BJOTIC FACTORS IN RELATION TO INDIVIDUALS 



239 



grass, leaves, leaf- tissue (tapped by leaf 

 miners), nectar, insects of various size 

 levels, herbivorous mammals at various size 

 levels, appear to have afforded the oppor- 

 tunities lor adaptational evolution; further 

 steps in the same direction are to be seen 

 in the development of the still more specific 

 monophagy frequent in the insect-plant and 

 parasite-host relation (see p. 614). 



The preservation of primitive types of 

 animals may be accomplished by the ex- 

 tremes of food specialization made possible 

 by the very fact that they have had avail- 

 able long periods of time for their evolution. 

 This is especially clear when their adapta- 

 tions are correlated with an otherwise in- 

 completely tapped food supply, as with 

 sloths and anteaters. 



Caenogenesis is partly a response to a 

 food supply available to the separately 

 evolving stage, as is evident in specific food 

 adaptations of such stages. Adaptive radia 

 tion into specific environments and for spe- 

 cific foods is as evident among larval insects 

 or tadpoles as in adult animals. 



Symbiosis, which we define to include 

 commensalism, mutualism, and parasitism 

 (see p. 243), is a further evolutionary ad- 

 justment to more complete utilization of 

 food surplus. Social habits and social or- 

 ganization likewise involve efficient exploita- 

 tion of food supplies, whether of great vari- 

 ety, as by man, or of extreme uniformity, 

 as by termites. 



As was remarked with reference to the 

 sea and the forest, the concept of an organ- 

 ized interlocking community of plants and 

 animals involves the idea of maximum con- 

 tinued utilization of the food supplies avail- 

 able in the given environment. Develop- 

 ment of a complex food pyramid or of a 

 food chain depends on a basic food supply 

 and on the preservation of continuity in 

 that food supply by means of controls on 

 the predator superstructure. The evolution 

 of communities in the direction of increas- 

 ing complexity appears to be a direct cor- 

 relation with fuller utifization of existent 

 and developing food supplies (Schmidt, 

 1945). 



Basic Nutrient Cycles 



A number of chemical cycles exist in 

 which inorganic material becomes a part of 

 living protoplasm and is later returned to 

 the nonliving, perhaps even to the inor- 

 ganic, world. The return follows release as 



a result of metaboUc processes or decay an6 

 occurs either directly or, after transfer from 

 organism to organism, as in a food chain. 

 The water cycle, which in part runs such 

 a course, has been given in some detail (p. 

 177), and important aspects of other cycles, 

 particularly of the nitrogen cycle, will also 

 be discussed (p. 497). It may be re- 

 peated that the nitrogen of the air is largely 

 unavailable either to plants or to animals. A 

 small portion becomes usable when com- 

 bined under the influence of electric dis- 

 charges to form ammonia, nitrites, or ni- 

 trates (p. 199). Fixation of nitrogen also 

 occurs under the influence of nitrifying 

 bacteria (p. 711); those symbiotically asso- 

 ciated with the root nodules or tubercles 

 of clovers and of legumes in general form 

 a particularly intimate part of the biotic en- 

 vironment. Animals are mainly dependent 

 on plants for their nitrogen, although some 

 protozoa lacking chlorophyll can build their 

 own protein from nitrogenous salts without 

 ingesting plant proteins (Heilbrunn, 1943). 



The carbon cycle is based primarily on 

 the processes concerned with the photo- 

 synthesis of carbohydrates by chlorophyll 

 and the transformations of primary sugars 

 into related substances by plants and by 

 animals. Carbon dioxide is taken from the 

 surrounding air or water, used in photo- 

 synthesis, and returned sooner or later to 

 the external environment. 



Chemical cycles also include those deal- 

 ing with oxygen, phosphorus, and sulfur, 

 as well as somewhat similar ones of such 

 substances as iron, calcium, sodium, potas- 

 sium, iodine, and silicon. In fact, all chem- 

 ical elements composing the bodies of plants 

 or animals come on last analysis from the 

 inorganic environment. Many become in- 

 corporated in animals only or mainly 

 through the mediation of plants. With some 

 substances the cycle may be short and fre- 

 quently repeated; others are bound for 

 longer periods, perhaps, as with coal, for 

 geological ages (cf. Rogers, 1938). 



Impact of Kinds of Food* 



The biotic food environment of the indi- 

 vidual animal has been of profound evolu- 

 tionary influence in the direction of special- 

 ization, with the result that an animal with 

 a high degree of specialization is rigidly 



"Franz Doflein (1914), especially Chapter 

 2, pages 21 to 326, serves as a general 

 reference. 



