Species and Species Change 



121 



is one of ecological zonation, for example the upland versus the 

 lowland species of Salix (willows). In animals the difference may 

 be in food habits, such as is found in many moths, sawflies, leaf- 

 hoppers, and certain other insects in which closely related species 

 feed on different host species ( Clarke, 1952 ) . If no other difference 

 is noted, each species has some difference in range or population 

 density, both of which are expressions of physiological differences 

 and their ecological manifestations (Fig. 47). Many of these 

 biological differences are modal rather than absolute, especially 

 those concerning range, density, or ecological segregation within 

 the same area. Only in the examples of food specificity and behavior 



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 Q. 



lawsoni arta 



usitata torella morgan! bella 



Fig. 47. Order of abundance of leafhopper species of Enjthroneiira coexist- 

 ing on sycamore in Illinois. In the tabulation of each species, the left column 

 represents collections made prior to and during 1954; the center column, 

 collections made in 1955; the right column, total of all collections. The per- 

 centage of these rankings gives a measure of the differences in population 

 density between the six species, all of which have greatly overlapping ranges. 



