160 AGGREGATION, COMPETITION AND CYCLES 



ing in the usual sense, though keen competition frequently occurs in 

 what is visually open spacing, as with desert shrubs and bird terri- 

 tories. Competition is regularly most marked between organisms with 

 the same or similar needs, as within a particular life form of plant or 

 animal, but it may even take place between plants and animals in soil, 

 aquatic, or parasitic communities. 



In general, competition is to be distinguished from all other co- 

 actions by the test of a common demand upon a limited supply. This 

 criterion applies even to the combat between two males for the same 

 mate. However, the active or passive contest between an animal and 

 its food organisms, as well as combat between two animal families, 

 belong to a different category. This non-competitive type includes in 

 particular the destruction exerted by carnivores, though an active com- 

 petition often exists among these. Similarly, for example, the para- 

 sitism of the cowbird is not competition in the stict sense, in spite of 

 the fact that passive competition occurs among the nestlings. More- 

 over, there is an element of competition in certain types of disopera- 

 tion, instanced by the examples of beggary and thievery cited in the 

 preceding section. Furthermore, though cooperation is the exact antith- 

 esis of disoperation, it is also antithetic to competition, since com- 

 petition is regularly harmful in its effects. On the other hand, when 

 competition leads to dominance and subordination, as it often does, 

 especially among plants, a certain degree of cooperation is established. 

 Consequently, while coaction is regarded as embracing all the inter- 

 actions between organisms, competition comprises only those directed 

 to a common need. 



Types of Competitors and Objectives. It is evident that organisms 

 compete with one another only when they make the same or similar 

 demands and typically at the same time, in the absence of an ade- 

 quate supply. This may be for one, for several, or for all the essential 

 factors of the habitat. However, there can be no significant competi- 

 tion between an oak and a forb of the forest floor after the latter has 

 become subordinated, though it may exist when the oak is a yearling or 

 when the ground layer is well developed. As a consequence, competi- 

 tion depends in the first instance upon the life forms and life habits 

 involved, and in the second upon the manner and degree of aggregation. 

 Plants and animals will compete least frequently with each other be- 

 cause of certain basic differences in their demands, but when they are 

 similar in size, as in microplankton, or in nutrition, as with leaf 

 parasites, competition does arise. Competition will be keener between 

 mammals than between mammals and birds, as a rule, but there is 

 little between carnivore and herbivore except in so far as some tend 



