BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



237 



adjustment to the use of food from organic 

 sources. We may point to the ubiquity of 

 microscopic plants and their evolution of 

 resistant stages whereby their dispersal is 

 accompHshed. Exploitation of food supplies 

 seems to be the common factor in such 

 diverse phenomena as the adaptations of 

 the plants of rock surfaces, of desert plants, 

 of epiphytes and parasites, and of the plants 

 of rich soils or especially enriched areas of 

 the sea. 



There is a striking and fundamental 

 correlation of density of marine life with 

 continued fertihzation from a specific 

 source, like the influx of plant food at a 

 river mouth, or hke the tapping of deep 

 water supplies of dissolved substances by 

 an upwelling current. The fertihzation of 

 the sea bottom with a rain of dead organ- 

 isms produced by the interaction of the 

 radically different Labrador Current and 

 the Gulf Stream may be said to produce the 

 cod and halibut fisheries of the Grand 

 Banks. 



The density of plant populations on land 

 depends largely on available food (using 

 the term food in a broad sense), availability 

 being dependent on water supply. The rich 

 plant cover of the tropical forests reflects 

 the maximum use of the available plant 

 food, and with water in excess, other factors 

 than the food supply may limit its develop- 

 ment. Aside from such considerations (see 

 p. 562), the evolutionary diversity of the 

 tropical forest may be thought of in terms 

 of increased utilization of food supplies, on 

 the basic principle of diflFerences in food 

 requirements from the soil as well as in 

 terms of occupation of all possible niches, 

 as by lianas, epiphytes, and parasites. 



Succession in temperate climates, in 

 the change from simple transitory communi- 

 ties to complex and stable ones, while based 

 in part on the toxicity of the wastes of the 

 earlier types of the series, reflects improved 

 utilization of food by specific evolutions of 

 plants toward improved use of available 

 food and by the filling of all available 

 niches in which a food surplus develops. 



Finally, it may be pointed out that bac- 

 teria in general and anaerobic bacteria in 

 particular tap otherwise wholly unavailable 

 food supplies. 



In the whole evolutionary development 

 of the plant matrix, the production of an 

 excess of material by the individual plant 

 may be regarded as a factor of safety for 



the individual. It has been pointed out 

 that trees normally bear something like 

 50 per cent more leaves than are necessary 

 for growth and survival under normal or 

 average conditions (Clark, 1927). This ex- 

 cess of foliage becomes of vital importance 

 to the plant under the extreme conditions 

 which may arise at longer intervals in the 

 climatic or biotic cycles to which the in 

 dividual plant is exposed during its life. 

 The excess of fohage, and of other plant 

 substance, is in turn the basic food supply 

 of many animals, and the excess itself may 

 be thought to be further increased by the 

 response of plants to the benefits received 

 from the wastes produced by their animal 

 "enemies" (p. 496). 



A still further surplus of plant food is 

 supphed by the vast excess production of 

 spores, pollen, seeds, and of mature individ- 

 uals necessary for the survival of the 

 species. Progressive evolution seems to be 

 in general toward the reduction of this ex- 

 cess in plants as in animals, but there can 

 be no question that plant species face in- 

 creasing hazards to their survival with re- 

 duction of their populations below an op- 

 timum level, and that the excess of num- 

 bers is in part a factor of safety for the 

 species. The total surplus of food is thus 

 the excess of the surplus of the individual 

 multiplied by the total number of individ- 

 uals. 



The surplus of plant food is reflected in 

 the quite similar derived surplus of anima! 

 food. Excess populations of animals further 

 elaborate the various food chains, food 

 webs, and food pyramids. The development 

 of surplus animal food goes hand in hand 

 with the evolution of predaceous controls, 

 which depend on surplus populations, and 

 with the invasion of the niches supplied bv 

 the individual animal to parasites, which 

 depend on the individual's surplus of body 

 tissues. Since evolutionary success in the 

 direction of utilization of food supply tends 

 to produce an excess of individuals beyond 

 the capacity of the base of the food pyra- 

 mid (either plant or animal) to support, 

 predaceous controls become beneficial to 

 the oversuccessful herbivore or intermediate 

 prey, and these benefits afford the founda- 

 tion for the development of complexity in 

 the communitv. Evolutionary success of ani- 

 mals in the direction of reduced rates of 

 reproduction can apply only to the final 

 elements of a food pyramid. In a food 



