uals. These groups may consist of plants of a single species, con- 

 stituting a population, but more often the groups comprise 

 individuals of several species, constituting a community. A great 

 deal is known about the ecology of individual organisms, the eco- 

 logical relationships of populations are now being intensively 

 studied, and much work has been done on communities as well. 

 It has been suggested that communities of interacting populations 

 may also behave as units in natural selection. ^^ These three levels 

 of ecological integration, the individual, the population, and the 

 community, are considered in the study of ecology. 



Seeds of many kinds of plants are carried into bare areas, where 

 many of the seeds germinate; but only those that are suited to 

 the prevailing conditions will grow and produce offspring. The 

 kinds of plants that can grow in a particular habitat must have 

 the ability to grow not only under the prevailing physical envi- 

 ronmental conditions, but also in association and competition 

 with neighboring plants. Some species are more successful than 

 others, as shown by the number, size, or behavior of the individ- 

 uals. Hence the potentialities of species, differing according to 

 their genetic constitutions, determine not only the types of hab- 

 itats that each can occupy, but also the nature of the interrela- 

 tionships that develop with other species. These potentialities fur- 

 nish the key to understanding how groupings are formed, the 

 nature of the groupings or communities, the processes occurring 

 within them, and how one kind of community can replace an- 

 other. The complexity of the community and the intricate rela- 

 tionships of organisms to one another and to the environment, 

 therefore, depend upon the habits and qualities of individual 

 species; hence knowledge of the latter will aid us in understand- 

 ing the community as a whole. When interrelations of the 

 individual species are emphasized, the study is often called aute- 

 cology, in contrast to synecology, the study of the community 

 ecology. The former is obviously incomplete because different 

 kinds of plants live together. 



In this book the nature of the species in relation to the phys- 

 ical environment and in relation to other organisms will be 

 treated first because knowledge of the organism is essential to 



In«i 



