ECOLOGY 281 



Ecological Value of Species Relations. — It is clear that when the 

 relationship between animals of different species is such that one is 

 dependent upon the other, that relationship becomes a factor in deter- 

 mining the distribution. This is best exemplified lay the parasitic forms. 

 If the host is debarred from or exterminated from a certain habitat the 

 parasite is also debarred or exterminated because of its dependence upon 

 the animal with which it is associated. The same is true of symbiotes; 

 and when prey is not to be found in a region a carnivorous form is likely 

 to be excluded from it. At this point ecology merges into the field of 

 zoogeography. With restricted application the interrelation in habits 

 between species is a factor in the formation of animal communities or 

 associations as well as in the determination of geographic ranges. 



Plants as Food. — Perhaps the most obvious relation between the 

 plants and animals in a region is that which exists between plant-eaters, 

 the herbivores, and their food supply. Sometimes this relationship is very 

 close, the animal being dependent upon a certain plant for food. Many 

 insects are thus limited in the food which they take. The water beetle 

 Donacia piscatrix, which inhabits the yellow water-lily, is found elsewhere 

 only occasionally on the white water-lily; and the larva of the butterfly 

 Papilio philenor feeds only on one plant, the Dutchman's pipe. In other 

 cases a particular kind of food furnished by various plants may be 

 required. Thus there are browsing animals which feed upon twigs and 

 shoots of trees and bushes, grazing animals which eat grasses and similar 

 plants, bark-feeders, flower-feeders, mushroom-feeders, etc. For her- 

 bivorous forms with a restricted diet the nature of the vegetation is an 

 environmental factor. 



Plants as Protection. — The vegetation may also be an environmental 

 condition in other ways. Animals which require shade, e.g., some tree 

 frogs, find favorable conditions in the forest. Some animals breed only 

 in trees; for example, the beetle Buprestis nuttalli consularis breeds only 

 in the decayed wood of one species of pine, Pinus rigida. Other forms are 

 structurally adapted to life in trees, as the flying squirrel, and the sloth 

 (see frontispiece) which cannot walk except when suspended from a 

 limb. Associations held together by various kinds of dependence upon 

 vegetation may be easily pointed out. Thus the prairie faunas are com- 

 posed of many grass-eating ground forms, while the forest areas are in- 

 habited by browsing or other types which feed upon or breed or live in 

 trees. 



Gall-forming insects usually derive both food and protection from the 

 plants they inhabit. Some stage of the insect, most often the larva, hves 

 in or on the tissue of the host plant and in some way stimulates the 

 development of galls in which the insects are enclosed. This relation is 

 often a very special one, since certain insects produce galls only on the 

 hickory, some only on oak, some only on goldenrod, some only on the rose, 



