212 The Evolution of Communities 



has been going on since the first ckister of species evolved on the 

 earth. On this premise, mixtures of a few primeval species came 

 into existence early in the evolution of life, and mixtures involving 

 more and more species have since occurred many times in many 

 places. 



THE EVOLUTION OF COMMUNITY RELATIONSHIPS 



The relationships between species in a community may be grouped 

 into three categories: (1) exploitation, in which one species lives 

 at the expense of another, including chiefly the predator-prey rela- 

 tionship, (2) mutualism, in which one or both species benefit from 

 the relationship but neither sufters, and (3) coexistence, in which 

 species live together in some measure of actual or potential com- 

 petition for the same necessities such as light, moisture, space, and 

 nutrients. 



Considering that the first mixtures of species contained only a 

 few kinds which were probably quite similar ecologically, the in- 

 teresting question arises: How many species would have to inhabit 

 an area before modern community relationships would evolve? 

 Investigations in this realm are necessarily speculative because 

 neither the primeval organisms nor the primeval communities of 

 the past are still in existence. In their stead it is necessary to seek 

 present-day species and circumstances illustrating seemingly similar 

 simple phenomena and to assume that the general selection and 

 survival factors now in operation would have been those in opera- 

 tion when life began. The evidence derived from these available 

 sources indicates strongly that coexistence and exploitation could 

 and probably did materialize as features of community structure in 

 mixtures of only a few species. 



Coexistence 



Broadly speaking, all the species living in the same community are 

 coexisting, although they exhibit many degrees of interrelationship. 

 The kind of coexistence discussed in this section is the relationship 

 existing between the members of the community which utilize the 

 same space and food. This would include, for example, all the 

 trees in a community, all the herbivores, or all the carnivores. 



Within each of these general types, the food-procuring or other 

 activities or habits of each species may be slightly different. This 

 results in a staggered utilization of the commodity in potentially 



