418 The Communitij 



and 84 per cent of the plant genera were represented by only one 

 species (Elton, 1946). The average number of species per genus 

 in these widely spread surveys was 1.38 and 1.22 for the animals 

 and plants, respectively. In contrast the average number of species 

 per genus for all British insects, for example, is 4.23. 



As a general rule the species of a genus are sufficiently similar so 

 that their demands on the environment are in serious conflict. During 

 the course of evolution this conflict has usually resulted in eliminating 

 all but one species from each genus represented in the area. In 

 contrast to a geographical region, a community is a functional unit 

 and each niche in the community structure is occupied by that species 

 which has been most successful in the competition for it. Adaptations 

 for ecological needs and accessibility determine which species will 

 survive and populate each part of the biocenose. The species that 

 succeed in establishing themselves are almost always sufficiently dif- 

 ferent to be members of separate genera. Competition within each 

 level of nourishment in the food chain thus produces a certain or- 

 ganization among the species of a community in addition to the 

 organization imposed by the relationships from one link in the food 

 chain to another and by other dependencies. The species composi- 

 tion of each biocenose is therefore determined by relations within 

 the same functional level as well as in successive levels from among 

 the species that are distributed to the area. 



To make a census of the inhabitants of a diversified natural com- 

 munity and to ascertain the role of the various species in any compli- 

 cated ecosystem is exceedingly difficult. Such tasks require the 

 combined efforts of at least several taxonomists and ecologists com- 

 petent to deal with the various segments of the biota. General ac- 

 counts of the "plant communities" and the "animal communities" in- 

 habiting the principal types of environment in the world are available 

 in such works as Costing (1948), Hesse, Allee and Schmidt (1951), 

 and Dice ( 1952 ) . Detailed studies of the inhabitants of specific 

 biotopes have for the most part been carried out by investigators con- 

 fining themselves to restricted categories of animal or plant life. To 

 obtain an idea of the total assemblage of organisms making up a 

 biocenose and their integrated activities, it is usually necessary to 

 piece together the results of several investigations carried out in the 

 same region. The reader may obtain rather complete information 

 on the composition of the entire biota of certain specific areas by 

 reference to the collaborative reports of members of expeditions, of 

 government surveys, and of field research laboratories. In many 

 instances the work of areal surveys has been expanded to include 



( 



