220 The Evolution of Communities 



aggregation of variables, the relations between these variables will 

 also change with time in proportion to the variables themselves. 

 If one competitor becomes more successful than another, or a host 

 becomes more resistant to a parasite, then their relationship within 

 the community will change. 



This dynamic quality of the community plus its diversity of rela- 

 tions makes its evolution difficult to analyse. Many of the problems 

 are in a sense similar to those encountered when explaining the 

 evolution of the species (Chapters 3 through 6), in which change 

 in genetic composition is integrated with natural selection and all 

 the while is coupled with species fission leading to an increase in 

 the number of species. Thus, within the community many sets of 

 changes may be occurring simultaneously, some types of change 

 involving the whole community, others only parts of it. 



The increase in taxonomic diversity and ecological intricacy 

 found in modern communities is accompanied by changes in the 

 processes of community evolution. The community reactions lead- 

 ing to coexistence and exploitation have been modified and, in at 

 least some circumstances, form a self-perpetuating host-prey system. 

 Selection pressures having their first roots in the community struc- 

 ture itself have led to new community relationships, exemplified by 

 the differentiation of dominants and subdominants, the origin of 

 mutualism, and the origin of protective devices and mimicry. These 

 facets of community evolution are illustrated by the simplest type 

 of community change, that is, the changes occurring in a com- 

 munity progressing through time in a geographic area that is rela- 

 tively stable ecologically. 



Indirect Competition 



One of the basic considerations in a community is that the amount 

 of nutrient material available limits the amount of living matter 

 which can exist in a unit volume or area. The greater the number 

 of species inhabiting that unit, the less average nutrient material 

 is available per species. In the aggregation of species in communities 

 many stratifications have evolved, each tending to divide in some 

 manner this finite total amount of food among the species de- 

 pendent on it. In time stratification, some species are active at one 

 time and others later. Each uses the production of, or material 

 available in a different period of the day or year. In other stratifica- 

 tions some species feed on different parts of the food supply. With 

 a grass, fungi will feed inside its cells, some insects will feed on 

 its pollen, others only on its leaves, others inside the stem, and 



