Table 11.1. Essential elements (Clapham, Jr., 1973):
Macronutrients: Elements used in relatively large quantities.Micronutrients: Elements generally needed in relatively small quantities.Micronutrients: Elements needed by certain species in relatively small quantities.
Some of these tabulated elements are used in relatively large quantities as the fundamental building blocks of organic tissues and are called macronutrients. Others, the micronutrients, are used in much smaller quantities.
The micronutrients are also known as trace elements. For instance, a crop such as corn will remove over 45 kg of nitrogen per acre from the soil but only 10-15 g. of boron. In addition to the nutrients required by all species, some organisms have special nutrient requirements.
Liebig-Blackman Law of Limiting Factors:
An organism is seldom, if ever, exposed solely to the effect of a single factor in its environment. On the contrary, an organism is subjected to the simultaneous action of all factors in its immediate surroundings. However, some factors exert more influence than do others and the attempt to evaluate their relative roles had led to the development of the law of the minimum by the German biochemist Justus Liebig in 1840.
Liebig while investigating the relationship between the available amounts of essential elements and plant growth, discovered that crop yield was frequently limited by elements other than those utilized in the largest quantity. Freely translated, a part of his statement on his experimental results is that “growth is dependent on the amount of food stuff that is present in minimum quantity”.
This statement has come to be known as Liebig’s law of minimum. It is now usually incorporated with a law of limiting factors developed by a British physiologist F. F. Blackman (1905), who at the beginning of this century investigated the factors affecting the rate of photosynthesis.
He listed five factors involved in controlling the rate of photosynthesis: amount of CO2 available, amount of H2O available, intensity of solar radiation, amount of chlorophyll present, and temperature of the chloroplast. Blackman discovered that the rate of photosynthesis is governed by the level of the factor that is operating at a limiting intensity.
Further work on limiting factors and substances has shown that a high level of one factor will modify the limiting effect of a second, a process described as factor interaction. The same principle of limiting factors applies well to animal functions.
Since the rate of a process may be controlled by too great an amount of a substance ; such as heat, as well as by too small an amount, and since the presence or abundance of an organism may be limited by a variety of environmental factors, biotic as well as abiotic, and since the limiting effect may be due to two or more interacting factors rather than a single isolated one (Shelford, 1932) the law of minimum has been restated by Taylor (1934) in broad ecological terms as follows:
The functioning of an organism is controlled or limited by that essential environmental factor or combination of factors present in the least favourable amount. The factors may not be continuously effective but only at some critical period during the year or perhaps only during some critical year in a climatic cycle.
Threshold and Rate:
Every environmental factor varies through a wider range of intensity than any single organism can tolerate. Characteristically, there is for each individual organism a lower and an upper limit in the range of an environmental factor between which it functions efficiently. For any one factor, different organisms find optimal conditions for existence at different points along the range, hence their segregation into different habitats.
The threshold is the minimum quantity of any factor that produces a perceptible effect on the organism. It may be the lowest temperature at which an animal remains active, the least amount of moisture in the soil that permits growth of a plant, the minimum intensity of light at which photoreceptor is stimulated, and so forth.
Above the threshold, the rate of a function increases more or less rapidly as the quantity of heat, moisture, light or other environmental factor is increased, until a maximum rate is attained. Above the maximum, there is usually a decline in the rate of a process either because of deleterious effect produced, the interference of some other factor or exhaustion.