Limiting
factors (nutrients, light, water, space etc) are the factors present in limited
supply.
The
three laws explaining the effect of different factors on organism:
1.
Law
of minimum- Liebig’s law
Carl
Sprengel developed a law, later popularized by Liebig, called as law of
minimum.
Other terminologies used for the law
are: Liebig law or Liebig’s law of minimum. According to this law, the growth
is regulated by the limited factors i.e. resources in scarcity and not by the
resources in abundance.
This
law was originated after studying and observing the crop and plant growth. The
studies reveal that if we increase the supply of nutrients already present in
enough amounts, it does not affect the growth of plants i.e. no further growth
happens.
But when we provide
the nutrients which are present in scarcity or in limited supply, growth
improvements are detectable. Hence, it is the limiting factor that affects the
growth of plants. Liebig law was explained by Dobeneck in his own thoughts by
an example called “Liebig’s barrel” (figure
1), in this barrel the capacity to
sustain water is limited by the shortest staves similar to the growth
regulation by most limiting nutrients.
Figure
1: Liebig’s Barrel
The principles of Liebig’s law conclude
as a concept, where “The availability of nutrient in scarcity is the limiting
factor which is equally important for plant growth as the nutrient in abundance”.
The scientific applications of “law of
minimum” are extended to ecosystem models or population. The organism or plant
growth depends on many factors (organic or inorganic/ biotic or abiotic
factors). At any given time, these factors are available in different levels
and one among all different factors, are present in minimum levels, thus
limiting than other factors. Liebig’s law of minimum explains that it is this
limiting factor whose rate of availability affects the growth.
Recently, this law is applicable on natural
resources management. For example, Phosphorous or other resources recycling is
required for the non-renewable resources, limited in supply without any other
alternative or substitutes. Biotechnological innovations including genetic
modifications extend the limits by altering the dependency on biological
limiting factors.
2.
Blackman’s
law of limiting factor:
Blackman’s
was a plant physiologist with his most study on limiting factor on plant’s
photosynthesis system.
He stated that a number of factors
regulate the biological processes but the factors in different amount affect
the process on the whole.
For
example, photosynthesis requires basic components like water, sunlight in
proper intensity, chloroplast temperature, carbon dioxide, chlorophyll present
in certain required amount. Any of these factors if present in scarcity will
affect the rate of photosynthesis. In the graph (figure 2) the rate of
photosynthesis is depicted on Y axis while CO2 concentration in
X-axis. At first when the concentration of CO2 increases, the rate
of photosynthesis is directly proportional to the amount of CO2 supplied
and the graph (slope 1) shows increase in rate of photosynthesis but after a
limit any further increase in CO2 concentration has no effect on the
rate and the rate become constant (Line 1 to a). Now at this time when the
increase in CO2 has no effect on rate of photosynthesis, the
intensity of light became the limiting factor. And now as we increase the
intensity of light further increase in the rate of photosynthesis is achieved
(slope 2). After a point any further increase at this intensity will not affect
the rate and it became constant again (line 2 to b). The rate reaches its
highest limits (slope 3) at high intensity of light and CO2 concentration
and again became constant (line 3 to c).
Figure
2: Graph illustrating Blackman’s law of limiting factor
3.
Law
of tolerance- Shelford’s Law
Till
now we are concentrating on the minimal limiting factors affecting the growth
or rate of biological process.
But
Shelford’s law states that it’s not only the factor present in limits/scarcity
but also the excess/ abundance of that same factor can affect the growth,
development of organism or rate of biological process.
For
instance all nutrients required for the growth and development of
organism/plant are equally important but any nutrient in abundance may limit
other nutrients absorption, thus indirectly restricting or limiting the growth
of organism/plant.
Thus
the law of tolerance by Shelford’s revealed that the growth and development of
organism depends on the maximum and minimum limits of factors involved in the
biological process.
Thus
every factor has its own maximum and minimal limits in every organism and the
“Zone of tolerance” is the range between these two limits.
Based
on this, the environmental factors have two zones: (a) Zone of Intolerance and
(b) Zone of Tolerance. Further the Zone of tolerance is sub divided into three
zones; (i) Optimal zone, (ii) Critical minimum zone and (iii) Critical maximum
zone.
(a)
Zone
of Intolerance
The Zone unfavourable for the growth and
development of organism is termed as Zone of Intolerance. The limit of
tolerance varies from species to species with respect to different factors.
Organism
survives best if have a wide range of tolerance and broad distribution range.
(b)
Zone
of Tolerance
An
organism grows best in the Zone of Tolerance, which is favourable for its
development. This zone is sub divided into three zones:
i.
Optimum
zones: optimum zone is the most favourable zone
in the range between two extreme limits thus supports maximum for the growth
and development of organism.
ii.
Critical
minimum Zone: it’s the lowest limit of minimum
below which the organism growth is inhibited.
iii.
Critical
maximum zone: it’s the maximum limit of tolerance
zone above which organism growth ceases.
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