Trophic levels and standing crop

Levels of feeding

Go back to the chain: grass, grasshopper, frog, snake. The grass makes its own food. The grasshopper eats the grass. The frog eats the grasshopper. The snake eats the frog. Each organism is one step further from the Sun.

A step in the food chain is a trophic level (from the Greek trophe, which means nourishment). NCERT numbers the levels from the producers upward.

• T1, the first trophic level: the producers, which are the green plants, the algae and the phytoplankton.
• T2, the second trophic level: the primary consumers. These are the herbivores.
• T3, the third trophic level: the secondary consumers. These are the primary carnivores, which eat the herbivores.
• T4, the fourth trophic level: the tertiary consumers, the secondary carnivores, which eat the carnivores of T3.

One species, more than one level

The levels are about the job in a chain, not about the animal. A sparrow that eats seeds is at T2. A sparrow that eats a grasshopper is at T3. A person who eats vegetables is at T2, and a person who eats goat meat is at T3. So in a food web, one species can sit on more than one trophic level. The program below finds the levels from the web.

Why are food chains short?

You rarely see a chain with more than four or five levels. The reason is the loss of energy at each step, which the third course takes up. NCERT states that only about ten per cent of the energy passes from one trophic level to the next. After a few steps there is too little left to feed another level.

Where are the decomposers?

The decomposers do not belong to one level of the grazing chain. They work on the dead matter from all the levels. They begin the detritus chain.

The standing crop

At every trophic level there is a living stock. The standing crop is the mass of the living organisms, called the biomass, or the number of them, in a unit area at a particular time. It is measured at each trophic level.

The biomass of a species can be given as fresh weight or as dry weight. Dry weight is the more accurate measure. A leaf in the rainy season holds much more water than the same leaf in the dry season, and the water is not the plant itself. The dry mass is what is left after the water is driven off.

You should find that two plots with the same plants but a different water content have different fresh masses, and their dry masses show the difference in the plants.

Try it in Python

The program takes the web of the last lesson and finds the trophic levels that each species can occupy. The level of an animal is one more than the level of its food.

Practise with new numbers

Common mistakes

• Numbering the herbivores as T1. The producers are T1. The herbivores are T2.
• Giving one animal one fixed level. An omnivore sits on different levels in different chains.
• Measuring the standing crop in fresh weight and calling it exact. The water content changes. Dry weight is more accurate.
• Placing the decomposers at the top. They have no single level. They work on the dead matter of every level.

Remember these

• T1 producers, T2 primary consumers (herbivores), T3 secondary consumers (primary carnivores), T4 tertiary consumers.
• A species can sit on more than one trophic level in a food web.
• Chains are short because only about ten per cent of the energy passes to the next level.
• Standing crop: the biomass or the number of living organisms in a unit area at a time. Dry weight is more accurate than fresh weight.

Check yourself

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

1. What trophic level do the herbivores occupy? What are they called?

2. In the chain grass, mouse, snake, eagle, which organism is the tertiary consumer?

3. How can a person be at two trophic levels?

4. Define the standing crop. Why is dry weight a better measure than fresh weight?

5. The fresh mass of a plot is 500 g in a square metre and 70 per cent of it is water. What is the dry mass?

References

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