210 The Evolution of Communities 



the kinds of order that occur in natural situations. In pursuing this 

 inquiry, examples have been drawn chiefly from continental life 

 because, in many respects, life in these habitats is known better 

 than life in the oceans. 



Under natural conditions, living continental species form different 

 kinds of interlocking landscapes in such areas as prairie, forest, 

 desert, lakes, and streams. The type of vegetation, that is, whether 

 it is desert, prairie, or forest, is determined by climatic factors of 

 temperature, rainfall, and evaporation. In the main, forests occur in 

 regions with high rainfall, prairies in regions having lower total or 

 seasonal rainfall, and deserts where rain is scant and evaporation 

 high. The type of vegetation in the landscape is therefore a reflec- 

 tion of the climate, and, in general, widely separated areas having 

 similar climates have similar aspects of landscape. These major types 

 are called biomes, and are the largest ecological units. Each of these 

 biomes contains smaller units. A forest, for example, may be well 

 drained and high with a preponderance of oaks and hickories; in 

 another place it may be low and swampy, with elms, gums, and 

 other trees different from those in the better-drained areas. Each of 

 these fairly uniform areas is considered by the ecologists as a bio- 

 logical unit of natural areas and is called a community. Each com- 

 munity consists of a set of species that persists year after year. The 

 species living in similar communities are practically the same. Thus 

 oak-hickory communities in Wisconsin, Indiana, Missouri, and Okla- 

 homa each contain very nearly the same species of organisms. 



Although an elm or gum forest community contains a fair propor- 

 tion of the species found in an oak-hickory community, it lacks many 

 species found there but possesses instead species distinctive to itself. 

 Going further afield, a prairie community has a species make-up 

 differing greatly from that of a forest community, and both have 

 almost no species in common with marine communities. 



Within each community there exists a vital relationship between 

 many species. In a forest many species of both plants and animals 

 feed on the forest trees, shrubs, and herbs, and are dependent on 

 them for existence. Certain insects feed on the parasitic fungi, other 

 insects feed on the plant-eating insects, and still other species may 

 parasitize these predators. The squirrels and birds have their bird 

 and mammal predators and their insect and tick parasites. Many 

 species of shrubs and herbs are able to live only in the shade of 

 the trees and in this way are as dependent on the trees as are the 

 species which feed on the trees. This relationship is obligator)^ only 

 in one direction. The trees would live just as well without the or- 



