The Evolution bf Communities 239 



favoring protective devices arise early in community evolution but 

 also that these pressures led to a wider and more varied expression 

 of protective devices as communities became more complex. 



CHANGE IN SPECIES COMPOSITION 



As the community progresses through time, even in a static geo- 

 graphic situation, its species composition changes, either by sub- 

 traction or addition. These changes in turn affect the ecological 

 complexity of the community and the evolutionary selection pres- 

 sures acting within the community. 



Extinction 



From both the historical and fossil record we know that certain 

 species once living are now extinct, but in many cases we know 

 little about the reasons or mechanism of the extinction. Great 

 numbers of plants and animals known only from fossils have per- 

 ished through the eons (Fig. 89). The Labrador duck Campto- 

 rhynchtis labradorius is one species which seems to have become 

 extinct recently without the influence of man. However, the great 

 majority of known species which have become extinct recently owe 

 their demise to man's activities, usually either by his reducing the 

 area of the habitat below that required by the species, as in the 

 possible extinction of the ivory-billed woodpecker Campephihis 

 principalis of southeastern North America, or by his introducing 

 competitors or predators, as with the dodo Didiis ineptus of Mauri- 

 tius. It seems safe to speculate that under natural conditions the 

 number of established species in a community suffers only small 

 or infrequent reductions due to extinction. 



Increase in Number of Species 



Although range fission may be the most important single factor 

 leading to an increase in the number of species, an increase can 

 and probably does occur in a community which is relatively static 

 geographically. The following mechanisms, most of them explained 

 in the preceding chapter, contribute to this increase. 



COLONIZATION THROUGH DISPERSAL FROM 

 A SIMILAR BUT DISTANT COMMUNITY 



Thus the present West Indian fauna of over 50 species of the leaf- 

 hopper genus Empoasca undoubtedly reached the Antilles chiefly 



