BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



235 



a species operates) as well as the more spe- 

 cific and limited elements in the larger com- 

 munities, such as a type of fungus or mouse 

 nest. Niche is not here employed in the 

 sense of a particular role in a food chain or 

 pyramid, though the two concepts may in 

 some senses overlap (see p. 232). 



Every modification of the inorganic sub- 

 strate by a single species of plant or animal 

 is seized upon by a series of successional 

 forms that exploit the gains made by the 

 first, and this successional series tends by 

 increasingly complex interaction toward the 

 organic balance of a climax. Modifications 

 of the organic substrate, and herbivore ex- 

 ploitation of the habitat niches provided by 

 the strata of the plant matrix (soil, humus, 

 dead leaf cover, plant thicket, tree trunk, 

 trunk cavity, forest roof, pp. 478-495), pro- 

 vide corresponding environmental oppor- 

 tunities to animal predators, which may be 

 temporary invaders of these niches or may 

 become completely adjusted to them. The 

 series of elements of the plant matrix has 

 an invading series of larger forms, such as 

 nematodes, earthworms, pine mice {Pity- 

 mys), cottontail rabbits, squirrels, and the 

 leaf-eating insects, with the secondary series 

 of soil mites, moles, weasels, foxes, martens, 

 and fishers, and the great number of insect- 

 eating birds to prey upon the insect horde. 



The basal biotic strata are characterized 

 by vast numbers of microscopic and minute 

 forms. These compose the edaphon of 

 France (1914, p. 111). It is such complexes 

 of vast numbers of minute plants and ani- 

 mals that most evidently form the "biotic 

 environment" of larger forms. In the eda- 

 phon, as in the plankton, there are larger 

 forms Uke the gophers and moles and per- 

 haps the badger. The plankton most evi- 

 dently composes a biotic environment for 

 such forms as the whale-bone whales or the 

 sieve-bearing appendiculates. In the marine 

 environment the balance between inorganic 

 food, microscopic and macroscopic plants 

 and the pyramid of predators in plankton 

 and nekton has reached a perfection that 

 doubtless corresponds with the age of this 

 environment. The much greater variety of 

 terrestrial communities, and their inferior 

 areal extent, may be thought to reflect their 

 relative youth. 



Among the terrestrial communities, the 

 blanket of edaphon grades insensibly from 

 its cUmax of complexity in the moist soil of 

 forests to the minimum of bare rock or rock 



desert and of the polar ice fields. Chemical 

 lactors produce local pessima. The edaphon 

 is not without direct similarity to the 

 fresh-water plankton or even with the 

 plankton of the sea. The concept of a biotic 

 environmental matrix thus logically supple- 

 ments that of the plant matrix, and leads 

 directly to the concept of the environment 

 as holocoenotic (p. 87). Indeed, the eda- 

 phon affords a biotic matrix for the plant 

 societies that draw upon it for support and 

 nourishment. 



The edaphon is in turn greatly affected 

 by the mechanical and chemical influences 

 of invaders from higher strata or from other 

 communities, such as burrowers whose 

 excavations are retreats or nests and do not 

 involve the active fife of the burrowing 

 animal. Though directly related to the 

 higher plants that root in it, and thus to 

 the whole of the upper communities, the 

 edaphon has perhaps a continuity that may 

 underly many of the more conspicuous 

 successional phases of the whole complex. 

 The independence and complexity of the 

 edaphon reflect great evolutionary age. 



BIOTIC MODIFICATION OF MEDIUM 



The biotic nature of the environment of 

 the individual animal is intimately affected 

 by adjustment to the biochemistry of its 

 own species and presumably also to that of 

 all the species of its natural communal en- 

 vironment. How deUcate the biochemical 

 balance may be is shown by the long series 

 of studies on conditioning of the water 

 medium by aquatic animals in causal rela- 

 tion to aggregation (Allee, 1931, 1938; also 

 p. 398). It seems evident that such condi- 

 tioning of the environment must extend to 

 the vast aquatic communities in nature and 

 to the edaphon, whose elements are largely 

 dependent on soil moisture. 



The complexity of biochemical relations 

 is further exemplified by the "odor envi- 

 ronment," to which many animals have 

 made elaborate adjustments. 



Biotic Pressure 



The concept of biotic pressure within the 

 environment of an animal, made familiar 

 by Chapman (1931) under the name biotic 

 resistance, includes the competition of any 

 given individual with its fellows of the same 

 population, as well as the competition of 

 other animals with similar food habits or 

 with similar shelter requirements (p. 648). 



