clianges occur as tlie result of organisms living in it. 

 Compared with unconditioned water, these changes 

 may have either a harmful or a beneficial eflfect on 

 organisms introduced into the water after the original 

 organisms have been removed. Water is saitl to be 

 hoiitotyf<i(ally conditioned when the changes were 

 previously produced by individuals of the same spe- 

 cies as being studied and licterotyf^ically conditioned 

 when the changes were produced by a different spe- 

 cies. 



Harmful effects of conditioning on longevity, 

 growth, or reproduction are more easily e.xplained 

 than are beneficial effects. Harmful effects are 

 often consequences of oxygen depletion, reduction in 

 food resources, accumulation of excreta, or secretion 

 of toxins or growth-inhibiting substances. Over- 

 crowding of frog tadpoles in culture dishes is asso- 

 ciated with the occurrence of peculiar round vacuo- 

 lated cells in the intestinal tract and feces that appear 

 responsible for curtailment of further growth (Rose 

 1960). 



Under laboratory conditions, so-called killer 

 stocks of Paravicciitw aurclia produce a toxin, para- 

 mecin, at the rate of one unit-particle per animal per 

 five hours. One unit-particle is enough to kill one 

 individual of so-called sensitive stock of the same 

 species as well as being lethal to other species of 

 Paramecium (Austin 1948). Conditioning that be- 

 comes unfavorable homotypically may sometimes be 

 favorable, or at least tolerable, heterotypically. Thus 

 in protozoan infusions there is a microsere of one spe- 

 cies succeeding another. 



Experimental studies, on the other hand, have 

 demonstrated that goldfish grow faster in water that 

 has been homotypically conditioned for 24 hours than 

 in unconditioned water. Both fish and amphibian lar- 

 vae also do better in water conditioned by the pres- 

 ence of mollusks than in unconditioned water (Shaw 

 1932). The marine flatworm Procerodcs zvheatlandi 

 will survive mucii longer when transferred to fresh 

 water conditioned by the presence of either live or 

 dead individuals of the same species or by fresh-water 

 species of flatworms than they do in unconditioned 

 fresh water. The longer survival in toxic solutions, 

 faster growth, and greater reproduction of protozoans, 

 snails, flatworms. cladocerans, amphibian larvae, and 

 fish occurring in aggregations rather than as isolated 

 individuals is attributable to water conditioning. 



Various factors are involved in producing favor- 

 able conditioning: minute organic particles in sus- 

 pension resulting from excreta, regurgitated food, or 

 disintegration of dead animals previously present 

 may become concentrated in the alimentary tract of 



tlic animals and serve as an unsuspected fuud re- 

 source (Allee and Frank 1949) ; mucus or slime 

 secreted by organisms may coagulate, precipitate, or 

 reduce the potency of toxic substances : salts liberated 

 from the body may change the osmotic properties of 

 the culture medium ; or there may be liberation of 

 growth-promoting substances from one animal that 

 affects other animals (Allee ct al. 1949). Many of 

 these effects, both favorable and unfavorable, are 

 doubtlessly at work in natural habitats and should be 

 carefully studied as part of the internal dynamics of 

 the biotic community. It may well be, for instance, 

 that during the course of evolution organisms have 

 become adapted to tolerate or take advantage of these 

 external metabolites given off by their neighbors with 

 the result that the metabolites have become an im- 

 portant part of their environment (Lucas 1947). 



SUMMARY 



The characteristics of soil are determined 

 by the parent rock material, the reactions of plants 

 and animals, and climate. The burrowing of earth- 

 worms, ants and other ground insects, crayfish, and 

 rodents brings subsoil to the surface where it be- 

 comes mixed with humus. Animal metabolic proc- 

 esses aid in the formation of humus by breaking down 

 complex organic matter into simpler compounds 

 which the animals then excrete. Bacteria, actino- 

 mycetes, and fungi are doubtless even more im- 

 portant in this respect. Animals require nitrogen, 

 carbon, oxygen, hydrogen as well as some 13 other 

 elements, and hence are involved in nutrient cycles of 

 these elements in the ecosystem. 



Climate is directly involved in the weathering of 

 soil particles, insofar as rainwater percolating into the 

 ground carries nutrients into the soil, and indirectly 

 in determining the kind and luxuriance of the vege- 

 tation and animal life that occurs in the area. As a 

 result, mature soils of climax stabilized ecosystems 

 have profiles characteristic both of types of vegeta- 

 tion and of climatic regions. The species composi- 

 tion and density of ground animals vary with the 

 profile horizon and with the various soil-types found 

 in various parts of the world. 



Nutrient cycles occur in aquatic as well as ter- 

 restrial ecosystems. Organisms may modify or con- 

 dition the chemical and nutrient characteristics of 

 aquatic habitats in various ways to affect the occur- 

 rence of other individuals of the same or different 

 species. 



Reactions, soil formation, and cycles 173 



