240 The Evolution of Communities 



by overwater dispersal from South and Central America, and some 

 of the same species may have spread from the West Indies to 

 become established in southern Florida (Caldwell and Martorell, 

 1952; Ross, 1959c). In many instances of this kind, including both 

 plants and animals, vagrant species new to the community may 

 be close relatives to and competitors with established species. Emer- 

 son ( 1949 ) points out that myriads of dispersals never become 

 established in the community because, for one reason or another, 

 the latter is unsuited ecologically to the vagrant. However, if the 

 vagrant does have ecological tolerances which suit it to the com- 

 munity it reaches, then the species stands a good chance of becom- 

 ing established. One of the best examples of this is the establish- 

 ment of northern European beetles accidentally introduced into 

 Canada (Lindroth, 1957). 



EXTENSION OF ECOLOGICAL TOLERANCE 

 IN NEIGHBORING SPECIES 



It seems inevitable that chance mutations or other genetic changes 

 would occasionally fit a species living in one community to extend 

 its range into a neighboring one. In the caddisfly genus Triaenodes 

 the more primitive members of certain lines are members of small 

 stream communities, but one species, T. tarda, has become adapted 

 to lakes also and is now a common member of northern lake com- 

 munities (Ross, 1959/?). Another species in the same genus, Triae- 

 nodes injusta, has also become almost completely restricted to 

 lakes (Ross, 1959a). In each instance a species, of Triaenodes suc- 

 cessfully extended its range from one distinctive aquatic commu- 

 nity to another. A striking example involving a climatic shift is found 

 in the mosquito genus Aedes. Its subgenus Aedimorphus contains 

 94 species which occur only in tropical and subtropical commu- 

 nities, and a single species, A. vexans, which has become adapted 

 to temperate and boreal situations and is an abundant Holarctic 

 component of many northern communities (Stone, Knight, and 

 Starcke, 1959). The hybrid population of Amaranthus in the San 

 Joaquin delta in California studied l^y Tucker and Sauer ( 1958 ) 

 is one of many examples in plants in which a hybrid population 

 has arisen and successfully colonized an adjacent community dif- 

 ferent from those communities inhabited by the parental forms. 



The Old World grasses of the Dicanthium anmdatum complex 

 afford another interesting example of species wandering due to 

 genetic change (Fig. 103) (Celarier, Mehra, and Wulf, 1958). 

 Present forms of this complex apparently began as a diploid species 

 in India. This species presumably hybridized with a closely related 



