One may argue with considerable justification 

 that animals such as aphids, gall wasps, some bugs, 

 and others that show host specificity are really para- 

 sitic rather than herbivorous in their feeding be- 

 havior, and hence are not good examjiles of free- 

 living animals of restricted diet. Actually it is very 

 difficult to find proven cases of animals that confine 

 themselves to a single species of food. It is more 

 common to have an animal feeding on a small group 

 of related species, as do aphids. The potato beetle 

 originally fed chiefly on the sand-bur Solanum rostra- 

 tiim in the Rocky Mountains until about the year 

 1859, when it began to infest the potato Solanum 

 tuberosum and spread across the country. There may 

 be a biochemical reason for the preference of animals 

 for related plant species, recognized by similarity in 

 taste, odor, or nutritional values. 



Herbivorous species may be classified in respect 

 to the diversity of their food into : 



Monophagous : restricted to a single food plant 

 Oligophagous : restricted to a few very definite 



food plants 

 Polyphagous : feed on many species. 



The restriction of animals to particular foods may 

 be the result of chemicals affecting odor or taste, or 

 to structural adaptations. Chemical stimuli are espe- 

 cially important with insects (Dethier 1947). The 

 crossed bills of certain birds (Loxia sp.) are well 

 fitted for prying seeds from between the bracts of 

 coniferous tree cones. The Siberian nutcracker has 

 special structures in its bill for cracking the nuts of 

 the Siberian cedar on which it depends almost ex- 

 clusively for food (Formosof 1933). 



In spite of the monophagy exhibited by some spe- 

 cies, many herbivorous species have a wide choice of 

 food ; the bobwhite quail in Georgia is known to feed 

 on 927 different food species, 107 of them regularly. 

 These are mostly seeds and fruits, although about 14 

 per cent of the food of this species consists of insects 

 and spiders taken chiefly during the summer months 

 (Stoddard 1931). Restriction of feeding to a single 

 or a few species is a specialized behavior. Feeding 

 on a wide variety of substances or prey usually rep- 

 resents the more generalized primitive condition 

 (Dethier 1954). 



FOOD CHAINS 



A single food chain should have at least 



three links to be complete : plant ^herbivore > 



carnivore. Very often, however, a small carnivore or 

 omnivore may be preyed on by a larger carnivore, 

 and so on until four or five links are involved. Rarely 

 are food chains longer than five links. An example 



of a three-link chain occurring on the North .Ameri- 

 can (ireat Plains is: grass »pronghorn ►coyote. 



A four link chain common to deciduous forest com- 

 munities might include tree foliage >leafhopper 



»vireo >hawk. A five-link chain would have to 



involve a number of small s]x-cies as bacteria > 



protozoan ^rotifer »small fish »large fish. A 



food chain does not need to start with a living plant ; 



consider, for instance : detritus »snail >shrew 



»owl. A strict predator need not necessarily be 



the last link, it could be an omnivore : flowers * 



bees »bear. Saprovores do not fit into food chain 



diagrams very well because they feed on all links of 

 the chain. Food chains occur in all kinds of habitats 

 and communities, even with the micro-organisms of 



the soil : detritus >nematodes »mites »pseu- 



doscorpion. 



The feeding coactions between the many species 

 that constitute the community are seldom as simple as 

 the food chains just described. The rotifer feeds not 

 only on protozoans but on bacteria. The small fish 

 feeds on insect larvae and many other plankton spe- 

 cies besides the rotifer. The shrew in the forest feeds 

 not only on snails but on a variety of insects, and is 

 fed upon in turn not just by owls but also by hawks, 

 foxes, weasels, and others. If all of these feeding re- 

 lations between species in a community were dia- 

 grammed, a complicated web would be formed — the 

 so-called food zveb. 



BALANCE OF NATURE 



Charles Darwin explained, a hundred years 

 ago, that there was a balance in nature between the 

 abundances of plants, herbivores, and carnivores. 

 Were carnivores for some reason to increase unduly 

 in numbers, they would soon exhaust their food sup- 

 ply and die of starvation. On the other hand, were 

 food plants or herbivores to fluctuate excessively, 

 then predators would vary in a similar manner. 

 There is no doubt that marked variations in abun- 

 dance in one link of a food chain will cause variations 

 in the other links. Rodent plagues in a local area will 

 bring an influx of foxes and hawks ; a spruce bud- 

 worm oubreak will result in an increase in the bird 

 population ; increases and decreases in soil bacteria 

 are correlated inversely with decreases and increases 

 in soil amoebae that feed upon them (Russell 1923). 

 As a case in point, the relation between the mule 

 deer population and its predators on the Kaibab 

 Plateau in northern Arizona is worth citing in detail 

 (Rasmussen 1941). When this area was made a 

 game preserve in 1906, killing of deer for sport was 

 prohibited. At the same time, there was a marked de- 

 crease in grazing by domestic sheep. During the 

 period from 1906 to 1939, there were 816 mountain 



Food and feeding relationships 195 



