RELATED ECOLOGICAL CONCEPTS

LIMITING FACTORS

A major reason why organisms do not spread everywhere is that each species and each individual organism of a species has a particular range of tolerance to variations in chemical and physical factors in the environment, such as temperature.
The tolerance range includes an optimum range of values within which population of a species thrive and operate most efficiently. This range also includes values slightly above or below the optimum level of each abiotic factor- values that usually support a smaller population size. When values exceed the upper limits or fall below the lower limits of tolerance, few if any organisms of a particular species survive.
The law of tolerance states that the existence, abundance and distribution of a species in an ecosystem are determined by whether the levels of one or more physical or chemical factors fall above or below the levels tolerated by the species.

Related to the law of tolerance is the limiting factor principle. Too much or too little of any abiotic factors can limit or prevent growth of a population of a species in an ecosystem even if all other factors are at or near the optimum range of tolerance for a species.
A single factor, such as temperature, water, light or soil nutrients in terrestrial ecosystems and
salinity, temperature, sunlight or dissolved oxygen content in marine ecosystems, found to be limiting the population growth of a species in such ecosystem is called a limiting factor.


The Law of the Minimum
Justus von Liebig, generally credited with being the "Father of the Fertilizer Industry", propounded the "Law of the Minimum" which states that if one crop of the nutritive elements is deficient or lacking, plant growth will be poor even when all the other elements are abundant. Any deficiency of a nutrient, no matter how small an amount is needed, will hold back plant development. If the deficient element is supplied, growth will be increased up to the point where the supply of that element is no longer the limiting factor. Increasing the supply beyond this point is not helpful, as some other element would then be in a minimum supply and become the limiting factor.

The figure at the left exlains that the yield potential of a crop is like a barrel with staves of unequal length. The capacity of the barrel is limited by the length of the shortest stave (in this case, nitrogen), and can only be increased by lengthening that stave. When that stave is lengthened, another one becomes the limiting factor.

The concept of the law of the minimum has been modified as additional elements have proved to be essential in plant nutrition. It has been extended to include other factors such as moisture, temperature, insect control, light, plant population and genetic capacities of plant varieties.

Carrying capacity refers to the number of individuals who can be supported in a given area within natural resource limits, and without degrading the natural social, cultural and economic environment for present and future generations. The carrying capacity for any given area is not fixed. It can be altered by improved technology, but mostly it is changed for the worse by pressures which accompany a population increase. As the environment is degraded, carrying capacity actually shrinks, leaving the environment no longer able to support even the number of people who could formerly have lived in the area on a sustainable basis. No population can live beyond the environment's carrying capacity for very long.
In fact, the criterion for determining whether a region is overpopulated is not land area, but carrying capacity.