Law of Limiting Factor: Blackman’s Law, Criticism

Limiting factors are those which affect the growth and distribution of various aspects of an ecosystem. More than one component affects the biogeochemical processes, but the factor that affects the most is known as the limiting factor for that process. The limiting factor can either be biological, chemical, physical or even geological which has a direct or indirect impact on the growth and development of an ecosystem. One of the important laws of limiting factors is the Blackman’s law of limiting factors which states that the rate of the slowest factor determines the overall late of the process. 

Keyterms: Limiting factors, ecosystem, biogeochemical processes, plant, factors

Read Also: Difference Between Mitochondria and Chloroplast

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Blackman’s Law of Limiting Factors

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• The law of limiting factors is given by a plant physiologist: F.F Blackman in 1905. 
• In simple terms, Blackman's Law states that when a process is reliant on several elements, the rate of the slowest factor determines the rate of the process. Based on his research on limiting variables in the photosynthetic system of plants, Blackman proposed this law.
• He stated that biological factors are affected by several factors, but the rate at which they affect the whole process is different.

Different Factors of Plant Growth

• To prove his point, Blackman used the example of photosynthesis.
• To carry out photosynthesis, plants require proper amounts of water, sunlight, temperature, carbon dioxide, and chlorophyll, as we all know.
• The rate of photosynthesis is affected by the scarcity of any of these components.

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Limiting Factor on the Rate of Photosynthesis

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• Assume that a leaf is exposed to a light intensity that allows it to use 5 mg of CO₂ per hour in photosynthesis.
• The rate of photosynthesis is limited if only 1 mg of CO₂ penetrates the leaf in an hour due to the CO₂ factor.
• However, as the CO₂ concentration rises from 1 to 5 mg/hour, the rate of photosynthesis rises along the line AB. 
• Any rise in CO₂ concentration will not influence the rate of photosynthesis, which has stabilized at the BC line.
• Only if the intensity of light is likewise increased from low to medium will the rate of photosynthesis increase farther down the line BD.
• The medium light intensity becomes the limiting factor again at point D, and the rate of photosynthesis becomes constant again along the line DE.
• Similarly, at even greater light intensities, an increase in CO₂ causes an increase in photosynthetic rate along with the line DF.
• Further increases in CO₂ concentration will have no beneficial effect on the rate of photosynthesis, which becomes constant along the line FG beyond point F, when increasing light intensity becomes a limiting factor.

As can be seen from the diagram above, the rate of photosynthesis cannot be improved just by increasing one element. 

• For a positive effect, the other components should also be increased in the right proportion. Other elements that affect the photosynthetic rate, such as temperature, water, and so on, might also become limiting factors under specific conditions.
• Blackman's law of limiting factors can be applied to any physiological process involving several components.
• It is vital to remember while teaching the principle of limiting factors that the absolute magnitude of the elements isn't as relevant as their relative magnitude.
• Because its requirement may be in traces, a factor that is present in the slightest quantity may not be limiting.
• Another element present in greater amounts, on the other hand, may become a limiting factor because it's required for the physiological function to be higher.
• And once the rate of a process has become constant due to a limiting component, it can only be enhanced by raising only that limiting element and nothing else.

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Criticism of Law of Limiting Factors

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• In his depiction of the limiting factor, Blackman showed abrupt interruptions in the rate of photosynthesis produced by low light intensity.
• James, Harder, and the rest of his coworkers slammed this illustration claiming that when one of the components becomes limiting, the rate of photosynthesis does not drop suddenly, but rather gradually.
• This is because not all chloroplasts are exposed to the same environmental circumstances.
• Deep-seated chloroplasts receive less light and CO₂ than exposed chloroplasts.
• If Blackman's illustration is correct and these effects are limiting, only some chloroplasts will be affected.

Read Also: Difference between Photosynthesis and Respiration

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Things to Remember

• Limiting factors are those factors which affect the growth and distribution of various aspects of an ecosystem.
• The law of limiting factors is given by a plant physiologist: F.F Blackman in 1905. 
• Blackman's Law states that when a process is reliant on several factors, the rate of the slowest factor determines the rate of the process.
• He stated that biological factors would either positively increase or decrease depending on all major contributing limiting factors.
• Blackman's law of limiting factors can be applied to any physiological process involving several components.
• Blackman’s law of limiting factor was criticized by claiming that when one of the components becomes limiting, the rate of photosynthesis does not drop suddenly, but rather gradually.

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