162 AGGREGATION, COMPETITION AND CYCLES 



prairie grasses. A similar adjustment or evasion is evident in the 

 insects and birds of tree trunks by contrast with those of the crowns, 

 and it characterizes also the subterrene moles and gophers. 



Evasion in time is perhaps even more general than that in space. 

 It is best illustrated by aspect societies of plants or animals, or both, 

 as a consequence of which the maximum demand is distributed through 

 three or four major phases of the growing season or the year. Some- 

 thing of the same advantage, only in a smaller way, accrues from 

 the nocturnal habit and in some degree also from estivation and 

 hibernation. Though less regular in operation, annuation may possess 

 similar value. This is particularly true of winter annuals in the 

 Southwest and desert regions generally, and must apply in a large 

 degree to the animals dependent upon them. It must also play some 

 part in the dynamic balance of animal populations. 



Similarities and Differences. With respect to the chief prerequisite 

 for competition, namely, a common demand in excess of the supply, 

 all communities, whether plant, animal, or mixed, are in complete 

 accord. But beyond this, striking differences arise in the process as 

 a result of the divergent demands made by autonomous plants and 

 animals. The plant, and hence the plant matrix, as the producer of 

 food from raw materials, makes demands, that are peculiar to it, upon 

 water, carbon dioxide, nutrient salts, and light. The need for oxygen 

 is felt by both plants and animals, and they are likewise in accord 

 as to the essentials of respiration. As to appropriate working condi- 

 tions, both groups of organisms are dependent upon temperature and 

 radiation in varying measure, but there is no actual competition for 

 these, apart from that for light. They may also exhibit competition 

 for certain solutes, as diatoms, chara, Infusoria, and mollusks for 

 lime and silica, though the movement of water or organism may obvi- 

 ate this to a certain extent. 



Competition for food is characteristic of animals alone, with the 

 exception of plant parasites and saprophytes. Animals are unique in 

 competing for mates, though the rudiments of such competition may 

 be seen in the motile gametes of a few algae. However, a process 

 with a certain similarity is that of insect pollination, in which flowers 

 compete with one another for pollinators. 



PLANT COMPETITION 



Nature and Kinds. By contrast with animals, the general lack of 

 motility among plants renders their competition passive and hence 

 inconspicuous, if not invisible. It is based upon reaction rather than 

 coaction and is consequently more or less indirect in operation. Ex- 



