color displays (a sort of saber-rattling) by tisii and 

 birds for defense of territories ; and in the singing 

 and calling of birds, some mammals, and frogs as bids 

 for mates. 



Indirect competition, sometimes called exploita- 

 tion (Brian 1956), is common among |)lants when 

 certain species monojxjiize the water and nutrient re- 

 sources of the soil or available light so that competing 

 species cannot maintain themselves (Clements, 

 Weaver, and Hanson 1929). .-Knimals may also ren- 

 der a habitat unsuitable by their excreta for a species 

 which otherwise would occur. Once an area is well 

 saturated with established individuals, it is often 

 more economical of energy for new individuals to 

 seek homes elsewhere, even in less favorable situa- 

 tions, than to intrude. To be successful by indirect 

 competition, a species needs to get established in an 

 area first, or if the invasion of various species is 

 nearly simultaneous, then to have a more rapid rate 

 of reproduction and growth, or a greater longevity, so 

 as to utilize the resources of the habitat to the fullest 

 possible extent (Crombie 1947). 



Competition is usually keenest between individ- 

 uals of the same species, intraspecific competition, 

 because they have identical requirements for food, 

 mates, and so on, and because they are more nearly 

 equal in their structural, functional, and behavioral 

 adaptations. Interspecific competition occurs where 

 different species require in common at least some ma- 

 terials or conditions. The severity of competition de- 

 pends on the e.xtent of similarity or overlap in the re- 

 quirements of difTerent individuals and the shortage 

 of the supply in the habitat. It is generally the case 

 that the more unlike the kinds of competing organ- 

 isms, the less intense the competition. Yet birds 

 compete with squirrels for acorns, nuts, and seeds ; 

 insects and ungulates compete for food in grassland : 

 the bladderwort plant competes with small fish for 

 entomostraca and other plankton. 



Competition has five important effects on the ani- 

 mal community : 



1. Establishment of social hierarchies 



2. Establishment of territories 



3. Regulation of population size 



4. Segregation of species into diiTerent niches 



5. Speciation 



The first two effects are chiefly intraspecific ; will be 

 considered in this chapter. Regulation of population 

 size involves both intra- and interspecific competition, 

 and many other types of coaction as well, and will be 

 considered in Chapters 15, 16, and 17. The last two 

 effects are interspecific ; they will be discussed in 

 Chapters 18 and 19. It is important to realize that, 

 when these effects are fully manifested, there is a 

 decrease in tension and intensity of competition as 



eacii individual or .species takes its place in the orderly 

 structure and organization of the community. Let us 

 see how this works out in the instances of social 

 hierarchies and the establishment of territories. 



Social hicrarrhii's 



When groups of individuals of certain animal 

 sjx-cies are confined to limited areas, frequent fights 

 or pecking of one another occur. By way of these 

 encounters, the more aggressive and successful indi- 

 viduals establish a hegemony to which the more sub- 

 missive individuals acquiese. A social hierarchy is 

 thus established ; the phenomenon was first clearly 

 described for the domestic fowl (.Schjelderup-Ebbe 

 1922). 



The so-called peck-order in the domestic fowl is a 

 linear one. Close observation of marked individuals 

 showed that, in a flock of 13 birds, one bird became 

 the supreme despot of the whole flock ; another bird 

 was submissive to the first but despotic over the re- 

 maining 1 1 : and so it went on down to the last bird, 

 which had the right to peck none but was pecked by 

 all. This type of social aggressiveness or despotism 

 is called peck-right. In practice, certain individuals 

 establish the right to peck others and not get pecked 

 back. In the middle of a series, the order is some- 

 times less fixed, and reversals or triangles occasion- 

 ally occur. Although most easily demonstrated in the 

 crowded conditions of captivity, peck-right has also 

 been observed to obtain under free natural conditions. 

 The peck-right type of social hierarchy has been found 

 to occur in several other species of birds, in several 

 species of mammals and fish, and in a few lizards, 

 crayfish, and insects. 



Possession of the following characteristics usually 

 gives an individual at least some advantage in gain- 

 ing a high position in the despotic order : strength ; 

 good health ; maturity ; relatively large size ; hegem- 

 ony over own territory ; responsibility of acting to 

 protect young; accompaniment by members of his 

 own group when meeting a stranger ; male over fe- 

 male, at least during the nonbreeding season ; female 

 mated with a strong male : the hormone testosterone ; 

 and innate aggressiveness (Allee ct al. 1949: 413- 

 414). A high position in the social order is advan- 

 tageous to the individual as it gives him priority 

 over food, mates, territory, and other resources of 

 the habitat (Collias 1944) and is sometimes, but not 

 always, correlated with leadership in the group. 



In some species the social hierarchy is not as overt 

 as that we've described. In peck dominance the indi- 

 vidual that is usually subordinate is successful in a 

 certain number of conflicts. Position in the despotic 

 order is a function of ratios of success in continuing 

 conflicts rather than on the results of the initial con- 



Cooperation and disoperation 



183 



