Criticism of Law of Limiting Factor

A limiting factor is a factor that limits the growth of a population’s size. It denotes the scarcity of essentials necessary for survival and growth. It could be non-availability of biotic factors like resources, food, mates, or abiotic factors like environmental imbalance, extreme weather conditions that stop or limit the growth of organisms in an ecosystem. Limiting factors are generally expressed as a lack of a particular resource that is essential for survival. It ultimately determines a habitat's carrying capacity, which is the maximum size of the population it can support.

Blackman’s Law of Limiting Factors

• The law of limiting factors was postulated by a plant physiologist- F.F Blackman. Blackman’s Law in simple language states that a process that is dependent on several factors, the pace of the slowest factor decides the rate of that process.

• Blackman postulated this law based on his study on limiting factors on the photosynthesis system of plants.

• He stated that the biological factors are affected by several factors, but the rate at which they affect the whole process is different.

• Blackman took reference of photosynthesis to prove this. As we know, plants require adequate amounts of water, sunlight, temperature, carbon dioxide, and chlorophyll to carry out photosynthesis. The scarcity of any of these components affects the rate of photosynthesis.

• He also illustrated that the rate of photosynthesis cannot be increased by increasing only one factor or altering only one essential component.

• Thus, other factors or components should also be increased in proper proportion to achieve a higher rate of photosynthesis.

• Therefore, Blackman’s law is based on a principle that the absolute magnitude of factors is less important over their relative magnitudes.

• This gives us a theory that any physiological process which is affected by more than one factor is governed by the law of limiting factor.

• It should be noted that the factor which is present in higher amounts may be a limiting factor in comparison to the one present in smaller amounts. This is due to more requirements of the factor present in higher amounts.

• Thus, when the rate of the process becomes constant due to a limiting factor, it is regulated by regulating the amount of only the limiting factor. For example- a leaf that utilizes 5 mg of CO2 per hour in photosynthesis is exposed to certain light intensity. If only 1mg of CO2 enters the leaf in an hour, the rate of photosynthesis is limited due to the CO2 component which is also the limiting factor in this case.

• The rate photosynthesis increases with the rise in the concentration of CO2. Any further increase in the CO2 concentration does not affect the rate of photosynthesis. It only increases if the intensity of light increases.

Criticism of Law of Limiting Factors

• Blackman in his illustration of the limiting factor exhibited abrupt breaks in the rate of photosynthesis that was caused due to the low intensity of light.

• This illustration was criticized by his coworkers James, Harder, and the others.

• According to them, the rate of photosynthesis does not decline abruptly, but gradually, whenever one of the factors becomes limiting.

• This is a result of all the chloroplast being not under the same environmental conditions.

• The exposed chloroplasts receive more light and CO2 than the deep-seated ones.

• If Blackman’s illustration is true and these factors are limiting, the photosynthesis will be affected only in some chloroplasts.

• This will result in the gradual decrease of the photosynthesis rate.