ECOLOGY 



769 



Figure 37.5. Diagram of a food chain in an Illinois deciduous forest. (After 

 Shelford.) (Courtesy of Dr. V. E. Shelford.) 



predators, and so on. The members of each assemblage are not deter- 

 mined by chance but by the total effect of the many interacting physical 

 and biotic factors of the environment. The ecologist refers to the or- 

 ganisms living in a given area as a biotic community; this is composed 

 of smaller groups, the members of which are more intimately associated, 

 known as populations. There is no sharp distinction between a popu- 

 lation and a community. 



The intermeshings of the food chains in any biotic community are 

 very complicated and are sometimes called a food web, or "web of life." 

 Some of the interrelated food chains of a deciduous forest in eastern 

 North America are indicated in Figure 37.5. The basic principles of the 

 ecologic relations of biotic communities have been elucidated by the 

 study of somewhat simpler communities such as the arctic tundra or 

 desert. The producer organisms of the tundra are lichens, mosses and 

 grasses. Reindeer and caribou feed on the lichens and are preyed upon 

 by wolves and man. Grasses are eaten by the arctic hare and the lem- 

 ming, which are eaten by the snowy owl and the arctic fox, which is 

 preyed upon by man for its fur. During the brief arctic summer the 

 food web is enlarged by many insects and by migratory birds which 

 feed upon them. 



345. Populations and Their Characteristics 



A population may be defined as a group of organisms of the same 

 or similar species which occupy a given area. It has characteristics 

 which are a function of the whole group and not of the individual 

 members; these are population density, birth rate, death rate, age 

 distribution, biotic potential, rate of dispersion and growth form. Al- 

 though individuals are born and die, individuals do not have birth 

 rates or death rates; these are characteristics of the population as a 

 whole Modern ecology deals especially with the community and popu- 

 lation aspects of the science, and the study of group organization is the 

 most unique part of the science of ecology. Population and community 



