BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



241 



while the appendiculates (e.g., Oikopleura) 

 are notable for the extreme elaboration of 

 their filter apparatus. The net-plankton 

 forms the immediate food of some of the 

 largest of the whales, or it may be fed upon 

 by fishes of various size grades, whose enor- 

 mous schools in turn provide food for larger 

 fishes, or for birds and mammals. Filter 

 feeding leads to extreme structural speciali- 

 zation. It should be noted that among filter 

 feeders the distinction between herbivorous 

 and carnivorous habits is not a sharp one 

 and that availability becomes the only 

 criterion governing the food supply. 

 Specialization in the direction of mono- 

 phagy, evident among land animals, is es- 

 sentially excluded by the conditions of 

 plankton feeding. 



In general, land animals fall rather 

 sharply into herbivores and carnivores, and 

 omnivorous types are the exception rather 

 than the rule. In some groups, however, like 

 the opossum, there may be no apparent 

 food preference, while in others, like the 

 pig, primarily herbivorous habits readily 

 give way if animal food is available. The 

 categories are in any case not absolute, for 

 extremely well-adapted herbivores may be 

 driven to animal food by scarcity (as the 

 reindeer may take to eating fish), while 

 carnivores, in the absence of suitable prey, 

 may eat a considerable proportion of plant 

 material. The specialization of feeding ap- 

 paratus in these two principal directions is 

 familiar in the giinding teeth of artiodactyls 

 and the flesh-cutting dentition of carnivores. 

 The more extreme limitation to plant or 

 animal food alone arises in connection with 

 specialization for feeding upon specific parts 

 or types of plants or animals (p. 701). 



Such further specialization for more spe- 

 cific tvpes of food leads to some of the most 

 remarkable and extreme adaptations and 

 transformations of animals. Thus, living 

 woody plants supply food to insects that 

 feed exclusively upon sap, such as aphids 

 and scale insects; other insects eat the 

 wood, either of the main stem or of the 

 twigs, and may depend on special layers 

 such as the bark, cambium, or the older 

 wood; still others feed exclusively on leaves, 

 and these are joined by a wide variety of 

 mammals and a few reptiles; minute insects, 

 the leaf miners, live between the surface 

 layers of the leaf, and thus feed only on the 

 softer part of the leaf tissue; hosts of in- 

 sects, birds, and mammals depend exclu- 



sively or primarily on the seeds or fruits; 

 still others are flower eaters, or are mi- 

 nutely speciahzed for feeding on pollen or 

 nectar; and the subterranean roots may fur- 

 nish food to burrowing animals.* These 

 adaptations are reflected in the systematic 

 categories of insects. 



Nectar feeding by insects, birds, and 

 bats, and occasional other mammals (espe- 

 cially the marsupial Tarsipes) , is enor- 

 mously developed in the insect group, hand 

 in hand with the evolutionary expansion of 

 nectar production by plants in correlation 

 with the benefits of cross fertilization. Othei 

 glandular secretions of plants are fed upon 

 by insects and may commonly be produced 

 by hypertrophied structures when some 

 benefit to the plant accrues (see p. 248). 

 The browse (twigs and leaves taken to- 

 gether) constitutes a special type of plant 

 food for the larger mammalian herbivores 

 and may be a sufiiciently exclusive food to 

 exhibit correlation of the food-taking struc- 

 tures, as in elephants, or in the African 

 black rhinoceros, whose finger-bke labial 

 appendage contrasts sharply with the square 

 lips of the grass-eating white rhinoceros. 



Herbaceous plants, except for the absence 

 of wood and bark borers and for the greater 

 number of root eaters, exhibit the same 

 series of specialized animal dependents as 

 do trees and shrubs. Ferns and their allies 

 appear to be little preyed upon. Fungi, on 

 the other hand, attract a great variety of 

 animals, including a number primarily de- 

 pendent upon them. Bacteria as food for 

 land animals are important only in the eda 

 phon, and the only specialists dependent 

 upon them are presumably the most minute 

 of single-celled animals. 



A further grade of food specialization 

 appears in the limitation of animals already 

 confined to a single type of plant food to 

 a restricted taxonomic group of plants (e.g., 

 a species, genus, or family). The distribu- 

 tional conditions set by the biotic environ- 

 ment for such monophagous forms are radi- 

 cally different from those set by the plant 

 environment for poh'phagous or omnivorous 

 creatures. 



The great group of scavengers that de- 

 pend upon the products of plant decay may 

 be mentioned in this connection, though 

 their food environment, while organic, is 

 essentially nonliving. Decay, however, is so 



• The extremes of insect food specialization 

 are discussed by Brues ( 1946 ) . 



