14. DISSOLVED SALTS AS LIMITING FACTORS 



An environmental factor exerts a limiting 

 influence when, despite the favorable na- 

 ture of the remainder of the environment, it 

 comes to control the habitat because of 

 scarcity or overabundance. The amount of 

 available moisture in arid regions is an ex- 

 ample of a limiting factor; in many places, 

 after irrigation, the xerocole biota of the 

 typical desert is replaced by a much more 

 abundant biota of mesocoles. Accumulated 

 water in swamps, bogs, ponds, or streams 

 is also a limiting factor for obligate xero- 

 coles or mesocoles. Low temperature acts as 

 a limiting factor in winter in the higher 

 latitudes or altitudes, as does heat toward 

 the equator. Oxygen is a limiting factor in 

 the hypolimnion of thermally stratified lakes 

 and in higher altitudes on mountains. 



Historically, this field was first crystal- 

 lized around the principle of the minimum 

 that was brought into prominence as Lie- 

 big's "law of the minimum" (1840) and 

 was later restated by Blackman (1905) as 

 his "law of limiting factors." The reaction 

 of organisms to these limiting influences is 

 sufficiently regular and widespread to be 

 summarized by Shelford's "law of tolera- 

 tion" (1911c). This generalization considers 

 the reaction to both minimal and maximal 

 quantities of difiFerent environmental fac- 

 tors. 



The principle of the minimum is well 

 illustrated by the efi^ect of certain nutrient 

 salts upon the density of basic plant popu- 

 lations, such as those of the plant plankters 

 in water, that form the broad foundation for 

 the food pyramid (p. 501) for aquatic ani- 

 mals. The use of dissolved chemicals to 

 illustrate and test this principle is appro- 

 priate, since Liebig originally worked with 

 the relations of mineral nutrients, especially 

 those in the soil, to plant growth. A state- 

 ment of this principle by Johnstone (1908, 

 p. 234) is a convenient point of departure 

 for the present brief discussion. 



"A plant requires a certain number of food- 

 stuffs if it is to continue to live and grow, and 

 each of these food-substances must be present 

 in a certain proportion. If one of them is 

 absent the plcnt will die; and if it is present in 

 minimal proportion the growth will also be 

 minimal. This will be the case no matter how 

 abundant the other food-stuffs may be. Thus 

 the growth of a plant is dependent on the 



amount of the food-stuff which is presented to 

 it in minimum quantity. Marine plants require 

 certain things— carbonic acid, nitrogen com- 

 pounds, silica, phosphoric acid, and certain 

 mineral salts. The carbonic acid and the 

 mineral salts are present in relatively large 

 amounts, but the proportions of nitrogen com- 

 pounds, silica, and phosphoric acid in the 

 water of the sea are very small. The density of 

 the marine plants will therefore fluctuate ac- 

 cording to the proportions of these indispensa- 

 ble food-stuffs." 



It is not easy to recognize a supposedly 

 limiting factor in nature. Whether the 

 limitation is imposed by the scarcity of 

 energy or substance, the needed information 

 includes: 



1. The amount of the limiting factor reg- 

 ularly present and the limits of its normal 

 and its occasional variations. 



2. The rate of input of the hmiting factor 

 into the ecological system under considera- 

 tion and its variations. 



3. The rate of consumption of the limit- 

 ing factor. 



4. The degree to which some vital activ- 

 ity, such as rate of population growth, is 

 affected by the low concentration of the 

 limiting factor. 



5. The time relations; usually observa- 

 tions must be continued for a considerable 

 period of time in order to understand the 

 causal relations underlying the observed se- 

 quences of pulse of increase and decrease 

 of the limiting factor in relation to the 

 process limited. 



Information concerning the amount of a 

 limiting factor that is regularly present is 

 relatively easy to secure within the limits of 

 accuracy of approved quantitative methods. 

 Estimates of the rate of use are characteris- 

 tically based on measurements and calcula- 

 tions dealing with accumulations in the 

 biota or in some part of it. Input of radiant 

 energy excepted, trustworthy data about the 

 rate of input are usually hard to come by. 



The rate of use may be so closely related 

 to the rate of input that the amount of the 

 limiting factor present in the eco-system 

 may remain constant or almost constant. 

 Sometimes, as with the amount of carbon 

 dioxide in the atmosphere, the small supply 

 regularly present— 0.03 per cent, or a total 

 of 1700 X 10' tons— compared with the 



198 



