Secondary productivity and what limits productivity

What about the animals?

A rabbit eats grass. The rabbit grows, and it has young. So the animals also make new living matter, even though they do not make food from light.

Secondary productivity is the rate of formation of new organic matter by the consumers. They make it from the food that the producers made. So the secondary productivity can never be more than the net primary productivity that feeds it.

What controls the productivity?

Not every place makes the same amount. NCERT says that primary productivity depends on three things.

1. The plant species that live there, and their photosynthetic capacity.
2. The environmental factors: light, temperature and water.
3. The availability of nutrients.

So the productivity differs from one type of ecosystem to another. In general, a tropical rain forest, a coral reef and an estuary make a great deal for each unit of area. The open ocean and a hot desert make very little.

Why is the ocean poor for its size?

NCERT gives the fact and does not give a full reason. The standard reasons in the textbooks are these.

• Light. Water takes up light, so the algae can live and photosynthesise only in the top layer.
• Nutrients. In the open ocean, the minerals that the algae need are scarce, and they sink with the dead matter.

On land, the plants stand in the light and take minerals from the soil. This gives the land a higher productivity for each unit of area.

The scarcest factor

When several factors are needed, the one that is in shortest supply sets the rate. A plant with plenty of light and water but too little nitrate makes little new matter. This idea is called the law of the limiting factor. The model below uses it. It is a simple model, and real plants respond in a smoother way.

You should find that raising a factor that is not the scarcest changes nothing, and raising the scarcest factor raises the productivity until another factor becomes the scarcest.

The energy budget of an animal

The next part is not in the NEET syllabus, but A-Level and IB ask it. An animal does not use all the energy in its food. It follows a budget.

• Some of the food passes out as faeces. It was never taken into the body.
• The rest is assimilated: $\text{assimilated} = \text{eaten} - \text{egested}$.
• A part of the assimilated energy is burned in respiration.
• What is left goes into new tissue, which is the secondary production:

Two efficiencies follow from it. The assimilation efficiency is the assimilated energy divided by the energy eaten. The production efficiency is the secondary production divided by the assimilated energy.

Try it in Python

The first program shows the idea of the scarcest factor.

The second program works out the energy budget of a herbivore (the extra part).

Practise with new numbers

Common mistakes

• Saying that the secondary productivity can be larger than the NPP. It is made from the NPP, so it is smaller.
• Averaging the factors. In the limiting factor model, the scarcest factor sets the rate.
• Giving only the area of the ocean as the reason. Light and nutrients decide the productivity, not the area alone.
• Counting the faeces as assimilated. They pass out of the body without being taken in.

Remember these

• Secondary productivity is the rate of formation of new organic matter by consumers.
• Primary productivity depends on the species, the environmental factors, the nutrients and the photosynthetic capacity.
• The scarcest factor sets the rate (the law of the limiting factor).
• A-Level and IB (not NEET): $\text{secondary production} = \text{assimilated} - \text{respired}$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Define secondary productivity.

2. Name three things that the primary productivity of a place depends on.

3. A pond has plenty of light, water and warmth but very little phosphate. What sets its productivity?

4. Why is the productivity of the open ocean low, although it covers most of the surface?

5. An animal eats $500\ \text{kJ}$ and egests $150\ \text{kJ}$. What is the energy assimilated?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, section on productivity.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems, section on energy flow through ecosystems.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy, sections on production.