 

 competition between species by affecting one more 

 than the other (Crombie 1947). 



It appears that when two species with similar 

 niche requirements meet in competition under natural 

 conditions, one of three things will happen (Lack 

 1944): 



1. One of the two species will be so much better 

 adapted that it will spread rapidly through the 

 range of the other and exterminate it. 



2. One species will be better adapted to a portion 

 of the range, in which it will eliminate the 

 other species, but the other species will be 

 better adapted to the remainder of the range 

 and will occupy it exclusively. Thus the two 

 species will occupy adjacent geographic re- 

 gions with perhaps a zone of overlapping occu- 

 pancy. 



3. Each species will be better adapted to a dif- 

 ferent portion of the niche, to which it will 

 become restricted ; with this separation, each 

 species will then spread through the range of 

 the other. 



Advantages of niche segregation 



Probably the major advantage animals gain by 

 occupying different niches is escape from continuous 

 intense competition. It is also true that the niche oc- 

 cupied is favorable to the species physically in fur- 

 nishing suitable substratum and micro-climate, al- 

 though many species have the ability to live elsewhere 

 were competition not involved. Automatic segre- 

 gation of a species into its niche through inherited 

 behavior patterns avoids the great expenditure of 

 energy and loss of time that would be required if this 

 segregation had to be worked out anew each year or 

 each generation. Segregation into niches avoids con- 

 fusion of activities between organisms in the com- 

 munity and permits a more orderly and efficient life- 

 cycle on the part of each species. Furthermore, the 

 segregation of each species into diiYerent niches per- 

 mits the occupancy of the area by a larger number of 

 species, since they will better divide the available 

 resources between them. Similarly the more distinct 

 the niche of a species is, the more it can avoid con- 

 flict with its neighbors and lead a life that is orderly, 

 productive, and efficient. Competition is thus a potent 

 factor in giving ecological structure to the commu- 

 nity. 



254 Ecological processes and dynamics 



