DISSOLVED SALTS AS LIMITING FACTORS 



205 



as a substitute for iron in lakes and usually 

 occurs in lower concentrations; it is more 

 abundant than iron in humus lakes (Ohle, 

 1934). 



The extent to which other elements pres- 

 ent in the sea in trace concentrations— cop- 

 per, manganese, cobalt, and vanadium, for 

 example— are limiting factors remains largely 

 for future investigations. There is little 

 doubt that these elements exert influences 

 on the distribution and other ecological re- 

 lations of at least some organisms. 



Though the ecology* of trace concentra- 

 tions is still largely an untouched field of 

 study, the eflFect of minute amounts of cop- 

 per on the ecology of some marine organ- 

 isms has already been established. The 

 settling and attachment ("setting") of oys- 

 ter veliger larvae (Ostrea virginica) in Con- 

 necticut are most pronounced at the stage of 

 the tide when the copper content of littoral 



Fig. 47. At Milford Harbor, Connecticut, the 

 more copper present within the eflFective range, 

 the greater the rate of attachment of oyster 

 larvae. ( Redrawn from Prytherch. ) 



waters is highest and within the range of 

 0.05 to 0.5 mg./L. At this concentration the 

 number of oyster larvae that respond to 

 copper stimulation is directly proportional 

 to the amount of copper present (Fig. 47). 

 Copper is leached from the land and 

 brought to the littoral region of the sea by 



* For a discussion of the physiology of trace 

 elements, see the review of Pirschle (1938). 



surface and underground waters (Prytherch, 

 1934), and occurs near Woods Hole at a 

 concentration of between 0.01 and 0.02 

 mg./L. (GaltsoflF, 1943). 



Another phase of the ecology of trace ele- 

 ments should be mentioned. Granted that 

 little of such substances as copper or cobalt 

 is needed by the organism, does the 

 environment furnish even these small 

 amounts? Sometimes it does not; often it 

 does. A rough calculation indicates that in 

 sea urchin eggs stimulated by 10" molar 

 solution of copper salt (Finkel, Allee, and 

 Garner, 1942), the copper is some twenty- 

 five times more concentrated in the eggs 

 than in the surrounding sea water. Other 

 cells are known to make concentration of 

 this order of magnitude (cf. p. 167). The 

 capture of ions of the heavy metals should 

 be no more difficult than that of lighter 

 metals such as magnesium and sodium and 

 may well be easier, since heavy metals are 

 readily adsorbed by organic surfaces. 



The density of a species population that 

 spends its life history in one habitat niche 

 depends on the most unfavorable environ- 

 mental factors in that niche during the part 

 of the Ufe history when resistance is lowest. 

 Low resistance frequently accompanies 

 some phase of the breeding cvcle or comes 

 during early development. The same gen- 

 eralization holds when emigration and peri- 

 odic migrations are considered, except that 

 then the habitat includes the geographic 

 range occupied by migrating individuals of 

 the population in question. 



The discussion of Liebig's 'law of the 

 minimum" and its various extensions, like 

 the treatment of different environmental 

 factors throughout this whole section, has 

 been on the assumption that each of these 

 factors acts independently of the others. 

 This assumption has been a matter of con- 

 venience known to be contrary to the facts 

 (p. 87). It has become increasingly dif- 

 ficult to maintain the fiction of the inde- 

 pendence of the difi^erent environmental 

 factors, and the need to do so no longer 

 exists even for pedagogical reasons. The 

 limiting action of one of the dissolved min- 

 eral nutrients mav be strongly affected by 

 the amount of others present. The applica- 

 tion of an incomplete fertilizer deficient in 

 potassium, nitrogen, or phosphorus, for land 

 plants, leads to an increased use of the other 

 two and may actually result in a decreased 

 absorption of the omitted element from the 



