POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



361 



the osmotic pressure is raised by the addi- 

 tion of sea water. This is a straight physio- 

 logical observation. Also, their survival in 

 such dilute media is increased by popula- 

 tion conditioning. This conditioning may 

 be either homotypic, as was seen when, 

 upon the early death of grouped worms, 

 needed products were liberated into the 

 water or in the "worm soup" experiments; 

 or it may be heterotypic, since it has been 

 demonstrated that living, fresh-water pla- 

 narians so condition the environment for 

 Procerodes that the latter survive longer. 

 Thus, the conditioning process appears to 

 have two principal aspects. The first is con- 

 cerned with osmotic regulation in the sense 

 that conditioning raises osmotic pressure 

 by liberating salts. This is a more general 

 aspect. The other is more specific and re- 

 fers to the demonstration that dead and 

 disintegrating worms, or, more slowly, liv- 

 ing worms, release calcium into the water. 

 This available calcium has a protective ac- 

 tion for marine animals put into distilled 

 water out of all proportion to its efiFect on 

 osmotic pressure. 



The Procerodes study illustrates popula- 

 tion conditioning (1) through the libera- 

 tion of some needed substance into the 

 environment, and (2) osmotic regulation 

 of the aquatic environment. Such condi- 

 tioning probably at times confers survival 

 value upon the worms in nature. 



Physical Conditioning of the Substratum. 

 Probably the most usual sort of condition- 

 ing in the sense of frequency of occurrence 

 involves the various alterations a popula- 

 tion impresses upon its physical substratum. 

 Although this usually is not thought of as 

 a conditioning process, it actually is one in 

 that it falls within the definition presented 

 earlier (p. 352). While the matter cannot 

 be developed in detail, in part because of 

 lack of space, but primarily because of lack 

 of quantitatively studied cases, a few com- 

 ments are appropriate simply to put the 

 issue in proper perspective. 



It seems self-evident that most natural 

 groupings, whether intraspecies or inter- 

 species populations or communities, modify 

 in one way or another the substratum upon 

 which or in which they live. The degree 

 of such modification obviously varies with 

 the size of the group under consideration. 



Many examples of such substratum con- 

 ditioning come to mind: the dissection and 

 tunnelling of soil by burrowing forms such 



as earthworms, insect larvae, certain ro- 

 dents; the modification of the sea bottom 

 by populations of annelid worms; the crea- 

 tion of special microhabitats in logs through 

 the decomposition and/or comminution 

 activities of such forms as bacteria, fungi, 

 and wood-boring insects (see Savely, 

 1939); or the modification of the substra- 

 tum by forest-floor forms living in the inter- 

 face between the soil and such objects rest- 

 ing upon it as leaves, logs, boards, and so 

 forth. This last niche is a modification of 

 such significance that a fairly particular 

 fauna or assemblage, the "cryptozoa," in- 

 habit it (see Cole, 1946). 



Investigations that approach this field 

 problem as a study in conditioning, and 

 that analyze the relations between the .size 

 and composition of a particular population 

 or set of populations and the amount and 

 pattern of substratum alteration that re- 

 sults, are to be desired. These findings then 

 should be appraised as they relate to the 

 subsequent life history of the populations 

 in question. As an extension of this topic 

 the reader should refer to Section II, par- 

 ticularly the discussion of soils (p. 216), 

 and to the pertinent botanical literature on 

 succession as aflFected by successive modi- 

 fication of the substratum. 



Microclimate Primarily Dependent on 

 Density 



The point has already been made that 

 for many instances the primary effects of 

 climate upon populations are essentially? 

 independent of density. There is evidence, 

 however, that the microclimate (p. 211), 

 i.e., the climate of the immediate environ- 

 ment in which organisms live, may be 

 secondarily aflFected by crowding. Whether 

 this is merely incidental for a particular 

 species or is actually significant in respect 

 of group survival becomes a matter for em- 

 pirical analysis. 



Michal (1931) worked with large larval 

 cultures of the mealworm Tenebrio in 

 which the larvae were dispersed, not at 

 random in the medium, but grouped into 

 aggregations or "nests." He was able to 

 show that the temperature within this ag- 

 gregation was higher than that of the air 

 inimediatelv above the culture surface and 

 that the former temperature tended to ap- 

 proach the optimum for development more 

 closely than did the latter. In other words, 

 when the external temperature was un- 



