206 



ANALYSIS OF THE ENVIRONMENT 



already deficient soil. Similarly the absence 

 of potassium salts from a nutrient solution 

 may produce an increased absorption of 

 nitrogen and phosphorus (E. C. Miller, 

 1938), and there is much other evidence 

 that, with plants, the "law of the mini- 

 mum," though finally effective in extremes, 

 is much influenced in its action by the 

 combination of environmental influences 

 operating at the time. 



Vitamins also act as limiting factors. Al- 

 though little work has been done upon the 

 action of vitamins in nature, there is evi- 

 dence that growth-promoting accessories are 

 necessary for the growth of certain diatoms 

 (Harvey, 1939) and that these become 

 limiting in their action only when the quan- 

 tity present is very small. Hutchinson 

 (1943) reported that the thiamine (vita- 



min Bi) content of unfiltered water from 

 certain ponds or small lakes in Connecticut 

 lay between 0.03 micrograms (7) and 1.2 

 7 per liter. In Linsley Pond he could re- 

 move from 61 to 93 per cent by filtration. 

 Even half the amount of thiamine present 

 might be ecologically significant for pro- 

 moting growth of planktonic algae. Al- 

 though seasonal variations occur, no accu- 

 mulation of thiamine was found in the 

 hypolimnion at the end of stagnation. Un- 

 consolidated mud contained 2 to 3 7 per 

 gram of dry mud. A variety of plankters, 

 both plant and animals, living in these 

 waters were rich in thiamine. Growth ac- 

 cessories are produced by other organisms 

 and are, therefore, derivatives of the biotic 

 environment and should be included in a 

 complete discussion of biotic factors. 



15. COMBINATIONS OF ENVIRONMENTAL FACTORS 



Only when a given aspect of the environ- 

 ment approaches maximum or minimum 

 toleration limits for an individual, a popu- 

 lation, or a community does it become suf- 

 ficiently important to assume virtual control 

 of the ecological situation. Normally, as we 

 have just said, each environmental factor is 

 only one of a number of influences in a 

 given habitat, and organisms react to the 

 whole rather than to parts hypothetically 

 dissected out of it. Even the limits of toler- 

 ation for a given factor are partially set by 

 the extent to which the remainder of the 

 environment is favorable. 



A number of important interactions exist 

 between pairs or groups of environmental 

 factors. Heat and light are often closely 

 associated; in fact, unequivocal separation 

 of them as regulators of seasonal succession 

 does not seem to have been demonstrated 

 (Hutchinson, 1941). Heat and humidity 

 effects are also closely intermingled, and 

 heat, relative humidity, and wind combine 

 to form the environmental complex often 

 called "the evaporating power of the air." 

 This complex unit would be more signifi- 

 cant if it could include also the vapor pres- 

 sure of water both at the evaporating sur- 

 face and in the air above (p. 182). 



A less expected temperature relationship 

 is found in the strong evidence that the 

 shells of marine organisms from warm habi- 

 tats tend to have a higher proportion of 



magnesium to calcium carbonate than do 

 those of the same general taxonomic groups 

 from colder waters. The analyses by Clarke 

 and Wheeler (1922) show this correlation 

 for crinoids and alcyonarians and suggest it 

 for foraminiferans, crustaceans, and cal- 

 careous algae. They comment that the facts 

 are definite but unexplained. 



Reaction to a vertical gradient in the en- 

 vironment is easily oversimplified and re- 

 garded as a response to gravity. It may be 

 just that, or it may be a reaction to any 

 other stimulus, or to some combination of 

 stimuli, possessing a vertical differential. 

 Vertical gradients in light, heat, substratum, 

 turbulence, or pressure, may affect or con- 

 trol the reaction. In air, these may include 

 also an evaporation gradient; and in water, 

 vertical differences in density, viscosity, 

 mineral nutrients, and dissolved gases are 

 readily recognized. It requires direct inves- 

 tigation to find which one of these, or what 

 combination of them, is responsible for ob- 

 served reactions toward or against the pull 

 of gravity and for the distribvitions that re 

 suit from these reactions. 



Diurnal depth migrations, such as are 

 common among aquatic animals and are 

 made by some forest invertebrates like the 

 hemipteran Menecles (Park and Strohecker, 

 1936) and other land animals, may result 

 from the interaction of change in light in- 

 tensity, or other diurnal changes, with the 



