396 Relations between Species 



Competition 



In the foregoing sections we have discussed certain categories of 

 antagonism in which one species harms another by parasitizing it, 

 by preying upon it, by poisoning it, or by taking some other direct 

 advantage of it. In this section we shall consider first the aspect of 

 competition that involves the "cold war" between species that are 

 contending for "lebensraum" or for "consumer goods," and then we 

 shall discuss certain generalities in regard to competition involving 

 both direct aggression and indirect rivalry. An organism competes 

 with members of its own species as well as with representatives of 

 other species for space, light, food, or other necessity, but the nature 

 of the competition between species differs because of the variation 

 in precise needs and adaptations of different species. 



Lichens compete with each other for space on a dry ledge and also 

 with members of other species; on a submerged rock barnacles simi- 

 larly compete for space with one another and also with oysters, mus- 

 sels, and other sessile animals. Tree seedlings vie with small shrubs 

 and herbs for light in the developing vegetation. The roots of a 

 forest tree engage in a continuing but unseen struggle with the roots 

 of other trees for water and for nutrients. Various species of para- 

 sites contend for the choicest tissues of their host. Grasshoppers not 

 only compete closely with other insects for grass but also contend to 

 some extent with mice and rabbits as well as with sheep and antelope 

 for the same food. Various carnivorous species similarly are often 

 rivals for the same prey. 



The more closely similar one organism is to another the more nearly 

 alike will be their needs— and hence the more intense will be their 

 rivalry in obtaining their requirements from a common environment. 

 This fact means not only that competition between individuals of the 

 same species is particularly keen but also that the intensity of com- 

 petition between species is directly related to the ecological similarity 

 between them. The rivalry between species of the same genus is 

 therefore usually more severe than that between species belonging to 

 different genera— as pointed out long ago by Darwin. 



Various aspects of interspecific competition have been investigated 

 with carefully controlled laboratory populations of flour beetles. 

 When Tribolium confusum and Tribolium costancum, for example, 

 are grown in the same "universe" of flour, one species always becomes 

 extinct, leaving the other in sole possession, although plenty of food 

 is available for both. The two beetles are very closely similar in life 



