106 A NATURALIST IN THE GREAT LAKES REGION 



The amount of oxygen contained in the water plays a very 

 important part in determining the distribution of the water- 

 breathing species. Some can live only where there is an abun- 

 dance of it. Thus, some sorts of insect larvae, snails, and fish 

 are found only in the rapids of the brook where the constant 

 turmoil of the water brings in an abundant oxygen supply; such 

 will not live at all in the nearby quieter reaches. Other kinds, on 

 the contrary, can get on very well even in the stagnant pools 

 where the decomposing organic debris is abundant, the oxygen 

 content low, and the carbon dioxide content high. The details 

 of such distribution will be considered in a later chapter on the 

 brook community. 



Another exceedingly important factor in determining the dis- 

 tribution of animals, probably the most important locally, is the 

 location of the food supply. Thus one sort of animal may feed 

 on a single species of plant or those of one genus, and then its 

 distribution is determined by that of its food plant. Anosia 

 plexippus, the monarch butterfly, is found the world over only 

 where the milkweed grows. This is not only because the butterfly 

 feeds upon its blossoms, by no means exclusively, however, but 

 because the young feed on the leaves. Similarly the young of the 

 anglewing butterflies are reared on violet leaves and locally they 

 are abundant along the borders of wood or on moist prairies where 

 the violet abounds. One hunts for the pawpaw butterfly, 

 Papilio ajax, and for the spicebrush swallowtail, Papilio troilus, 

 only in the climax forest, though of course occasionally one may 

 wander some distance from its customary habitat. This mobility 

 of animal life as contrasted with the fixity of most plants makes 

 it more difficult to fix the limits of animal associations; and yet, 

 making due allowance for the wanderer, they are quite as definite. 

 So important is the food plant that one could almost name animal 

 communities after the plants that serve as centers of attraction, 

 either directly as a food or indirectly by harboring animals that 

 are the prey of the carnivorous types. The common milkweed 

 has such a host of visitors nearly four hundred, though not all 



